Is a FFL required for work on non-serialized parts?

[WeaponOutfitters]

I assumed that working on non serialized parts is ok for non FFL holders.

I am interested in selling 14.5" upper receivers, but will need a friend to pin/weld muzzle devices for me. Given that an AR-15 upper is not a serialized part, or technically considered a firearm... would it be ok for him to work on said parts without a FFL?

[tenpercentfirearms]

Yes. Make sure he doesn't have any AR15 lower receivers in his possession while he works on them or that can be constructive possession of a short barreled rifle. If he has some AR pistols, then he would be legit.