Does it bind hard enough to cause a stoppage?
Does it fail the
tilt test with the stock off?
I have an HRA SG that won't return the bolt to forward during the tilt test. It only takes a light bump to start it, but it won't go forward at over 45 degrees. All my other oprods work in this receiver, and this oprod works on all three of the other M1s. Hmmm.
This receiver/barrel/oprod combination appear uncycled/unfired; it shows very little wear. The oprod shows minor differences in the curvature compared to the others, as the others do among themselves, but nothing really stands out. What's a fella to do?
Measure the tab, that's what.
The tab on the culprit oprod measures .324" height (should be .320" max). It displays no up and down slop in the tab race at all. The other oprods "wiggle" very slightly. This one does not. Also, inside the race there are two machining marks on the bottom showing a polish. None of the other receivers show this. The top of the offending tab has a highly polished spot on top. The other oprods do not.
Think we're getting close here?
I think Gus called this "tolerance stacking."
Upshot:
don't bend that oprod! Check for tolerances, scuff marks and crud.
Then, if all else fails, Jim Swartz is still your friend.