MAS Defense stripped upper receiver, RTB 16 inch barrel, standard mil-spec barrel nut. Will attach a Samson Evolution rail that fits on mil-spec barrel nuts. Have molybdenum grease.

Does it need a barrel nut shim? How much does it need to be torqued down? Do I need an upper block in a vise, as for a 5.56 mm or other centerfire upper?

For the bolt assembly- I have a barely used Ceiner Atchisson conversion, and a RTB stainless collar. Any tips on removing the conversion adapter/chamber insert, and replacing it with the collar, without breaking or damaging the rails on the assembly? I plan to polish the feed ramp and chamber according to the advice in the tacked  thread above, or wherever it is..

Thanks for your assistance.

Barrel nut shims resolve timing issues for the nut and gas tube, the latter of which you will not use.  I don't foresee how you could need one.



I like to end on 50 to 80 pounds after the third tightening, just like a regular AR.



People have built uppers without a vise and block, but I wouldn't try it.



Be gentle with the frame.

If you are going to install a Samson Evo handguard, indexing is required, however; the two pieces that go around the barrel nut and inside of the rail allows a little bit of rotational play before you tighten down the screws at the bottom.  It needs to be indexed pretty close where a gas tube would normally sit in one of the barrel nut notches but can be off a little because of the play in the fitment of the handguard.  When I installed a Samson Evo on my 9mm PCC SBR, I did not need any shims.  I did have to torque the nut to just over 70 ft-lbs to get it to index close enough.  I have had no issues with my build. See pic below...
Attached File

You won't be able to use a bev block or reaction type rod on the assembly. Just buy one of those plastic clam shell vise blocks to hold your upper receiver in the block.

To undo your chamber adapters just gently pull apart the two rails enough to rotate out the adapter.

See at 00:45 here

Pro Tip...
When assembling the collar on the conversion rails.
Place one collar slot completely on a rail cove.
And rotate the collar, while slightly spreading the other rail.

It is easy.

Or, if a flat-top and you don't have a fixture:

Very helpful replies so far, everyone. Thanks !