Very interesting thread, folks. Good reading for sure.
I had moved from relatively 'stock' ARs like a Bushmaster V-Match 16 or 20, to 'building' my own from parts, etc. It wasn't uncommon to put 3-5 shots in groups we could cover with a dime.
Shot NRA Service Rifle and Match Rifle for a while, using White Oak Armament uppers. Nothing too fancy; Wilson barrels if I recall correctly... but very well executed. They shot great - easy sub half MOA with dang near anything I put thru them.
Around that time, I also had a 'parts' gun that I'd put together, misc. lower, RRA carbine upper... stupidly accurate with the Weaver 1-4x scope I had on it at the time. 3/4 moa easy with the same handloads I used for short yard lines in my Match Rifle space gun. Then I put a suppressor on it
This was back when 'adjustable gas block' meant that big honkin' monstrosity from JP Enterprises, which wouldn't fit on my build. A lot of the other toys and fixes available now weren't, back then. Broke some pins, and the gun shot like crap. Fixed everything, took the can off. Still shot like crap. Started a long upgrade / tinker path... one of those where you mess around with it for a bit, get irritated, set it aside for 6-8 months, then dust it off and try again. And then stick the bloody thing back in the corner again. Etc. etc. etc. Changed almost everything out on that gun (including the barrel), never did get it to group worth a damn again. It's still in the corner waiting for me...
More recently I got an MSR15 Recon from Savage. Nothing too fancy, just a run-of-the-mill 18" AR carbine. Put a Geissele 2-stage SR trigger in it, which helped immensely. But again, it never has shot 'great'. After tracking down some optics/mounts issues (which I half-way expect to have been the problem with the 'other' problem child, since I swapped one scope/mount between the two) it's doing 'better'... but 'better' seems to be a little under MOA with H75BTHP, ~24 gn Varget, mixed LC brass and CCI 41 primers. Tried a few other loads, and thats the best so far.
I've got lots and lots of components and all winter to kill, so I'll keep after it but I sure have been surprised at how much of a PITA it seems to be to get half-ways decent groups out of these last couple ARs. Every once in a while I pull out the old Match Rifle with it's 26" heavy barrel and run some loads through it and go "Nope, it's not the trigger puller" as it still puts pretty much everything in 1/2-3/4 moa without much effort. Granted, it's got a 'match' barrel and a lighter trigger, but still...
Fun & games, for sure