Gun Review: The SIG Sauer P556 Pistol


The AR-15 pistol is the Rodney Dangerfield of the semi-automatic 5.56 mm world. It just doesn’t get any respect. It’s considered tough to shoot accurately, being too small and light to manage a rifle cartridge with no shoulder stock. Some claim that it has too much recoil, and other than being a range toy that eventually gets tossed into the back of the safe or sold, it has no real use (unless of course you’re planning to make it into a short barrel rifle). Being the unabashed SIG whore that I am, though, I decided to procure a SIG Sauer P556 pistol. My main reason for doing this was that I wanted an SBR and I wanted something to play with during the interminable wait for the ATF to get around to approving my application (eight months now and counting). At the time . . .

I didn’t own an AR-15 lower receiver that had not come as part of a rifle and due to the vagaries of the various federal gun laws, it turns out that while you can convert a rifle lower into an SBR, you can’t first convert it into a pistol to shoot while you wait.  You can purchase a full AR-15 pistol and later convert it to a SBR and you can purchase a stripped lower and build a pistol out of it, but since I would have to spend money anyway, I decided to go the SIG 556 route.

Why?, you may ask. Well, it’s pretty simple. The SIG 550 line of rifles uses a different operating mechanism than the direct impingement (DI) operation of the classic AR-15.  It’s operating system is also different from the short stroke piston designs found on other AR-15 style rifles including SIG Sauer’s own 516 line. I’d heard good things about the 550 line and wanted one in my collection, so I figured it might as well be my SBR.

The P556 is built on SIG Sauer’s well-respected SG 550 platform. The 550 itself employs a long stroke piston-operated rotating bolt locking mechanism reminiscent of Mikhail Kalishnakov’s original AK-47 design. As a result, one of the first differences that you notice between the P556 and traditional AR-15 pistols is the absence of the buffer tube protruding from the back of the gun. It’s not there because you don’t need it.

This has the immediate advantage of allowing you to use a folding stock to reduce the overall length of the rifle when transporting it or using it in close quarters. The original 550/551 line of rifles used a proprietary magazine, but when SIG Sauer brought the gun into the U.S., they developed the 556 model that used the standard STANAG magazines found in your favorite AR-15.

The long stroke piston design affords two improvements over the AR-15. First, as it uses a piston to reset the bolt rather than showering the carrier with filthy gas, the gun runs cleaner. Whether this makes one whit of difference to the reliability of the gun is a matter of significant debate. What it unquestionably does is to keep the innards cleaner than what one would find on a DI gun, so it makes maintaining your gun a bit easier.

The long stroke piston is also superior from an accuracy perspective to the short stroke piston design found on guns such as the HK 416 and SIG 516. Short stroke pistons strike the bolt carrier group hard to create enough momentum to cycle them back to eject the spent cartridge and load a new one. This high impact collision occurs while the bullet is still in the barrel and can result in a shock sufficient to throw the aim off just a bit. While this really doesn’t matter at the intermediate ranges the 5.56 mm cartridge is designed for, if you’re trying to make a long range precision shot, the short stroke piston design is not likely to be the most accurate system. A long stroke piston by comparison starts very close to the bolt carrier so when it strikes the bolt carrier, the shock is reduced.

Second, the long stroke piston is designed to remain in contact with the bolt carrier during the entire rearward travel, so it operates more as a gentle push rather than a powerful strike. There’s minimal if any shock to the rifle when the piston is operating so the rearward travel of the bolt carrier is much smoother and the chance of throwing off the aim of the gun much less. For this reason, the SG550 line of guns are well respected for their medium- to long-range accuracy.

If the long stroke piston design is so great and the AK-47/AKM/AK-74 line of rifles uses it, then why are AKs so inaccurate at distance, you may ask? The reason is that they’re manufactured to very loose tolerances. The SIG 550 line, though, are manufactured to relatively tight tolerances which is why they have accuracy that AKs can only dream about. If you doubt this, go shoot an AK side by side with a SIG 556R rifle that fires the same 7.62×39 mm round and you’ll see the difference.

The P556 is sold in two flavors; the first (and the one that I bought) comes with the Swiss-style hand guard. The other version (SWAT model) features a quad rail. Purchasers of the Swiss-style version get three short rails that can be mounted to the bottom and sides of the hand guard allowing you to mount a weapon light, laser, etc. Both versions have an integrated receiver rail which is where you can mount your optics.

Included in the box is SIG Sauer’s standard red dot. It’s not a terrible sight, just not a particularly good one. While I’ve never had a problem with one, folks who know tell me that in any given multi-day rifle class at the SIG Sauer Academy, at least one or more of these die. Then again, since you’re not likely to be popping targets 200+ yards away with this pistol, it should be fine for this application.

The P556 also features back-up iron sights which take the form of a permanently mounted front sight and a small rear sight that folds completely down into the rail.  These are BUISs in the truest sense of the word as the rear sight is a very thin piece of metal and wouldn’t likely stand up to daily use and abuse for very long, so you are going to want to use something else as your primary aiming device.

I initially mounted all three of the short rails to the hand guard, but it wasn’t particularly comfortable to hold, so I removed the left and bottom rails. Currently my gun is only running with the right rail, which is perfect for mounting accessories. I initially mounted my Streamlight TLR-2 laser/flashlight combo to the gun and it made the P556 an awesome home defense gun. The activation switch for the TLR-2 was in easy reach of my support hand without having to take my hand off of the rail. And the 10-inch barrel makes the gun very easy to maneuver in close quarters. Plus, once I had my laser dialed in, I was able to fire from the hip using the laser for sighting and consistently placed rounds in a reasonably tight grouping at 16 yards.

The P556 is capable of accurate fire well beyond 16 yards, but I had to transition to the red dot to accomplish this as the laser isn’t bright enough to illuminate my target beyond 16 yards when I tested it at the local outdoor range (and I ain’t about to test fire it in my basement.).  In an attempt to get better distance for the laser, I swapped out my TLR-2 for a Beamshot GB9000G which uses a green laser, but it didn’t really buy me much over and above the TLR-2. The activation of the light on the Beamshot isn’t as good as the TLR-2, so I will likely switch back in the not too distant future.



The P556 in stock configuration is plenty comfortable to hold. As I mentioned, I had initially mounted all three of the included rails to my P556, but that made the gun cumbersome to hold as the rails on a 10 inch barrel are so short that you have to wrap your hands around them and they protrude too far to make this very comfy.

Obviously, mounting a fore grip on the bottom rail would give you a good place for your hand, but that would make the gun an AOW in the ATF’s eyes, so so don’t do it unless you have tax stamp in hand. Instead, I went with a single rail mounted to the right hand side which is where I positioned my laser. Once I get my SBR paperwork back from that ATF, I may revisit that.

I ran into a problem when I discarded the SIG red dot sight for something else. I first tried my Lucid red dot, but the nuts that secure it to the rail are mounted on the right side and can’t be switched. Unfortunately, this gets in the way of the charging handle so that was a non-starter. I then tried my Strike Fire. That worked, but is still a bit on the cumbersome side. The ideal would be an Aimpoint Micro T1 red dot, but at $600, I’ll deal with cumbersome for a while. FWIW, the SIG red dot fit with no issues, but if this is to be used as a personal defense weapon (PDW), I’m not willing to trust my life to it.

Contrary to what I had been led to believe, recoil on this gun is rather manageable. When using the red dot, I can keep the back of the gun mere inches from my nose and when firing, the gun lightly taps me in the face. Rapid fire and accurate follow-up shots are child’s play. When you consider the fact that you’re loosing full-power rifle ammunition from it, you quickly come to grasp the effectiveness of this gun as a PDW.


One thing that I really like about the P556 is that it cheerfully accepts any 30-round magazine that I want to shove into it. I’ve noticed that on my SIG 516 patrol rifle, some mags take an inordinate amount of force to seat. Not so with the P556. Every single magazine snapped in, no problem. Several hundred rounds of milsurp penetrator ammo resulted in not a single misfire (and a huge, stupid grin on my face). And I’ve yet to clean my P556. As long I keep it lubricated, it runs like a champ.


Between the integrated rail on the receiver and the optional small rails that can be mounted to the hand guard, you can pretty much load this gun up with anything you want.  Uncle Mike’s makes a great little gun case that, while designed for sub-machine guns like the H&K MP5, fits the P556 perfectly. Bonus: you can even have a 30-round magazine loaded and locked into the gun and it still fits into the case. That makes this an excellent trunk gun, especially in places like New Hampshire where you can’t carry a loaded rifle in your car, but since this is technically a pistol, that prohibition doesn’t apply.


For a pistol, accuracy is pretty decent. The funny thing was that the laser enabled me to get a tighter group firing from the hip than I was able to manage using the red dot simply because I was much better able to brace the gun against my ample midsection. Nothing like a few extra inches of soft beer belly cushioning to keep the gun rock steady. My first group was fired with the red dot from 25 yards, and while nothing to write home about, would have kept every round in the bad guy and inflicted a fair amount of damage in the process.


Firing from the hip with the laser, I got an even tighter grouping on my head shots from 16 yards (the farthest I could still see the dot).


Now, it would certainly not fare as well as a rifle for long range engagements, but it’s not intended for that use. If the bad guy is beyond 25 yards, your chances of evading are much better. And if he is within 25 yards, he’s a corpse.

All of that said, however, I still see this gun as a niche weapon. If you plan to convert it to a SBR, it makes sense. The funny thing is that on the SIG 550 series rifles and pistols, it’s the upper receiver rather than the lower that carries the serial number. That means to convert it to SBR use, you register the upper receiver and then simply swap the pistol lower for a rifle lower (which you can mail order from SIG Sauer for $369 list).

If you want to do it on the cheap, you also have the option of removing the end cap of the pistol stock and attaching an AR-15 style buffer tube to which you can mount a standard stock. I’m going the route of the dedicated SIG stock because it folds and I want to keep this gun as compact as possible. Plus, the cost of the P556 plus the SIG stock is about $300 less than if I had purchased the SBR version of the 556 and I get to shoot it during the waiting period for my ATF paperwork.

SIG now offers the P556 in both Classic and SWAT flavors with the SB15 Stabilizing brace  pre-attached. Those versions cost about $130 more than the base model which is about what you would pay for the brace separately (but it’s still a good deal as it includes the necessary buffer tube for mounting the brace).

Beyond the whole SBR thing, though, this gun is a PDW and nothing more. As I noted earlier, its small size and awesome firepower make it a true nightmare for the bad guy in a home invasion situation, but you do need to remember that you’re firing rifle rounds, so undesired wall penetration is a risk if you miss.

While we’re on the topic of penetration, I decided to shoot my P556 side-by-side with my 16″ SIG 516 rifle and chronograph the results. While the six inches of reduced barrel size did make a difference, it wasn’t as bad as you might guess. I tested both 55 grain Fiocchi match ammo and 62 grain milsurp penetrator rounds. I shot 20 rounds from each gun and averaged the results. The mean speed of the 55 grain ammo was 2,806 fps while the P556 managed 2,431 fps, a 13% reduction. The 62 grain ammo out of the 516 clocked at 2,986 while the P556 managed 2,615 FPS for a 15% reduction in speed.

Is this a gun I’d recommend? That depends on you usage plan. It’s a wonderful platform on which to build an SBR and you can build it cheaper than buying a ready-made 556 SBR from SIG. I’ve never shot an AR-15 pistol before, so I can’t comment on how it would compare to one of those, but bear in mind that a standard AR-15 is going to add about 6 inches to the overall length for the buffer tube, which isn’t needed on the P556. If you are looking for a good home defense weapon or trunk gun that packs the power of 5.56 rounds in a short form factor, this is the way to go.

Overall, I like this gun. Given what I know now, if I were starting from scratch, I’d probably go for a 300 BLK SBR because it packs more punch and can fire subsonic rounds which is a benefit if you have a suppressor. Keep in mind that even with a 5.56 suppressor, the P556 is still going to be a louder gun as there are no 5.56 subsonic loads that I know about that will reliably cycle a semi-auto rifle and are available to the general public.

That said, 5.56 rounds are available for a lot less money than any 300 BLK rounds and the subsonic version of 300 BLK will cost even more than the supersonic ones. I have a reloading rig so I can build all the subsonic 300 BLK rounds I need. But if you don’t reload, you need to evaluate what’s available to you and 5.56 rounds are likely to be much more available and a lot cheaper than 300 BLK for the foreseeable future.


Caliber: 5.56mm /.223
Barrel: 10”
Sights: Sig Sauer Red Dot / BUIS
Finish: Matte black
Overall Length: 20.5”
Overall Weight: 6.3 lbs (without magazine)
Cost: $1,207 list, street price under $1,000

RATINGS (out of five):

STYLE * * * *
Say what you want about SIG Sauer, they know how to build nice looking guns.

It’s a pistol that shoots a rifle round. It’s better than an AR-15 pistol, but you’re still shooting a pretty powerful cartridge.

Hundreds of rounds through it over several months, never cleaned, never a misfire or a failure to feed or extract.

The Swiss style stock limits you a bit, but other than a laser/light combo and a red dot, what more do you really need to hang on an AR-15 pistol?

It’s great for what it does. If you have a need for it, you’ll love it,  If not, then just keep walking.