question on BCA upper assembly

[LRRPF52]

I dunno. I once used Permatex's green "Penetrating Grade" thread locker on a gas block. The kind of locker that's super runny and designed to be applied to parts after assembly. I figured, if it's not drilled for taper pins then this stuff will help. (Was a set-screw type attachment.) Boy was I right. A couple of year later I needed to get the barrel nut off so I could change out the hand guard. And I did remove the set screws before starting. Heat gun and tapping with a hammer would not get that gas block to move. It eventually took my MAP gas torch plus some POUNDING with that claw hammer to get it moving. Petty horrible. Ruined my gas block. I may as a well add that the gas block, a Troy Industries basic lo-pro, went on originally with a nice slip fit.

Learned a lesson. No more locking compounds under gas blocks. I've told this story to people and it's come back, "That's not possible. Everything melts at 400 degrees" or similar. Doesn't change the fact that mine needed to get effin' hot. And even then it was a mm by mm process.

I use locking compound on gas blocks and gas tubes as a rule on my builds, after establishing the smoothest mate-up with the carrier key on a stripped carrier with the upper inverted.

Last thing you want is gas leakage from the gas block. The Military TDP calls for a tight fit of the gas block to the barrel for this reason.

In a large manufacturing setting, they can select gas blocks that will have the tightest fit to the journal from the parts bin, and reject the leftovers to the parts market with sales to distributers with profit.

Barrel nuts:
Les Baer used to use Red Loc-tite to secure their barrel nuts, and several other companies have done it as well on rifles where every bit of accuracy was being wrung out.

Not really helpful on a garbage barrel made from low quality steel that doesn’t stay true under heat, but it is helpful on higher end barrels to eliminate as much deviation from the mate-up as possible.

Bill A. did fleet testing with this across many rifles and found that about 50% of the rifles would shoot no matter what, whereas the others were hit-and-miss even with high-end cut-rifled barrels and all top-notch components. He applied some of the techniques the AMU used to accurate their rifles, which included Loc-titing the gas blocks and bedding the barrels, and the remaining rifles tucked into the accuracy window he was looking for.

There are some great videos on this with a former AMU armorer who goes into much greater detail, including balancing the barrel nut torque harmonics (heavier barrels required less, lighter required more), tuning sets crews, progressive rifling, receiver face squaring, sights, etc.