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FM 9mm carbine upper
I have a FM 9mm carbine upper that I'd like to use with a mil-spec lower with endomag; but I also want a buffer/spring set-up that will perform with my standard 5.56 upper without changing the set-up (laziness). I've though of some options to keep from breaking the bolt stop with 9mm upper while still maintaining the lower's dual functionality. Would like comments on what would be best option to have good functionality with both 5.56 & 9mm uppers:
1. 9mm standard buffer (around 5 oz.) with .308 carbine spring)
2. same 9mm buffer & spring as #1 with spacer added
3. same 9mm buffer but with standard carbine spring using spacer
4. extended 9mm buffer (around 6 oz.) with standard carbine spring
5. extended 9mm buffer with .308 carbine spring
Thanks for any advice. WB
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I can’t help ya on the lower. I’m running a dedicated 9MM lower.
Right now it’s built up as an suppressed SBR, but I was thinking about putting a FM 16” upper on it to run PCC.
Off topic, but how is that FM upper? Which length do you have? Is it the side charger?
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What you're looking to do will not work very well.
Do yourself a favor and get a dedicated 9mm lower like the FM Glock.
Use the 45 heavy buffer with standard spring and enjoy shooting.
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Should have mentioned that both my 9mm upper and 5.56 upper are 16". I think option 1 or 2 above will be my starting point(s). Any of the options will work with the 9mm upper- the question is really will the same setup provide acceptable reliability with the 5.56 upper?
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6. 9mm extended buffer (7.5 oz) w/ 308 spring for the 9. And when you switch to 5.56, just do a buffer swap to a carbine or H1. A 9 w/o a spacer or the extended buffer will not run right...
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Would the A5 stock tube and spring work for this application?
You can get buffers from 3.8 to 6.8 ounces. Combines with different Sprinco Springs maybe?
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To those interested in my project to have an AR lower that will do dual duty (9mm & 5.56) with the same buffer/spring combo the results are as follows: I never got beyond option 1 above (standard 9mm buffer with .308 carbine spring) because I got perfect function and lock-up with both the 5.56 and 9mm uppers. An added bonus was that subjectively I felt that both the 5.56 and 9mm uppers on the same lower shot "softly". Of course, the loads used were not "hot" (.223 Fed. black box 55 gr. and my 9mm 115gr. moderate power handloads). I may go back to the range with some 5.56 ammo and some 9mm factories, but for now I'm satisfied. WB
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Combines with different Sprinco Springs maybe?
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Well I do swap out my SBR from 5.56 to 9mm upper regularly. I use a "Colt" pattern magazine well adapter, heavy A5 buffer / extended buffer for 9mm use and I use SpringCo Springs as being used suppressed most of the time. FYI.. I did add a 9mm Glock magazine lower too later it's still a "AR Pistol" yes it's a pos reddot sight. I prefer to use longer buffers with 9mm not only to stop bolt catch breakage but no brass to catch in hammer pocket and It will remind me to change buffer for 5.56 when charging it.
