by Bmt85 » Mon Nov 12, 2018 3:09 pm
I saw your post, but didn't have a chance to respond.
What I've noticed is that most 450 AR's (most AR's in general, actually) are over gassed, especially in the carbine length gas systems. Some have been under gassed, but that doesn't seem too common. What you do with an AR that won't lock back is figure out which it is, because they can both show similar symptoms. Another thing to check is the extractor . You want to make sure the corners aren't biting into the rim of the case because that can cause the case to not eject when its suppose to, so when the next round comes up, it can cause stove piping, double feeds, or it won't pick up the next round from the mag. It can sometimes cause the bolt to not lock back properly.
This is what I do on ANY new AR build. Apply plenty of oil to the BCG, inside of upper receiver, and trigger assembly (in new). I usually use a lighter gun oil for break-in. With no mag or ammo around, cycle it by hand quite a few times. Also check trigger function. Make sure it's not hanging up on anything. Next, take the BCG out, hold it straight up, with the bolt facing up. Now, slip a round under the extractor, and straighten out the cartridge (your depressing the ejector). There should be a bit of force to do this, if not, check extractor spring, plug, and o-ring/d-ring (if you have one), and also check ejector spring, too. If there is a decent amount of pressure, the next step is to slowly release pressure on the round. The round should have no issue dropping out of the bolt. If it doesn't that usually means your extractor is biting into the case and holding on to it, when it's not suppose to be. Lightly round the corner's on your extractor if needed. If everything check's out, clean out the chamber of any oil and go to the range. When I go, I take multiple weight buffers with me. When ready, load ONE round in the mag. DON'T START WITH A FULL MAG! Reason for this is if you end up with a trigger issue, and it wants to try and go full auto, your not dumping a whole mag (read DANGEROUS!). Now load mag, rack, and shoot. Check ejection pattern, and see if the bolt lock's back. If ejection pattern is good and bolt locks back, load 2 rounds in mag, and repeat. If not then, see what your ejection pattern is. Usually if it's at 1-2 o'clock, it's over gassed, If it's 5-6 o'clock it's under gassed. If over gassed, swap in heavier buffer's. If it's horribly over gassed, you want to get an adjustable gas block. If under gassed, it needs the gas port opened up a little. If I think it's under gassed, I try swapping the heavier buffer's in, lightest to heaviest, and see how it cycles. It should get worse as the weight goes up.
I try to stay away from heavy springs because they have a lot of force coming back when loading the next round. Usually not too much of an issue, but in this cartridge, bullet's moving forward because of momentum is a concern. The heavy springs will only make this worse. I only have one build that required a heavy spring. It's a 5.56 pistol that is horribly overgassed. It has a pinned gas block, and I didn't feel like putting in an adjustable. It doesn't cause any issues with it, so I run it.
Edit - I keep adding thing's.