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The SIG Sauer Adaptive Carbine Platform (ACP) is an interesting beast. While it’s not the first device to convert a pistol to carbine-like weapon, its predecessors generally worked with a very limited range of pistols. SIG’s likely the first company that can claim their carbinizer is multi-vendor friendly. They offer the ACP in three flavors – the standard ACP is just a frame and a Quick Disconnect (QD) sling mount (you supply the sling). For only $130 more . . .

The Enhanced model adds a detachable bungee sling and SIG’s red dot sight which normally carries an MSRP of $199 all by itself. In my mind, this is the configuration that makes the most sense for those unwilling to go the NFA paperwork route.  The final flavor is the Law Enforcement version which, for $30 more than the standard version includes a folding shoulder stock. Unfortunately, the fascists at the ATF see that scary stock as magically creating a short barreled rifle (SBR) which means you’d have to register it on a Form 1, pay the $200 tax stamp fee and wait six or so months to get permission to use it.

This, incidentally, adds a little confusion to the mix. In keeping with the ATF’s method of determining what exactly a firearm is, it’s highly likely that they would consider the pistol itself to be the firearm, meaning that if you later wanted to use a different pistol with the ACP, you’d need to re-register and re-pay the transfer tax. With that in mind, I just can’t see any reason why someone would want to go through all the ATF hoopla just to get a folding stock.

To understand what the ACP is, perhaps it’s best to first understand what it isn’t. It isn’t a firearm in and of itself. It’s really just a frame into which you strap a compatible pistol (more on that later). It also isn’t a true carbine. The intended benefit of the carbine is a longer barrel (which increases accuracy and muzzle velocity) in a still relatively compact form factor.

A good point of comparison is the line of Sub-2000 folding carbines made by Kel-Tec.  When folded, these guns have similar dimensions to the ACP and use standard pistol mags. When unfolded, they have the longer barrels and shoulder stocks of true carbines.  The ACP in its non-NFA versions lacks both the longer barrel and the shoulder stock, so what you really end up with is pistol accuracy in a somewhat more cumbersome form factor.

So, what’s the ACP really good for? Good question and one that I’m determined to try to answer. For the purposes of this review, I procured the Enhanced version of the ACP.  Given the fact that you’ll need some sort of rail-mounted sighting device anyway, it made sense to go with the Enhanced model as it came with both the red dot sight and the bungee sling.  One note here: I paid for my ACP with my own hard-earned cash and SIG Sauer had no idea that I’d be using it as a review piece (which is probably a good thing as we’ll see later).

Accessory Mounting Options

Since you’re going to be using a railed pistol in the ACP (a requirement for the attachment method), you’ll already have the ability to attach one device (a laser, light, or combo laser/light) to your gun. The ACP gives you the option to attach more than one. Whether this is of any value to you depends on your needs and tastes. I will admit that it’s nice to have a red dot sight on a pistol, but there are other ways to do this if you really want depending on your gun. The ACP allows you to mount a pistol scope if that floats your boat. Even if you just wanted to mount a simple set of iron sights, the ACP increases the sight radius pretty significantly, so that might be a deciding factor for you.

In theory, the ACP has the capability of giving you more stability. If you purchase the NFA version, you get a folding stock which would very likely enhance your accuracy. For those who don’t wish to get into the NFA morass, the bungee sling offered on the Enhanced version might improve your accuracy some. The added weight of the ACP helps to manage recoil, too. This in turn may help you bring your gun back on target more quickly than with a pistol alone.

A final advantage is that the ACP allows you to carbinize (yep – just coined that term) any caliber pistol that will fit the ACP including .22s, 9mms, .40s and .45s.

One thing that makes the ACP so, um, interesting is how easy it is to get into trouble of a Federal kind. As you can see in the picture above, the ACP has a bottom rail at the front of the gun. This rail is great for mounting things like lasers, lights, and, oh yeah, how about a fore grip?

Go ahead and try this only if you think a stay at pound-me-in-the-ass Federal prison sounds like something you’d like to try. According to a 2006 open letter by the ATF, adding a vertical fore grip to a handgun-based platform creates an item that is regulated under the Gun Control Act of 1968 as an Any Other Weapon (AOW).

The ATF can’t realistically call it a SBR, but they don’t consider it a pistol anymore since they define a pistol as being a weapon designed to be fired by one hand.  While you are certainly welcome to fight this distinction out in court, most shooters don’t want to risk felony charges, so the guidance is usually followed. It is, of course, legal to add a fore grip assuming you do it the proper way:

Option A: File a Form 1, Application to Make and Register a Firearm, pay the $200 transfer tax, and wait about six months.

Option B: Find someone who holds a Special Operational Tax (SOT) designation that allows them to legally make firearms. Have them screw on the fore grip and register it, then transfer it to you on a Form 4 as an AOW which only carries a $5 tax (but still requires the weapon to be held by the manufacturer for 6-7 months while you wait for approval).

Obviously, you’d have to be a nuts to go through option A (unless you live in a state that forbids SBRs but permits AOWs). If you’re going to go through the effort and expense to file a Form 1, you might was well register the damn thing as as SBR and call it a day. Cost to register is the same and you’ll have more accessory options.

In either case, the question is still open as to what part actually carries the NFA registered serial number. The ACP frame itself isn’t serialized, so do you engrave a serial number on the ACP itself or do you register the serial number of the host pistol? Chances are no one at the ATF will have the stones to go on record to answer this question definitively, which means you choose one or the other and then patiently await your arrest and court date.

UPDATE 9/28/2012

One of the glossy gun mags had a “first look” at the ACP in their November 2012 issue.  In that article, they covered the rules with respect to properly registering for the LE version of the ACP.  Basically, it is the pistol itself that is registered as the SBR.  This means that you cannot use any other pistol in the ACP (unless you register those as well and pay the $200 tax per gun).  Plus, if you ever decide to sell your pistol, you must first write to the ATF to have it de-listed as an SBR, otherwise the buyer will have to transfer it using a Form 4 and pay the $200 tax.

There has been some discussion on the Internet around the possibility of mounting a bipod on the lower rail (which is presumably permitted) and then folding down one leg to use as a fore grip. That might work, but then again it might get your clever ass tossed into the pen. Me, I’m way too pretty to go to jail so I’ll be keeping that lower rail clear of anything that might get the ATF’s knickers torqued.


There are essentially two main shooting positions. Position one makes use of whatever sighting gizmo you decided to bolt onto the ACP. Ideally, you’d hold your pistol using a standard two-handed grip. Assuming you’re using the bungee sling, that extra point of contact, and the increased weight of the ACP should help you to keep the pistol on target more easily.

The second position would presumably employ a laser attachment to one of the rails. With the laser in use, you could then hold the pistol grip with your strong hand and the forward section of the lower rail with your support hand and fire from chest or waist level using the laser as the aiming device. This might be useful to someone who’s doing bodyguard work and needs to get a gun in play fast.


Now we come to the main question – does the ACP improve your accuracy at all? To test this, I stacked the deck as much as possible in the ACP’s favor by using my trusty SIG P229 in 9mm. I had planned to show the targets from my testing, but guess what – the ACP made very little difference in group size over simply shooting a naked P229 at 20 and 50 feet.

While you might think the extra anchor point – the sling – would give you some added stability, the problem I found is that any benefit provided by the sling was more than offset by movement of my whole body, which when transferred to the ACP via the sling didn’t improve matters much. If I had to postulate the reason for this it would be that while a traditional rifle sling pulls the gun towards your body, the bungee cord sling of the ACP forces you to use your muscles to push the gun away from you in order to get tension.  Doing this fatigues your arm muscles, which in turn rapidly causes you to lose the steady aim that you might otherwise get with the pistol alone.


According to SIG’s marketing video, “The Adaptive Carbine is unique because it accepts virtually any full sized pistol.” The video goes on to show an M&P, a railed 1911 and a Glock slipped into the ACP. Since I don’t own any of the aforementioned guns (sadly my 1911 lacks rails), I broke out my meager collection of railed guns. Besides the P229, I also have a P220 Super Match, a Beretta 92-A1 and a Heckler & Koch USP Tactical.

First up was the Beretta. Everything seemed to be working well right up to the point where I tried to release the slide – it refused to go completely into battery. A look at the internals of the ACP using a mirror (no way I’m looking down the barrel of my gun)  delivered a horrifying sight – I could see the plastic recoil spring guide rod flexed up at an angle – the gun couldn’t completely move into battery as the bent guide rod was preventing it from doing so.

I compared the Beretta’s guide rod location relative to the barrel of my P229.  Whereas the center of the guide rod on the Sig is a little more than a quarter of an inch below the bottom of the barrel, on the Beretta, the center of the guide rod sits almost a full half inch below its barrel. Strike one. That extra ¼ inch or so excludes the Beretta from being an ACP host weapon.

Next up was my Sig Sauer P220 Super Match. I figured that since it’s a SIG it should work, right? Things looked good – no issues with the guide rod and everything seemed to seat okay. It was a struggle getting the back of the ACP clipped into place because, at just under 9 inches, the Super Match appeared to be too long (for comparison purposes, the regular P220 is an inch and a quarter shorter). And I had a problem when I tried to bring the gun into battery. The fit was so tight that the slide simply wouldn’t lock forward. Strike two.

My final option was the H&K USP. In some ways, the deck’s already stacked against the H&K. For some reason that must make sense only to Germans (much like their predilection for shouting, pounding on tables and invading France), H&K decided to go with a non-standard lower rail. Fortunately, a number of companies make adapter rails that slip on the H&K and convert it to a standard rail format. Unfortunately, the extra height added by the rail adapter combined with the very high front sight made for a pistol that was just too tall for the ACP. But it really wouldn’t be fair to call this strike three given the modifications necessary to bring this pistol into line with the railed pistol standard.

Fortunately, since I recently went shooting with one of my friends, I had one final candidate to try. He owns a SIG SP022 with a rail. As I had hoped, his pistol fit the ACP, so we finally have another hit on our hands.

The moral of this story is not to rely on SIG’s marketing materials and assume that just because you have a gun with a standard rail it’s going to fit the ACP. If you own one of the three mainline SIG pistols (P226, P229, P220) you’re good to go. Deviate from this short list, though, and you may find your pistol doesn’t work. SIG Sauer would do itself a service if they published a list of models known to work with the ACP. As of this writing, no such list exists, so caveat emptor.


As Foghorn pointed out in his first look at the ACP back in January, the charging handle sucks. It’s heavy and far from smooth. Looking at the underside, you can see gobs of lubricant that have been liberally spread around to try and reduce friction, but you are still left with the fact that you’re trying to drag a large piece of aluminum across another large piece of aluminum while under pressure from both the gun’s recoil spring and a second one built into the ACP to keep the charging handle forward.  Using my highly scientific and precise measurement approach (picture below) I clocked the pull weight to rack the slide on my 9mm P229 at about 35 pounds or so.

On the plus side, you don’t have to do this too often – generally, you’d need to do it once to chamber the first round and then occasionally if it became necessary to clear a malfunction. The other issue is that the charging handle is only about ¾ of an inch long.  This makes it tough to grab as you can’t get more than a finger or two on it. A good modification might very well be a longer handle, but good luck finding something like that.

The ACP does have appropriate cutouts along the length to make accessing the pistol’s controls fairly easy – if you own a SIG or something with a similar manual of arms.  Reaching the de-cocking lever and slide catch release levers were no problem. But if my Beretta had fit, it would have been impossible to reach the Beretta’s rear slide mounted safety/de-cocking while it was wearing the ACP.

I do have some concerns around clearing malfunctions as the standard “tap and rack” motion would need to be modified to a “tap and work the charging handle” motion.  Furthermore, I found pulling back on the charging handle and manipulating the slide lock to be cumbersome should I want to lock the slide back for some reason.


In the end, I’m left scratching my head as to the ultimate utility of this gizmo. Street price is about $310 for the standard model with a $110 up-charge for the Enhanced version.

Who is going to use it?  Well, aside from a SIG whore like myself who will likely plunk down their cash for just about anything they make (yeah, I’m still trying to rationalize that 716 rifle I picked up a couple of months ago – like I really needed another .308), I’m not sure who would want one. As I noted earlier, if you want the benefits of a carbine, the Kel Tec Sub 2000 seems to make a lot more sense and you get a full gun for about the same price.

Another issue is that unless you own a SIG, you can’t be sure that your will fit this thing. It was clearly designed around the P220/P226/229 railed form factors and will likely work best with those guns only. Everything else is a crap shoot.

I’m told that SIG is selling these things like hotcakes at the moment. It took about 3 weeks from the date I ordered an ACP to get mine. I think that may be related to its coolness factor. People initially think it’s a great gadget to have without asking themselves the question, “What is this really going to do for me?” Then again, a quick search of doesn’t show any used ACPs for sale, so it seems that – at least for the moment – the people who’ve bought these things aren’t in a huge hurry to unload them.

In the final analysis, I’m left with a feeling of self-loathing. I love SIG products. I own five SIG pistols and four of their rifles and I love every single one of them. The SIG line of guns are definitely my weapons of choice and if SIG had fanboys the way Glock does, I’d probably be president of the local chapter. That said, journalistic integrity demands that I look at this through somewhat more jaded (objective) eyes and assess its actual utility.

Adaptive Carbine Platform (ACP)
Length: 7.66″
Width: 2.75″
Height: 5″
Weight: 17 oz
MSRP: $369 – $499 depending on configuration

This review is based on the Enhanced Version

Ratings (out of 5 stars): 

Ergonomics:  * * 1/2
In the plus column, it affords more control over recoil and a faster way to get your gun back on target. On the downside, you’ll need to use an accessory sight when the gun is strapped in. Add to this the fact that it takes 35 pound of force using a 3/4 inch knob to rack the slide and locking the slide becomes a bit cumbersome. Depending on your pistol design, its controls for the safety and/or de-cocking lever might not be accessible.  If you have a malfunction, it might also be more difficult to clear.

Build Quality: * * * *
A pretty solid piece of kit. Initially getting the pistol mounted into the ACP is a bit of a job, but once you’ve inserted and removed your gun a few times to wear down the plastic insert, the gun goes in and comes out pretty easily.

Coolness Factor: * * * * *
Some folks will think the ACP makes you look like a tool, but if you saw some serious looking bodyguard type sporting one, you might change your mind. It does look pretty futuristic and when I hauled it out at the range, everyone wanted to give it a try.

Compatibility: * * *
Like the man says, if it fits, it ships, er, shoots. The trick is to make sure that one of your guns really does fit before you plunk down your cash. On the other hand, if you like the idea of a carbinizer for your pistol, the fact that it works with non-SIG guns and enables you to carbinize any caliber pistol you can get to fit is a pretty neat trick.

Overall: * *
It’s expensive for what it offers. If the base model was priced around $200, I could see this being a nice toy for the gadget-obsessed. Yes, it looks cool, but it didn’t really make my shooting any better. As noted earlier, slung on the shoulder of protection detail folks, it might be useful, particularly where rifles or carbines might not be permitted. If the version with a shoulder stock was available without all of the NFA silliness, I could argue that SIG might have a small winner on its hands, but the non-NFA flavors simply lack any real utility for the average Joe.

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  1. IMHO… Absolutely pointless unless you SBR it, and even then it’s a waste of money and time. A dedicated pistol caliber carbine makes so much more sense.

    But I guess if you’ve got a few hundred bucks lying around that you have no idea what to do with, and this looks cool to you… It’s your money and I won’t judge… Too much…

    • Wouldn’t there also be an issue with the removal of the pistol? Or is that just to pistol from a carbine classification?

      • If the gun was originally manufactured as a pistol, you can go pistol->rifle->pistol as many times as you’d like. If the gun was originally manufactured as a rifle, you’ve got an SBR. Yes, that is stupid, but that’s the current ATF interpretation. Still handy for those pistol->carbine kits, though. (I should get one for my Neos one of these days…)

        While I appreciate Sig trying to cater to the SBR/AOW crowd, this is such an amateur-hour product that it’s embarrassing. The FAB Defense KPOS G2 is so much better in every way…

  2. Thanks for the honest review… I’m a SIG fan myself, but I had a suspicion that the ACP isn’t all sugar and rainbows.

    I’m thinking if you had a chance to try a version with the folding stock and a foregrip, it would definitely make the ACP a lot more fun to use, but like you said… who’s wants to go through all the crazy ATF paperwork to get those things?

    Seems like the ACP and RONI are only for people with the $$$, time, and patience to do through the official hoops… and even then, it looks you’re just better off getting a dedicated carbine that accepts pistol mags.

  3. I have a co-worker who has a similar product designed for Glock pistols. As explained to him by the BATFE, if you possess the carbine adapter (unit with shoulder stock) you don’t have a NFA weapon. As soon as you procure a pistol that fits the adapter, you are in possession of a SBR. The pistol is registered as the NFA SBR and it will always be registered as a SBR. If you sell that SBR registered pistol to another person, the new owner would be in possession of a SBR unless the firearm is transferred using the appropriate forms. If you already own a pistol that you want to convert to a SBR, that weapon has to be registered as the SBR before you can purchase the carbine conversion thing-a-ma-jig.

    My co-worker followed the first route, he bought the carbine conversion unit online from Natchez Shooting Supplies. Then he submitted the paperwork to buy and register a Glock 22 as the actual SBR.

    • Thanks for the clarification on this. I suspected that might be the case and as such limits the utility of the NFA ACP even further as you have to choose which pistol you are going to register (unless you want to register and pay for several of them). It also makes me wonder what happens if down the line, you ditch the ACP and a year or two later want to sell the pistol that you registered. Would you be able to sell it as a regular pistol, or would the buyer have to pay the $200 transfer tax stamp since it is now registered as an SBR with the ATF.

  4. Speaking of the Kelly-Tek sub2k- anyone know where to find one? Can’t find any in stock online.

    • Try gunbroker. Generally there are ones coming up every couple of days. Pretty happy with the Sub2k. Lots of kick in the .40 cal shooting heavy bullets, but pretty accurate and fun.

    • Those, like the RFB and KSG are difficult to find in stores due to demand. As Gon2fish said, check gun broker, or if you want a used on, the forums at; you need 15 or 25 posts to access the trading post there.

  5. to me the holy grail is a longer barrel while still preserving delayed blowback, and anything less than that is entirely pointless

  6. Another MBA marketing genious provides a solution to a non-existant problem. Bet they don’t recover the R&D, tooling and setup costs on that one.

  7. I agree…absolutely pointless unless it has a stock and it’s SBR’d…but not going to happen in CA.

  8. I’m a Sig and H&K fanboi, myself, and even I agree that a Kel-Tec Sub2k would be a better choice. And I hate Kel-Tec for their inability to keep up with supply and piss-poor materials quality and fit/finish.

  9. Will it fit a Sig P239??? I have tons of Sigs and would like this as a toy only. Just wondering. Would be fun to have and play with out in the pasture…

  10. Maybe I’m reading wrong, but you seemed to miss a third shooting position. Did you try using the handholds from position 2, and utilizing the reverse sling tension and and optics from position 1? You seemed to imply that using the extra real estate with your support hand was only good for essentially hip firing, but isn’t that how you would hold any carbine that lacked a foregrip if you were going to aim down the sight? Granted you have the inverse of a shoulder stock, but its still a point of contact as you said, Sight up for head shots and when your arms get tired just drop em, flick on that laser and go center of mass. Multiple targets ideally, The only potential utility of this thing strikes me as being slightly increasing target acquisition speed and follow up shot accuracy/speed, which would shine the most with multiple targets, preferably zombies. Seems like a fun idea anyway, maybe not entirely worth the cost/legal hoops to jump through, but a fun new way to spend some time with your trusty old pistols. Thanks for the thorough evaluation, there really isn’t enough hard data on this thing, just conjecture and flaming.

  11. Yes, I own a Sig Sauer P239 and wondering would this ACP (any of the 3 types) fit or work well with my model (P239) of hand gun?

    Thanks In Advance for your immediate reply !!! (Where’re the best purchase places?)

    Frankie Cole, Sig Sauer Owner

  12. FYI: I purchased a brand new Sig Sauer P226 .40 Cal. tacops. The beaver tail is longer than the standard P226 in which the Adaptive Carbine Adapter polymer adapter does not allowed the rear end cap to fully engaged forward. Although it comes with 2 polymer adapters neither one works.

    Does anyone have a suggestion? However; I have not called Sig Sauer yet but will do ASAP.

  13. Mike I don’t think its a wise idea to ever modify this type of accessorie by your self. On the other hand if you have the nuts and the intelligence, the reality is that this platform is perfect project for the experimentalist. Take a look at the the plastic mold….. How hard would it be to properly mold the correct rail holder for virtualy any handgun you own. NUMBER 2- If you are intelligent enough to figure out the process of melting the right plastic for the the rear mount and then figure out the correct angle to hold it then you are almost set. NUMBER 3- THIS IS THE MOST IMPORTANT FUCK UP SIG MADE AND AT THE SAME TIME THE SMARTEST….. The charging handle does not lock onto the actual slide so that the charging bolt will not move unless it is you charging the slide. This was in my opinion a mistake that makes it very hard to load ammunition into the chamber due to the fact that you are charging the weapon from the very front of the gun making it very hard with the midget one finger bolt. There should be no spring, but only a clamp type device connecting the charge and the slide together. If you wratch the ACP video the mans thumb would then be FUBAR with my new design… Thus a smart design for the idiot that would end up breaking there thumb(Lawsuit).

  14. What a thorough, awesomely informative and entertaining review. The writer is definitely in the right line of work. I was being entertained and laughing my ass off while at the same time learning some important and thought provoking information as a consumer considering this product. I sincerely appreciate it as I was considering this for my SP2022. Now I can find a more practical use for that cash. Thanks. Still laughing.

  15. I attempted to use a Glock 22. Using two of the three slots on the rail adapter, the ejection port was within the housing. This would be an issue when shooting. Using the first notch of the adapter the pistol frame was too long to allow locking the rear adapter. I even switched the plastic piece but too long to allow lock. So I will try my S&W 9 SV or a FNP 45, I’m hoping one of these 2 bad boys will work. Great article and info.

  16. Just purchased it yesterday I like it for the tote factor….with the single point sling its much easier to carry around and have readily available than a holstered weapon…I like the red dot on the top because I am a gadget junkie and once I sighted it in then I had a group better than my average. That’s just me I am a little weird especially being left handed in a right handed world.

  17. I wonder, once you register your handgun as SBR, whether you cannot carry it concealed anymore, even without the ACP? Would be logical, but highly annoying, of course. What about “shall not be infringed” ? With the same reason they disallow but-stock on a pistol, they could disallow any caliber greater than 22. The constitution says nothing about different weapon types, rifles vs hand guns, ammo, etc. How did we screw up a beautiful guarantee of freedom – unique compared to other countries – into a bureaucracy nightmare?

  18. I love my acp the only thing is I bought a gun I said I’d never own to use in it and that be a Glock 17. It works great with the forehand grip and also gives it the feel of an AR with the angle on the glock grip. I would have used one of my sigs but the 33 round magazines from glock work better then the sig high capacity mags and have more rounds. For room clearing it fit the group very well and gives you some advantages over a plane pistol.

  19. I got mine for $129.00. Its the enhanced version with the laser and single point sling. I was able to put my Glock 22 on it and heard my buddy put his FN 5.7 on it. Taking it to the range soon to see how it works.

    • I found a cheap one only to discover my 5.7 is too long for it. Even after removing the spring in the rear stabilizer and having it all the way back, once shoehorned in there it pushes the slide back a few millimeters. I’m not comfortable test firing under those conditions, but maybe your buddy won’t notice and he can report back to you.

      Now for a quick review: I was not a fan with compression fit of the adapter piece. For one anything with the instructions “just lube it up and whack it for a while” shouldn’t need instructions. Really though when the manual called for a mallet I was floored. I thought the appeal of this would be to have a quick trunk SBR/AOW conversion, but trying to work that thing on and especially back off (due to limited surface area and approach angles) over each rib of a picatinny rail is WORK. I mean I was genuinely concerned I would have to break it to get it off my 5.7. I realize they probably haven’t sold nearly as many as they would like to have by now, but they really should have made a version 2.0 at this point, or at least a 1.5 compatible with the original, with new parts addressing some of the known issues.

  20. What they neglect to mention is only certain versions have the picatinny rail section on the rail so for people who have had the idea to use the Sig brace good luck figuring out how to attach it

  21. CHEAT?
    What if you put a brace rather than a stock on the non-NFA model? Couldn’t you run any pistol without ATF issues that way?

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