Prototype multi-caliber lower for the SCAR rifle

“Troubleshooter” is a gun designing genius who hails from Germany. His latest creation is a prototype lower receiver for the FN SCAR rifle.

STANAG 5.56mm magazine.
7.62mm NATO M14 magazine

Troubleshooter has to work within Germany’s firearm laws. He used an AGM airsoft SCAR replica for the upper reliever and built his own replica bolts and barrels.

7.62x39mm magazine
Magpul PMAG
5.45x39mm magazine

Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.


  • snmp
    • snmp, thanks, the video was there but I had made an error in the embed code.

  • Dakota

    That is very cool, having access to common types of ammunition that would be likely found in the field is always a plus.

  • vtb


    Unfortunately your comment about using ‘A-soft’ parts is misleading: One could think that airsoft toy could be modified into firearm (which is not true) – and it could affect large group of people.

    I think that it will be correct to point that it’s a replica gun made with plastic parts from toy-guns (instead of mentioning the exact model and manufacturer) for demo purpose only and incapable to shoot any real round.


  • Cobetco

    isn’t this what the scar was supposed to to in the first place?

  • ctc833

    Wouldn’t you need a barrel change as well (if the FN decides to look into it?) for example you are changing from a 5.56 to a 7.62? Unless there is a mechanism whereby the barrel caliber can be manually changed on the go, otherwise you would just need to bring different barrels with onto the field.

    one last question the pressure from the 5.56x45mm and the 7.62x51mm is that much of a difference, so i would assume that the bolt and the chamber can handle that kind of pressure? (from wikipedia search, the 7.62NATO actually produces less psi than the 5.56 which I always thought would be the reverse)

  • Jake

    I’m not quite sure what’s happening here…. Is he able to feed multiple types of rounds through the same gun without modification? Or is he swapping the barrel around?

  • John C

    Hmmmm… I think youre right cobetco – this was supposed to be one of the main features of the SCAR early on, but I have heard nothing about a different caliber barrel or bolt from FN. They are really behind of stuff like this.

  • Other Steve


    Yes, of course it would need a new barrel. It would also need a new bolt regardless of pressure. A 556 bolt face, extractor, ejector, and mass will simply not work for 762.

    The appeal is not changing caliber depending on what magnyou may find on the street, it’s that one registered gun can be mission-configurable. For civs, this means you only need to file for one sbr stamp, for mil it means the armouror only needs to know one platform, for maintenance and training it means commonality.

    Yes, this is what the scar was/is supposed to be from day one, but FN is almost as slow as HK…. It’s truly baffling.

  • Other Steve

    As for pressure goes between 556 and 762. Be careful because mil uses pressure at mouth iirc, instead of the saami check point. This can account for large differences.

    Also, if 556 has the higher number that doesn’t mean there is more energy. The 308/762 case has a lot more volume. Meaning that to get to even the same pressure as a 556 round it needs more powder and that there is more energy in the bullet.

  • Chris

    +1 To the M14 mag idea. I’m quite tired of proprietary magazines that cost significantly more than what’s not only common, but tried and true as well.

  • Mu

    This shows two funny effects of German gun laws. First, high capacity magazines for semi-auto rifles are strictly off-limits, hunters are alowed 2 rounds, collectors with special permit 5. That’s it, your standard PMAG gets you 5 years for violation of the war weapons act. Secondly, smaller caliber barrels don’t count towards your license’s number of guns, so having multiple calibers from one systems means you only have one entry for the biggest caliber, and the rest are bonus.

  • ZomBkiller

    If FN makes this, and makes it work, I will have no choice but to give in and get one. To be able to shoot both my ComBloc rounds from a single rifle, just by a few parts swaps would be wonderful for the range and a SHTF bug out scenario. My $0.02. Great info, great idea. Thanks Firearm blog and its readers.

  • JK17

    Hi, read the blog for a long time, my first comment… am i the only one who is a bit suspicious that he never shows the barrel change or anything in the video, its all done off screen? its a clever design no doubt, but if its so brillient why is filmed so poorly? (not to critisize, if it is real ill be as impressed as the next man)

  • Chortles

    This reminds me of “bringing the CM901 to the SCAR”…

  • zincorium

    Well, the SCAR-H could almost certainly use the smaller rounds- it’s a question of the length of the action more than the pressure.

    But that’s not particularly productive, since you’ve got a heavier rifle than the SCAR-L.

    I can see the benefit of FN producing this lower, since it means they can sell the same lower to different countries, and maybe the benefits of changing calibers for someone who shoots as a hobby, but militarily? Not any sort of priority.

  • vtb, I don’t see how I have been misleading. I used the word ‘replica’ many times.

  • Lance

    My money goes to the Colt CM 901 way over this. I prefer to switch upper than mess with my barrel and upper receiver. This is not a official FN anyway this is a independent fan made version anyway.

  • Likvid

    So, it’s like CZ 805 now. SCAR with copied feature from it’s own copy, lol 🙂

  • Nadnerbus

    Wouldn’t swapping out the upper be more efficient than barrels and bolts? I mean, the lower is serialized, so making that part modular for different caliber types makes tons of sense. But the upper is going to be specific to caliber as far as the sightings in of Irons and Optics. So swapping a different caliber, then swapping bolts and barrels means you would have to go down to the range anyway and re-zero.

    Also, I can see how having different calibers feeding out of a modular magwell would be a nightmare as far as ensuring reliablity. It’s hard enough to find a gun design that feeds reliably from one type of magazine.

  • Aurelien

    @ Chris : the magazine for the SCAR-H/Mk17/SCAR 17S is the same as the FN FAL, which i believe has been through trial by fire. The difference is that the design has been slightly modified to lock like the AR platform, not the FAL platform.

    @ JK17 : every engineer i know, however brilliant, cant type a good article to save its life. The guy probably only knows the basics of camera/light works, but he seems to understand the basics (and more) of workmanship and gun function. Thats not bad.

    @ Mu : they probably are afraid that if they get more than 3 rounds they’ll go ahead and invade Poland again.
    The guy should move to Belgium, the country where you can own any functional weapon you want (up to howitzers) without any papers as long as you dont own the live ammo to feed it.
    And then with the right papers you can get the ammo.

    • noob

      oh my god. Belgium implemented Chris Rock’s “bullet control”?

  • Maverick Moore

    I have literally never heard of a 5.45mm. What gun uses ammo like that?

  • peter

    i like the idea of a 7.62×39 scar then being able to switch to another round like the 7.62 NATO for longer range

  • Jay

    Of course you have to change the barrel, but why is that such a big deal. The barrel on the SCAR is held in place by six screws. All you have to do is remove the screws, change the barrel and retighten the screws.
    Since this video was intended to show the rifle actually cycling all this different cartridges, he maybe didn’t used all the screws.
    I know High Desert Dog is already working on caliber conversions for SCAR. 9mm, .22LR and 7.62×39 are in the works right now. What’s nice about this multi caliber lower, is that uses real AK mags.

  • …and real?

    ” …The CZ 805 BREN A1 features a unified and modular weapon system manufactured in 5,56×45 mm NATO and 7,62×39 mm calibres (with the prospective introduction of the 8×43 mm SPC Rem), which has been designed as personal weapon for any combat branch. From the functioning point of view, this is a gas operated automatic weapon with locked breech having rotary breech block,. Its modular concept and the unified system of interchangeable barrels attachment, using exchangeable breech block and gas tube, allows easy and quick conversion to a different calibre. The design of interchangeable magazine well enables utilisation of the standard M-16 magazines implemented by NATO, including high-capacity magazines for company automatic weapons…”

  • Cobetco

    @Maverick Moore- the Ak-74 series of weapons uses 5.45

  • Lance

    @ mavrick moore the 5.45mm round is the current Russian Military cartridge of the Ak-74 and AK-74M. It is used by Russia the CIS stae around Russia and some Eastern European countries.

    Thw MK-17 uses differnt mags than a FAL it has hole in it to use in a AR style mag well.

    The Colt CM-901 uses AR-10 mags which the M-110 uses in the military.

  • greasyjohn

    Mr. Moore, the 5.45×39 is a Russian cartridge used in the AK-74, RPK-74, AN-94, and others.

  • kvalseth

    Maverik Moore— the AK-74 uses the 5.45×39 cartridge. Google is your friend.

  • xamoel

    Sorry to correct you, but high cap mags are no problem at all in Germany. everyone can buy pmags, if you want betamags as well, noone bothers. Though WHILE you’re hunting, only a two round mag may be with you. The two round limitation of course doesn’t exist for sport shooters, during the 3gun stages you’d be f’d with a 5 round limit.

  • jdun1911

    Maverick Moore,

    5.45×39 are mainly use by the Russian, ie AK74 and variants. It is their primary small arm ammo and


    The SCAR serial number placement is on the upper and not lower. I believe this is the standard practice for most of FN products (non-AR).

  • subase

    Well maybe the military disagrees. If caliber conversion is a fairly a simple process with the SCAR, then converting the SCAR H to 5.56 AR mags and AK mags shouldn’t be difficult at all. This would make SOCOMs backtrack on the 5.56 SCAR appear less dumb. (smaller money layouts for the caliber kits, which are only useful in specialist situations, would be much easier, politically speaking)

  • jdun1911

    Why would you want to downgrade a 7.62 gun to a .22 for combat is beyond me. I can see .22 to 7.62 but not the other way around for combat.