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Posted: 3/2/2010 9:11:00 AM EDT
| I just received my FA Titan lite 308 upper assembly and after mating it to a CMMG G3 308 lower I'm finding the bolt won't lock back. Everything seems to all fit fine but the bolt catch just won't quite grab the bolt carrier. I'm running a UBR stock with a slash buffer and armalite AR10 spring if that makes any difference. Anyone got any ideas? thanks! |
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Remove the spring from the buffer tube, reinstall the buffer and try to lock the bolt back.
If the bolt moves further back to where it would lock (travels 1/4" past the bolt catch), then you are experiencing coil bind from the spring. The solution is to clip a coil or 2 from the spring and try again. If the bolt still won't go back far enough to lock back, then the buffer is too long or there is something inside the tube that is stopping the buffer from full travel. You can trim the bumper on the buffer if it is close. ETA: didn't notice that it is a G3 lower 
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Quoted:
Sorry guys, my mistake it's a MK3 lower (not the Mk 8 G3 mag model). I have the shorter Slash buffer in there but I'll check what you guys mentioned. It's a side charger and it appeared the carrier was going all the way back however. Make sure the BCG and buffer bottom out in the buffer tube without the bolt handle hitting the back of the slot in the upper. Is it equipped with a charging handle too? If so, can you lock it back with the charging handle? |
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Good call you guys, the slash buffer is 2 5/8 and I'm getting 3 5/8" travel with the spring as it stands and that's only 6 1/4 out of the 7" available in the UBR tube. That missing 3/4" is not letting the carrier back enough to allow it to lock up. My current spring is 12 3/4" it's the shortest of the 3 AR10 springs I've got. What I can't tell is whether the spring is fully compressed or not when it stops at the measurement above. There doesn't appear to be any problems with the tube and the spring slides freely over the buffer so I'm assuming I'm at full compression. So the fix is to remove coils until it locks? or til I get the full travel with the buffer kissing the back of the tube? The guy at Slash said his AR10 buffer was recommended for use with a UBR so I don't think what I'm attempting is fundamentally not going to work.
Rich |
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Having the buffer bottom out at the proper time is a good thing. A buffer of the proper length is needed to protect the receiver ring from being cracked. (The receiver ring is that threaded loop at the rear of the lower where the buffer tube screws into.) A buffer that is too short and doesn't bottom out on the tube will allow the rear of the gas key or shoulder of the 308 bolt carrier to hit the receiver ring. If it hits it hard enough or often enough, the receiver ring will crack and ruin the lower. This problem is usually found where a carbine buffer was mistakenly used with a rifle-length buffer tube. |
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thanks again for the help.
I'm not sure if my buffer is currently bottoming out on the spring or the back of the tube. It's allowing the carrier to travel around a 1/2" past the point needed to lock up the bolt but there's still a ways to go before the side charging handle impacts the end of it's groove in the receiver. Does it matter what's stopping the buffer/carrier as long as the gas key isn't hitting the receiver ring you mentioned? I figured taking off the minimum spring necessary would be best because I figured with too much spring removed the buffer could possibly start slamming the back of the tube (on the violent side) and that I might not have enough spring rebound left to properly force the bolt/carrier forward when feeding/chambering the next round. Does any of that make sense? I guess I should measure it and see if I'm getting the full travel if it's important the buffer makes it all the way to the back of the tube. I haven't had a chance to shoot it yet, hopefully tomorrow. |
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