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Posted: 6/15/2026 12:14:29 PM EDT
[Last Edit: MegaTarsal][Edited]
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Trying to simplify this but be as detailed as possible. I can't figure out why this setup works in one upper and not the upper. Upper Build A generic 9mm mil spec upper 5.5" 9mm barrel FM9 BCG (older production but has the removable weight and i upgraded the firing pin spring) mil spec charging handle Upper Build B FM9 billet 9mm upper 5.5" 9mm barrel FM9 BCG (New production BCG with removable weight that I left in place) Raptor charging handle Troubleshooting: Upper A + Lower: Flawless functioning in both semi and FRT mode with the super safety Upper B + Lower: multiple failures including failure to eject and light primer strike Upper B + Upper A BCG + Lower: multiple failures as noted before. So what could the issue be or what would make the same BCG work perfectly in one upper and not in another if all factors besides the receiver and barrel are the same??? Both BCG's have the same rear cut. I did not try BCG B on Upper A since Upper A already works great. The lower being used is an 80% AR9 Lower that takes glock mags -mil spec trigger -Super Safety FRT with centering block -mil spec buffer tube -Kaw Valley 10oz blowback buffer with standard carbine spring |
"she's not going to think that you've embarked on a secret life of crime... sketchy underworld figures, like gang members, cartel members, INTERPOL fugitives, and the lizard people.
And it's that last part as to why we need a vet. "L_JE"
And it's that last part as to why we need a vet. "L_JE"
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This Reddit thread might be relevant: https://www.reddit.com/r/AR9/comments/1hry37o/fm9_super_safe_combo_that_worked/ It describes that there's a slight difference in how much the firing pin protrudes the rear between the new and old bolts. The newer one protrudes less and when the hammer releases in SS mode it could fall just too early that it hits the bolt instead of the FP, this could cause the light strike/FTF issue. |
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Originally Posted By D_Man: This Reddit thread might be relevant: https://www.reddit.com/r/AR9/comments/1hry37o/fm9_super_safe_combo_that_worked/ It describes that there's a slight difference in how much the firing pin protrudes the rear between the new and old bolts. The newer one protrudes less and when the hammer releases in SS mode it could fall just too early that it hits the bolt instead of the FP, this could cause the light strike/FTF issue. Thanks. I've read that and a few others which helped me end up where i'm at. FoxtrotMike also sent me a document with recommendations for FRT success. My big question is why would two uppers with the same bcg and lower yield different results? |
"she's not going to think that you've embarked on a secret life of crime... sketchy underworld figures, like gang members, cartel members, INTERPOL fugitives, and the lizard people.
And it's that last part as to why we need a vet. "L_JE"
And it's that last part as to why we need a vet. "L_JE"
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I have a guess. Does the barrel in Upper A have a muzzle threaded 1/2-36, while the barrel in Upper B have a muzzle threaded 1/2-28? I was having hammer follow w/dead trigger w/live round chambered w/ light primer strike issues on one of two nearly identical FM uppers, and I sent the troublesome one back. FM Customer Service told me there was nothing wrong with it, but I DID find out that Foxtrot Mike changed their chambering reamers when they changed the muzzle threads. The chambering reamer used on the older ones was NATO spec, and it is SAMMI spec on the newer ones. (Or vice-versa...?) Anyway, my newer one is the one that (STILL) gives me feeding issues using a FRT, even swapping lowers and bolts around. I ordered a Tubb 300 BO flatwire lightweight carbine spring to try next. Haven't tried it yet, as I am getting worn down and disgusted with this project. 9mm blowbacks and FRT's are very finicky. When you get one to work, don't change ANYTHING! |
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Originally Posted By Ken6PPC: I have a guess. Does the barrel in Upper A have a muzzle threaded 1/2-36, while the barrel in Upper B have a muzzle threaded 1/2-28? I was having hammer follow w/dead trigger w/live round chambered w/ light primer strike issues on one of two nearly identical FM uppers, and I sent the troublesome one back. FM Customer Service told me there was nothing wrong with it, but I DID find out that Foxtrot Mike changed their chambering reamers when they changed the muzzle threads. The chambering reamer used on the older ones was NATO spec, and it is SAMMI spec on the newer ones. (Or vice-versa...?) Anyway, my newer one is the one that (STILL) gives me feeding issues using a FRT, even swapping lowers and bolts around. I ordered a Tubb 300 BO flatwire lightweight carbine spring to try next. Haven't tried it yet, as I am getting worn down and disgusted with this project. 9mm blowbacks and FRT's are very finicky. When you get one to work, don't change ANYTHING! Originally Posted By Ken6PPC: I have a guess. Does the barrel in Upper A have a muzzle threaded 1/2-36, while the barrel in Upper B have a muzzle threaded 1/2-28? I was having hammer follow w/dead trigger w/live round chambered w/ light primer strike issues on one of two nearly identical FM uppers, and I sent the troublesome one back. FM Customer Service told me there was nothing wrong with it, but I DID find out that Foxtrot Mike changed their chambering reamers when they changed the muzzle threads. The chambering reamer used on the older ones was NATO spec, and it is SAMMI spec on the newer ones. (Or vice-versa...?) Anyway, my newer one is the one that (STILL) gives me feeding issues using a FRT, even swapping lowers and bolts around. I ordered a Tubb 300 BO flatwire lightweight carbine spring to try next. Haven't tried it yet, as I am getting worn down and disgusted with this project. 9mm blowbacks and FRT's are very finicky. When you get one to work, don't change ANYTHING! Originally Posted By Ken6PPC: I have a guess. Does the barrel in Upper A have a muzzle threaded 1/2-36, while the barrel in Upper B have a muzzle threaded 1/2-28? I was having hammer follow w/dead trigger w/live round chambered w/ light primer strike issues on one of two nearly identical FM uppers, and I sent the troublesome one back. FM Customer Service told me there was nothing wrong with it, but I DID find out that Foxtrot Mike changed their chambering reamers when they changed the muzzle threads. The chambering reamer used on the older ones was NATO spec, and it is SAMMI spec on the newer ones. (Or vice-versa...?) Anyway, my newer one is the one that (STILL) gives me feeding issues using a FRT, even swapping lowers and bolts around. I ordered a Tubb 300 BO flatwire lightweight carbine spring to try next. Haven't tried it yet, as I am getting worn down and disgusted with this project. 9mm blowbacks and FRT's are very finicky. When you get one to work, don't change ANYTHING! Originally Posted By D_Man: Does the problem upper run fine with a standard semi FCG? Both barrels are 1/2x28. I picked one up used in the EE so the date is unknown. I am the original owner of the FM9 barrel and its probably 5 or 6 years old. The barrel makes sense that it's the culprit, I just didn't undestand the "why". Both uppers work great with a standard (non super safety) lower with mil spec FCG. |
"she's not going to think that you've embarked on a secret life of crime... sketchy underworld figures, like gang members, cartel members, INTERPOL fugitives, and the lizard people.
And it's that last part as to why we need a vet. "L_JE"
And it's that last part as to why we need a vet. "L_JE"
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My troublesome upper runs pretty good in regular semi auto too. I think it slows cartridge feeding down just enough to cause issues when the FRT is releasing the hammer just as the bolt closes. Thus, the hammer ends up down on a chambered round in FRT mode. It probably doesn't take a lot of slowing to cause problems! |
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