I lapped the front of the receiver (looked like it needed it).
I know, we had a long drawn out discussion on lapping.
I do it. All the time.
I picked the BCA right side charging upper since I would be prone, No-Gas, and it seemed stronger.
There where other but more costly options.
I check for squareness by lapping a little with polish. Break through the anodize.
Then lap for 360 contact. Not the 28" barrel in this pic.
For diameter fit I measure OD of the extension, check extension slop in the upper, then mask and spray a thin coat of High Temp paint. NO ONE Does that. Really thin shim stock (half thou or one thou) is hard to work with and if you do a thermal fit, LockTite is messy.
Let that dry well and burnish with 800 grit paper for about a .002" increase (about 0.001" of paint).
Do a warm/cold fit up then lock in place with a barrel nut with just a heavy hand tight. Let it sit.
Pic of another AR extension.
I found a super cheap round hand guard (heavy, sturdy, simple) on ebay but it came with an aluminum nut. Swapped out for a steel nut and saved it for a lighter build. The 15" Keymod was $32 shipped. The steel nut was $7.
Did the lube, 50% 75%, 90% full torque thing and stopped at 50 ftlbs with a 'calibrated torque wrench' (and used the 'lever arm correction' No real reason for 50, just my target with a steel nut.
After the nut was installed (No worry about gas tube alignment) I used a little Locktite on the tube, let it set then ran the lock ring up tight. It has a row of keymod holes on each side and the bottom but not on top.
Caught a 20 MOA mount on sale ($75) and put it on the receiver.
I originally started with a Bald Eagle mount and a home made 3.5 pound bag rider (lead filled), but now use a home made bipod (pics of that later).
In the next installment
what I did to make charging and extracting easier with a straight pull no-gas AR.