Short Stroking: Trim Buffer Spring?

Sapper33

Sapper33

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So I'm having an issue with my AR, it's firing, but not reliably ejecting the spent case, let alone picking up the next round:

The catch is it's a 7.62x39 upper, shooting Czech ammo.

out of 50 rounds (10 mags):
-0 successful last round hold opens
-less than 5 shots picked up the next round
-~20 rounds would not eject

All the rounds that failed to eject got stuck in the same place, the bolt would cycle back so that about 1" of the case was visible in the ejection port, but would get stuck there.

Cycling by hand everything works hunky-dorry, even shot decent groups. Originally I thought it was the mags, but cocking by hand would lock the bolt open every time.

I know 7.62x39 is a slightly lower pressure round than .223 (50k vs 52k CUP, Speer #14), and seeing that this is cheap ammo, may be even lower. So was thinking I may need a lighter buffer/spring combo to cycle properly.

So I picked up a spare buffer spring this weekend ($5 @ gun show, same type of butt), I was thinking that I could trim the spring and stretch it to make up the length. This should decrease the spring weight.

Anyone have experience doing something like this, or fixing this kind of problem?
 
If you trim the spring, there may not be enough force to properly chamber a round... There is a consequence to this action :D

It sounds like you have the wrong sized buffer or a gas port alignment/sizing issue.

Was your upper built or bought as a whole?
 
Bought mostly together. I got the receiver, barrel, gas system and bolt as an assembled unit. I had to get a bolt carrier for it, but an using a non-FA carrier.

Seeing that it did work, albeit infrequently, I'm thinking a slightly lower spring tension wouldn't be totally detrimental.
 
damn.. I misread your post. I presumed it was .223 that you were shooting.

I do that... I reply before I finish reading the full post and make an ass out of myself, just like now.

Well then I'll have to agree with you on this, you can try trimming the spring one segment at a time and see what happens. But if that is the case in which the pressures are lower for 7.62 russian then your problem would make sense with a standard spring.

Personally I would probably see if increasing your gas port size is an option. Only because you want good spring tension to chamber a round and you want enough gas to push the bolt all the way back for proper ejection. Problem is, if the port is too big and you get too much pressure pushing the bolt you run into other problems.

Not to many people up here have 7.62 russian uppers, I would suggest posting your question on www.thehighroad.us as well. You may get an answer if no one here can chime in.
 
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