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Thread: compatibility

  1. #1
    Gunco Member gassaway's Avatar
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    Default compatibility

    do all the HK lower g3/mps/usc/ump take same fire control groups?

  2. #2
    Gunco Good ole boy kernelkrink's Avatar
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    No. The modernized "plastic fantastic" UMP and such use totally different lowers and internals.

    The "old style" G3/MP5/HK33, etc. generally all accept the same type of lower assembly, the .308 rifles have a longer "tail" on the back because the stock mount is longer and is held with 2 pins. The 9mm and .223 variants have only one stock pin and are shorter back there. They interchange as a unit otherwise, and the shorter ones work fine on the .308 rifles. The only internal differences are the ejector is caliber specific and some calibers used a different hammer and/or spring, so those need to be changed out if the caliber is changed.

    The one exception is the MP5K and SP89 lowers, they do not have a tail at all and use a pin at the upper rear of the lower instead, unique to those firearms. Internals are the same though.

    Now, there are several different varieties of "old style" lowers. The standard one found on most older rifles is the metal "SEF" type that used a separate plastic grip on a stamped metal lower. The internal FCG box was the same on all these, Three positions, Safe, Economical, and Fun! (safe/semi/full)

    Later versions used an all plastic one piece lower that was injection molded over a sheetmetal liner. They used the same SEF type box inside, but the selector was longer to compensate for the thicker sides of the lower. These had the finger groove grips similar to the stamped ones.

    Now we come to the burst lowers, the ones with 4 or more positions and a different internal setup. The burst lowers use internal detents in the FCG box, so the outside of the lower is smooth sided. Different selector, they are also ambi models with a selector on each side. There were two versions of the burst FCG box, one had the ratchet mech in front, the other in the rear. Dunno if they interchange or not, but the rear mounted ones are the easiest/only ones to use with a registered auto sear. These usually have the smooth grip assembly, commonly called the "navy" type.

    There are literally dozens of variations in the "old style" HK lowers and internals, especially in the burst pack lineup. Basically, as long as you buy a "matched set" of lower and internal box you can get it to work on any of the old style HK models.

  3. #3
    Gunco Member gassaway's Avatar
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    next question I got a stripped metal lower mp5 can i use Cetme internal on it. I want to use it as the basis for a tube gun
    Im trying to be cheap

  4. #4
    Gunco Good ole boy kernelkrink's Avatar
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    The whole internal trigger pack will fit in as a unit, but the individual parts inside a CETME pack are totally different, as is the semi auto only conversion. From what I understand, CETME US made and semi auto conversion parts are scarce right now, while the HK stuff is everywhere.

    Be aware, any gun that can accept an HK/CETME type lower will need the front mounting point made as a shelf like on the semi auto HK91, the original G3/MP5 type front push pin attachment allows an unmodified full auto unit to mount and the resulting gun would most likely be considered full auto by ATF even if no full auto parts were present. The lower must also be converted to semi auto only BEFORE it is modified to attach to the semi auto shelf styled receivers. A full auto pack modified to fit a semi auto shelf mount is considered a conversion device all by itself and is a machine gun even if there is no gun it can fit present.

  5. #5
    Gunco Member gassaway's Avatar
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    so whats best place to find a trigger ack assembly?

  6. #6
    Gunco Good ole boy kernelkrink's Avatar
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    Robert RTG has the G3 packs for $85, you will also need the selector @$5.

    http://www.robertrtg.com/g3.html

    These have the .308 specific ejectors, since you are building a tube gun you should be able to make it work by figuring it into your bolt design. MP5 specific stuff is getting harder to come by and is very expensive:

    http://www.hkspecialist.net/trigger.htm

  7. #7
    Gunco Member gassaway's Avatar
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    thanks I saw those spanish sub guns in SGN love to try my hand at it. probably should have looked for a cetem trigger group fist would have saved $$$$. If I remember a few years back these items were going for like 30 bucks times change!

  8. #8
    Gunco Good ole boy kernelkrink's Avatar
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    You might also look into buying a whole rifle kit and parting out the stuff you don't need. Back in the day I bought a bunch of those $100 HK kits from FAC and parted them out on Ebay in those wonderful days before they banned black rifle parts. I dunno why, but people on there were willing to pay about twice that when broken down into sub-assemblies. I actually built my rifle and picked up a set of spares for free simply by cherry-picking what I needed from about a dozen kits and selling off the rest. The profit paid for my HK receiver and a lot of other FAL stuff I was into at that time. Nowdays the kits prices are up there where turning a profit is almost impossible, but the CETME kits are still fairly cheap enough you might break even.

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