searcher
Gold $$ Contributor
You are going through much of the same as most do when trying to get an A/R really dialed. I've been shooting and building them for over 40 years, nearly all for precision varmint - and a few for military match. The most important things on the rifle for accuracy are barrel, trigger, tight stock/lower receiver fit. A/R's are like a can of nuts and bolts rattling around when fired. The idea is to eliminate the rattles that are not consistent - like the stock fit. Every minute change in pressure or the way you hold the stock will change the point of impact. Other vibes - like loose carriers- tend to repeat the vibe on each shot - and are not "as" critical, however loose they may be. The plastic plug you use for upper/lower gap control is adequate to get the groups you want in respect to tight upper/lower contact. Frankly - I disagree with much of what has been said by a lot of people about quality of parts - as many of them don't affect accuracy as much in an A/R as one might expect. A quality bolt is more about safety than accuracy. I prefer the JP Enhanced bolt - but this is not about recommendations on equipment - so much as loading technique for A/R's. The best things for A/R accuracy loading in my experience are keeping headspace to .002". Most recommend .003" to .005". You are not in combat. If your headspace ends up .001" short on a round due to brass irregularity - believe me - it will get pushed back to where it needs to be when the bolt slams it home. The tight range on headspace is key to small groups. I'd not waste money swapping out bolts, triggers, etc.. Powder is a bigger deal than those.
Consistent bullet release is second. That is achieved by a combination of methods - some of which - or all of which can be combined. I do them all on any ammo I want to get the most out of. First is neck turning to at least 80% clean metal. If you don't do this - there will be huge deviation in release. Next is using a bushing die to control the seating pressure. You want no more than is necessary to slam a round home without the bullet slipping in the case. Trial and error are the order here. Next is application of moly, HBN or other slippery coating to bullets. This serves the purpose of aiding a bullet release that is more uniform than without such aids, will help the round load more smoothly on the feed lips, scratch the bullet less, help prevent "cold bonding" between bullet and case if you load your ammo weeks or months before a match. A very mile crimp can work after cases are neck-turned, yet that adds to the repeatability problem of making ammo the same every time. Turning necks does, as well - but it creates a much larger return. Do your bullet slippage tests with the moly bullets - not before you apply moly to them.
To test your loads - it helps a lot to mount a forearm sled to your rifle and fire on sandbags, a toggle rest or what have you to counter movement and make your test rounds count. I affix a very powerful, proven scope just for these test firings - usually something like a Nightforce Competition. I have multiple, very accurate A/R's with the same lowers on my varmint rifles (PRS). If I accidently put the wrong upper on the wrong lower - the groups will usually shift anywhere from an inch to 2 inches, despite the upper/lower tension devices. That is because each lower may have a different mainspring suited specifically for a certain load on an upper or just because another very minor difference is at play - such as hammer thrust on the trigger, the lateral force difference in the upper/lower assembly pins, etc. It really all does make a difference.
The biggest thing one can do after checking all vibes on the gun, and getting the brass properly prepped is not being afraid to test a LOT of powders with a particular bullet. The vibration ranges on an A/R are FAR greater than on a bolt gun of any kind because of the looseness of the gun as compared to a bolt gun. And while there are a few powders that tend to work well with a particular bullet (like Varget and 69 SMK's), I have ALWAYS found something better than the old "standbys" for my rifles. When you run across the powder your tube likes - there is a good chance it will shoot more than just that one brand of bullet well in the same weight range. If you are doing a ladder with your Hornady and Sierras and see that both shot better groups with a particular powder - that would be a powder I'd go back to as a finalist among any such others found. Changing primers usually does nothing more than adjust your velocity by 20 FPS to 40 FPS. Powder adjustment usually accomplishes the same thing unless one has a weak hammer spring, Just stick to primers with thick cups. If my "killer" powder resulted in a velocity 150 fps less than the "old standby", but gave me 1/2" better groups, I'd take it in trade. I can always improve my wind reading.....
I could have just said control vibrations, go crazy on necks and headspace, try LOTS of powder.
Good Luck!
