Frankenstein Gun Of The Day: HK91-M4 Hybrid Abomination

HK-AR

Some people think that any rifle can be improved by slapping an M4 buttstock on it. Got an AK? Slap an M4 stock on it! A 10/22? Slap an M4 stock on it! A converted semi-auto RPD? Slap an M4 stock on it! When it messed with the HK91, this M4 buttstock fetish went a little too far. The HK91 may be an obsolete, overweight, brass-chewing monster that kicks like a mule, but it has earned its place in history. It should be allowed to keep its dignity . . .

An M4 Stock? Really?

The fixed buffer tube of the AR platform has always been one of the weaknesses of Eugene Stoner’s versatile design because it can’t fold for storage or transport. It also makes that annoying ‘clunnnggg’ sound when you fire it with ear protection.

The collapsing M4 stock is an improvement over the original A2 fixed stock for most military purposes, but it’s still not a very good design by modern standards. What? How dare I blaspheme the name of Gene Stoner? 

Let me explain: because of the mandatory buffer tube, an AR stock can never fold out of the way. And because of its rear charging handle, the AR can never have an adjustable-height comb for a perfect cheek weld with optics.

Don’t take me for an AR hater: I’ve tested several, I own one, and I’m currently waiting with mounting impatience for my second one to arrive at my FFL.

There are much better options out there if you absolutely must replace an HK stock. But I’m going to tell you, flat out, that you still shouldn’t.

Don’t Mess With The HK

Sturmgewehr_45_reproduction

The PTR-91 is a semi-auto version of the German HK91, which was based on the Spanish CETME. The CETME itself was designed after the experimental StG45 shown here. If you think you see a family resemblance from the StG44 all the way forward to the MP5, you’re right.

Internally, all these guns use a roller-locking breech to keep the chamber closed until after the fired bullet has left the muzzle. Externally, they all share a similar stamped-steel receiver construction and humpbacked appearance.

The StG44/45 sired a great family of firearms in the second half of the 20th century, but the line has lost its vigor. The CETME and the HK91 along with their smaller sibling the 5.56mm HK93, are no longer fielded by any major Western armies. The MP5 has served on into the 21st century by virtue of its extreme reliability, but the limitations of the full metal jacket 9x19mm cartridge have become glaringly obvious in the age of battlefield body armor.

The roller-locking design tends to produce exceptionally stiff recoil from full-powered rifle cartridges, and those rifles are also fairly heavy. Civilian shooters love the history around these guns, but hate that their fluted chambers make their once-fired brass unusable for reloading.

It may be functionally obsolete in comparison with the evolved Stoner platform and other newer gas-operated designs, but the HK pattern has earned its place in history. Let’s not do a George Lucas-style hack job on it, trying to make it hip and modern.

Image courtesy IMFDB.org

If you really need your hard-kicking, brass-chewing battle rifle to wear a collapsible stock, spend a few bills and find a surplus H&K OEM collapsing stock like this one. Is it comfortable? Hell no. Does it give a good cheek weld? Hell no. In fact it doesn’t give any cheek weld.

But it’s part of the history of the rifle, and you definitely wouldn’t shoot any HK91 for its wonderful ergonomics anyway.

Whether or not you like H&K rifles or AR-style stocks, I think we can all agree that this genetically-modified PTR-91 is an abomination in the eyes of Eugene Stoner and Ludwig Vorgrimler alike.

comments

  1. avatar Michael B. says:

    It doesn’t look all that great but it’s more functional than the original.

    1. avatar AK says:

      Agreed.

      Besides, It can be swapped back with an HK stock with a few push pins.

      Since nobody is permanently defacing historical firearms, why the hell would I care what another person spends their money on?

      1. avatar Jay Wolf says:

        This is one of the few times I agree with slapping an M4 stock on a rifle. My shoulder can only take so much these days.

  2. avatar Rusty Owen says:

    Technically, the PTR is an American made clone of the HK91, which is a semi auto variant of the G3.

    This has been your daily firearms pedant service announcement.

    1. avatar Dryw says:

      ‘pedant’. Great word.

      That moniker in mind: “roller-delayed-blowback” is a more precise description of the function, as opposed to “roller-lock”. There is no external device (such as gas pressure) which forces a separate mechanism to unlock the rollers ala the MG42. The rollers simply timing delay the bolt until overcome by pressure.

      Since few have access to an MG42 or MG3, an excellent small-scale example would be a comparison of CZ52 and HK P9 mechanisms. The CZ52 is a great example of roller-lock as there is a clearly visible 90 degree shelf into which the rollers recess when ‘locked’.

    2. avatar Rad Man says:

      I have a PTR 91 and wouldn’t dream of putting a collapsible on it. Where would I put my take down pins? Plus, if I want to start trashing bozos (boot to the head) I’ll need a solid fixed stock with a sling mount.

      1. avatar David says:

        Omigod. Is that ‘boot to the head’ from a Dr demento skit? I thought no one else knew that!

  3. avatar 2Wheels says:

    Yeah… Although it’s sometimes a semi-practical addition (cheap and easy way to get an adjustable stock)… I’m not wild about slapping M4 style stocks on EVERYTHING.

    On an HK rifle, give me 2 slim pieces of metal for a collapsible stock or give me death!

  4. avatar Lemming says:

    What the heck? I’m not sure what Chris is worked up about. If someone was altering vintage G3s I’d _almost_ understand.

    These are PTR-91s. The m4 buttstock may look a bit odd, but is a practical way to adjust pull between different sized family members, and for summer and winter clothing. It sure seems more practical than the HK version he endorses.

  5. avatar PhoenixNFA says:

    Ahhhhh if inly HKFan were here. You guys could swap stories about how original your HK guns are.

    Utility > form. Keep your original. Great. Fantastic. Enjoy your bruised shoulder.

    But if I want a better stock that offers *gasp* cheek weld and wasn’t made for a 5 year old with freakishly large arms, I’ll get one.

    Or

    I bet FN FAL owners didn’t complain….. :D. Superior design in my opinion anyways.

    1. avatar Michael B. says:

      I love my FAL. <3

  6. avatar The Last Marine out says:

    I had a H&K 91 the stock is too short for it ,added on too, great strong rifle but it’s also hard to use the sights too.

  7. avatar Chaotic Good says:

    The HK91 was derived from the CETME rather than the other way around? I learn something new every day.

    1. avatar PhoenixNFA says:

      Yessir.

    2. avatar Michael B. says:

      Ze Germans were involved in inventing the CETME.

  8. avatar JaxD says:

    Abomination indeed.

  9. avatar Roll says:

    Ugh, the original collapsible stock on the G3 hurts, for some reason instead of a buttpad, HK put a meat tenderizer. Fun but painful, the good kind of hurt… 🙂

    1. avatar Michael B. says:

      Kind of like the steel buttplate on a mauser. They have a thing for pain, I think.

      1. avatar int19h says:

        Which WW2 rifle didn’t have a steel buttplate?

    2. avatar Stu says:

      Because WE are right and hate YOU

    3. avatar Dryw says:

      Working from memory here, but I recall the explicit design purpose of the A1 collapsible was to better facilitate launching grenades from an under-slung HK79.

      Never having used it for its intended purpose, I would agree it falls far short in the ‘comfort’ category.

    4. avatar defensor fortisimo says:

      Because you suck and we hate you

  10. avatar Mike says:

    Really? You don’t like Ohio Ordnance’s Tacticool B.A.R. with one?
    http://truthaboutguns-zippykid.netdna-ssl.com/wp-content/uploads/2014/01/630x420xP1020765-900×600.jpg.pagespeed.ic.8CqqmdVfaL.jpg

    I think an FN FNC with a UBR is pretty sexy:
    https://imagizer.imageshack.us/v2/800x337q90/547/wkh9q.jpg

    When it comes to franken guns, nothing beats the blasphemy that used to be an M1941 Johnson:
    https://imagizer.imageshack.us/v2/800x190q90/404/zevg.jpg

  11. avatar Julian says:

    I slapped an M4 style collapsible stock on my AK because I needed an inexpensive, US made part that was long enough to make the rifle comfortable. I’m not a tiny, underfed Russian peasant, and I hate the short stocks on AKs. It’s more comfortable to shoot, both because it is longer, and because it is has a larger contact area with my shoulder.

    1. avatar jwm says:

      The stocks are short on Russian service rifles to accomadate the extra thick clothing the Russians wear in their harsh environments.

    2. avatar slicer87 says:

      Lucas or any other film maker can chage their films to better suit their vision, people can change their guns however they want to better suit their needs or wants, and if you don’t like it, TS! Now I am going to go put M4 stocks on everything. People like them because you can adjust the length of pull, duh.

  12. avatar Dryw says:

    I cherish my “obsolete, overweight, brass-chewing monster that kicks like a mule” over all other semi rifles in my collection.

    Too much plastic and quad rails around these days. However, to address your contentions:

    1) Obsolete: Hard to foul, simplicity of design, easy to clean. I’ll take ‘obsolete’ then
    2) Overweight: Backpack gun it isn’t, nor would I ever use it as such
    3) Brass-Chewing: No argument here. Delayed-Roller. It happens. Though not nearly as mangling as one would be led to believe
    4) Kicks: Meh. Subjective. Its a .308

    In the interest of fairness: I would never consider the above stock arrangement. S’like a VW dune buggy Baha tailpipe on a Ferrari 348.

    1. avatar Dryw says:

      Addendum: better stock option is PSG-1 style.

    2. avatar James says:

      Agreed. The original recipe is a classic design.

      The pictured stock modification is like putting pearl grips on a P7 PSP or a Plymouth Road Runner Superbird rear wing on the back of a Daimler Double Six.

  13. avatar Noishkel says:

    Heh… Well I decided to throw a HK collapsing stock on my CETME a while back. And you’re right… forget about a cheek weld. But I DO like the rifle.

    That being said it really does aggravate me to see a M4 stock stuck on everything any more. As I think about it it seems that about the only reason they are sticking an M4 stock on everything because their relatively cheap to made. Why make a nice AK wood stock when you can just use some cheap injection molded thing. Plus that takes care of the 922r issue as well, if we’re talking about something made from a parts kit.

  14. avatar Azman says:

    Really?
    Does it improve the functional aspect of the gun? Yes. It’s easier to use for more people. Aesthetics has always been last on my list for a firearm. That said, there are better looking versions of the stock out there.

    That said, I do agree with Julian up above. Just because its not what you prefer or think looks ugly doesn’t mean it’s fecal matter.

  15. avatar Shire-man says:

    I shoot my PTR one handed with a Sig stabilizing brace. My delts and traps are hooooooge.

  16. avatar A-Rod says:

    Damn Robert, you are scaring the kids with picture. Put it away.

  17. avatar alex gama says:

    well, if you want to get technical the stock on the rifle in the first pic is an HK designed variation of an M4 stock that they put on the 416’s & 417’s which are a variation of the M4/AR10 anyway. huh, strange how things work out.

  18. avatar Sam Spade says:

    Frankenstein Gun Of The Day…Hybrid Abomination

    How about we pick a Gun Abomination of All Time?

    I nominate Red Jacket’s Tommy Gun.

    1. avatar JaxD says:

      Have you seen the chromed and flamed 1919 they did for OCC, or the BAR for Jesse James? IMO those are much worse than that tactifool Thompson.

  19. avatar emfourty gasmask says:

    I better go hide my PTR-91 with its Magpul PRS stock from purists then, geeze.

  20. avatar DisThunder says:

    Ah, CDNN. I miss getting the print catalogs. It was the closest you get anymore the Sears X-Mas one you’d pour over with a marker as a kid.

  21. avatar Taylor Tx says:

    This would look pretty cool with a Magpul UBR stock on it.

  22. avatar Bill says:

    can I put a pistol tube on it and and SB15 brace?? That’d be HOT

  23. avatar lolinski says:

    Magpul PRS stock is what you use if you want to be modern or “tactical”. I like the G3 (popular here in Norway due to the reservists). Gives you twenty rounds that will reliably stop someone in a reliable package. Only thing I don’t like is the brass mangling. Can the brass be reloaded?

    1. avatar Dryw says:

      Never done it myself, but I know of at least three fine gents who clamor for my ‘mangled’ brass every range day. According to them, its a simple matter of reshaping/re-sizing.

      At least one of them uses his reloads in an original 1970’s era HK91.

      Truth be told most of my personal mangling is a reverse impression of the fluting at the case neck. Albeit, a fairly deep ‘impression’ of the fluting. The brass is definitely not destroyed. So I can see the feasibility.

      1. avatar Tom in Oregon says:

        I leave my brass in a tumbler for a whole weekend. Looks like crap, but it reloads.

        1. avatar lolinski says:

          Forgive me for prying but how many reloads do you get with your brass?

        2. avatar Southern Cross says:

          If you are full-length sizing the cases after firing in a loose chambered service rifle, you might get 4 or 5 reloads per case. It does depend on the brass. With S&B cases you get about 1 or 2 reloads, and only the latter if you are neck-sizing. Any more and you are risking head-separations. YMMV

    2. avatar AZRon says:

      Lolinsky,

      I have reloaded for my 91 ever since I bought it in 1981. The first extra that I bought for it was an ejection port case buffer. It keeps the extraction from ruining the cases. (and makes them easier to find) I have never found the marks left by the fluted chamber to be an issue. After two hours in my vibratory cleaner, followed by normal reloading procedures, I can easily get 7 to 10 reloadings from decent quality brass. In fact, I was able to go from small base dies to neck-sizer only dies with no loss in function.

      Keep in mind that I’m not reloading “star busters”. My loads have 147 to 168 grain bullets that leave the muzzle at ~2400-2700 fps using medium-burn powders (4895 [both], 3031, 4062…etc.), I inspect, clean, and trim as needed.

      I wouldn’t trade my 91 for a new prostate. It just works…everytime.

  24. avatar Bill says:

    And what the f is the point of a 308 with a 16 inch barrel?? The 308 needs those inches to reach its potential. If you’re going to ballistically hamstring your 308, you may as well go with a light package and go AK/SKS/300 BLK

    1. avatar MMinSC says:

      I read some testing when the Ruger Scout rifle came out. Turns out the 16″ only loses about 100fps over a full length rifle barrel. Less than I expected. Probably a handloader could even make up the 100fps with a faster powder.

    2. avatar seans says:

      Maybe you should read some ballistic reports of what happens when you take off barrel length. This isn’t the 50s anymore. I ran a SCAR H(one of the few ones we didn’t have problems with) with a 13inch in the stan that could hit 800 yard targets all day.

      1. avatar rlc2 says:

        There was a great article from “the Warehous” results here – no time to find the link, but I think the bottomline was somewhere around 20″ was the sweet spot for .308, and if you didnt mind giving that up, for less accuracy, and more flash, you could go as low as 18″ and still hunt effectively at the ranges most hunters would.

  25. avatar SD3 says:

    Sierra Papa now adds M4gery adjustble stocks to the Beretta Cx4. For a fee.
    *Guh*

  26. avatar Samuel Leoon Suggs says:

    I’m not sure if a buttstock and adapter constitutes hybridization; kinda like saying a saddle on a person constitutes the first human equine hybrid.

    1. avatar lolinski says:

      Didn’t know you were that kinky.

      Actually I don’t know you at all, I just wanted to write a funny comment.

  27. avatar Alex Peters says:

    Before we get all over PTR, remember that they’re one of the few companies that took action and voted with their feet following the flurry of gun control laws enacted post-Sandy Hook. They relocated to South Carolina from Connecticut. Or, as I like to look at it, traded in Dan Malloy for Nikki Haley. 😉

  28. avatar slicer87 says:

    This article reeks of internet hateboy nit picking.

  29. avatar rlc2 says:

    Great review. I find the history on the designs very interesting, and well informed users comments on best way to improve the utility for relative newbs, to be one of the many good qualities of TTAG.

    Hey, give HKfan a break- yea, he was an opinionated fanboy, but he had done some useful homework, and he shouldnt have felt like he had to run away like a little girl…

    HK, where are you?

  30. avatar TheSleeperHasAwakened says:

    Love my PTR-91…I’ve slayed many a hog with her.

    Can’t beat the rugged/reliable design as well as the $2/$3 surplus mags!!!

  31. avatar Bob S says:

    I refinished some surplus wooden G3 furniture (lovely West German walnut) in Tung oil to give my PTR-91 that vintage look. Next I’ll get a rail forearm and a M4 buttstock. Swapping furniture is just a few push pins, when the shooter’s mood changes from period correct to Teutonic Tacticool.

  32. avatar Charlie says:

    I carried an HK-91 for deer between 1984 and 2012. It was a pretty decent rifle, and once I installed the ejection port buffer it didn’t chew up the brass nearly as bad. The scope mount was excellent and maintained zero when removed and replaced. I liked the rifle a lot!

    Negatives were:
    1. No last round bolt stop.
    2. Crappy trigger.

    In 2012 I bought an AR-10 for my boy’s deer gun, and when we were zeroing the rifles at the range the AR shot a 1.5 inch group at 200 yds, and the HK was all over the target (I was so disgusted I didn’t even measure the spread). So the HK got replaced with an AR-10BNMF, and I never looked back.

    I can’t wrap my mind around putting a crappy M4 butt stock on an HK. The OEM HK collapsible stock is a work of art!

    Charlie

  33. avatar Joey says:

    Almost everytime I go to the local gun show I come across a Cobray M11 frankencarbinesharpedgedbarrelcoolerwearing abomination. The last two times I went I saw a couple with and M4 stock…. With some saggy pants wearing fools salivating over them. Something like this… http://usmachinegun.com/prodimages/a%20Mini%20stock2%20002.jpg

  34. avatar victor says:

    I owned a 1980’s HK91 for many years. I reloaded my brass with “zingers” as I used to call them, basically a 308 loaded with 110 gr round-nosed 30 carbine bullets. They were really fun to shoot, almost no recoil, but nothing I would take deer hunting, but really fun on Texas jack rabbits. I never had any problem with the original stock, it fit me well. The only faults I had with my set up was #1 scope mounts, I had a b-square see thru scope mount, it would constantly move aft, It did not affect the accuracy but it messed up the eye relief. #2 a problem with the rifle’s design. The magazine receiver is too tight and makes rapid changing of magazines difficult if you are in a hurry or under pressure. This was pointed out to me by a friend of mine who carried a G3 as a mercenary in South Africa. I traded it for a M1A-1 even up and I still think I made a good choice.

  35. avatar dubbs says:

    Keep checking out various PTRs at lgs, but I’m thinking FAL for the same price built, or less if I buy the receiver and have a buddy who worked as a gunsmith, put it together.

    As far as a”purist” standpoint? The PTR isn’t a true HK anyway, it would be akin to a Taiwanese version of the Colt M4-“looks the same,shoots the same, BUT NOT the same”. This is a .308 plinker and if I,had to depend on .308, I have a tried and true M1A
    Scout rifle…..

    1. avatar HotDiggity says:

      A Springfiield Armory Inc M1A is no more a “true” Springfield Armory rifle than a PTR 91 is an HK. That’s a terrible argument. Springfield Armory Inc bears no relation to the Springfield Armory of Garand and M14 fame. At least PTR uses Former HK tooling and drawings.

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