About 15 years ago, I ran into a gunsmith at a local shooting range who had squared and trued-up everything he could think of for a Grendel build.
He squared the barrel extension, the receiver/extension/barrel nut faces and bearing surfaces, trued-up/cut the threads himself, and de-edged/blended/polished his feed ramps.
He was shooting .5” groups out of the gate with it. I think he had a 22” barrel of one of the pedigreed barrel shops, maybe a Shilen.
He’s where I got the idea for the barrel extension work I do for reliability/feeding/babying brass.
As to the question about bolt carriers, most people don’t realize the bolt carrier actually rides high inside the raceway due to cartridge stack upward pressure from the magazine spring. The rails-in-contact are the top rails on the carrier, while the bottom rails kinda bounce off the inner lower raceway track. The initial contact area for the bottom of the carrier is the center spine at 6 o’clock that rides on top of the cartridge in pre-presentation.
Once the carrier clears the cartridge stack, the bottom rails are free to fall into-position with gravity, then they lift above again once the bottom spine rides over the next cartridge in the magazine and the stack lifts the carrier back up into tension against the inner top of the raceway.
What 45-90 is doing with the 4 hemisphere bolts on the carrier tail is controlling some of that banging around that happens normally with AR-15s, centering the carrier tail throughout its travel inside the RET, if I’m interpreting his placement of those bolts correctly ( I might be totally off my rocker as well.)
Basically one follows in the footsteps of those armorers willing to discuss, in print or video, the things they do to make match winning ARs.
Then one adds, subtracts, and mixes their findings into what is learned.
It's a never ending endevor, always learning.
Here is a visual difference...Same match ammo, same day...2 rifles
the top group is a factory Areo Precision upper fully assembled by them...
Bottom is first 5 shots fired out of a new Proof barrel assembled in an upper with all the gunsmith accuracy enhancements listed.. with 5 into half inch right out of the gate, and is to be expected.
This is normal accuracy, with the full process, and a quality barrel...5 shots will hit .25" to .6" with a variety of ammo...and sometimes LC brass with cheap varmint bullets will shoot .3" & .4" 5 shot groups...so it doesn't always take match ammo to be accurate.
Yes, it all makes a difference. Unfortunately some of it requires machine tools, and specialized equipment, but do everything you can with hand tools and glue in or shims to improve accuracy.
There are other things never discussed but were taught, that do not require machine tools to improve accuracy.
I do not use them in my build/ assembly, but others might. ARs are accurate.
