A carbine lower w/16" RRA upper fails to feed ~once per mag. The round moves halfway from the lips of the mag to being chambered, and the bolt is left over the round, with the lugs denting the casing halfway to the shoulder.

The upper is 100% with my other two lowers, and the lower repeats the conduct (much less, once every 3-4 mags) with my 20" upper. I have switched the buffer and spring out with my other carbine lower (a known 100% lower) with no increase in functioning w/the bad lower and no worse performance from the known good lower.

The mags also function 100% with the other lowers.

Help requested.

Thanks

Either tighten or loosen the mag catch assy one turn.  Then try it out.

Tried the mag catch tightening, it didn't help.

I'd try putting calipers on the mag well and measuring specs.  something does'nt sound corrcet.  Either the bolt is not coming back far enough or the mag is sitting just a tad low.  Since you tried different buffers and springs, something could be out of spec somewhere.

my 308 does this often Im gonna try a new gas system.
Jp gas block or adjustable gas tube

Which round does it fail to feed?  Closer to empty mag?  I have this problem before with my old, old and used 20 rounder.  When it is getting close to empty, sometimes a round will jam when bolt contact the side of the case instead of the rear.

I use the same mag in other AR and some works fine and if I hold the gun via bottom of the mag, then it seems to work better.

Turns out the spring in the mag was getting weak.  I fixed it by pulling on the springs (not really recommended, replacement is better).  

It also seems the lowers dimensions are a bit different.