So you bought a USGI Parts kit, and you want to know if you should buy a fresh rifle and part it out, or build off what you have.
The general consensus on the internet is that the “golden age” of good receivers is at an end. But that doesn’t mean the receiver you have is trash.
Most of the problems with the current crop of receivers are the bolt. I could write a thesis on that, but what we were talking about here is the receiver.
I am assuming that you have read and followed the inspection FAQs in the MBR forum stickies. That the safety bridge is in spec, etc.
First, ensure the rifle is cleared (unloaded). Next you want to strip your rifle, as if you were going to give it a full detail clean.
Slide a stripped USGI bolt into the receiver, you will want the firing pin and extractor out of the way. This test can be done with-out the bolt being stripped, but you will have to "de-code" if any "hitches" that are from the bolt or firing pin etc. Now slide the bolt fully to the rear of the receiver.
Your bolt should sit “high” in the receiver. If it drops down then this receiver should be culled. if you are doing this test with a Chi-com bolt, then it is likely that the bolt is garbage, but it is still possible that the receiver could be out as well.
Now move the bolt slightly forward, softly and gently. You are feeling for “hitches” or sticky/grabby feelings. Move the bolt forward and back a few times and stop where the leading edge of the bolt is above the safety bridge.
The bolt should not “impact” on the safety bridge. A little touch is OK, but an impact is reason to cull that receiver from contention.
Now slide the bolt fully forward so that it drops down into the bolt lug recesses. Is it tight? Good bolt lug contact? If it is impacting on something, is it the receiver or the barrel?
You want to ensure that the bolt is fully to the bottom of the bolt lug recess.
You want to ensure that at least minimum contact is achieved with the bolt. Lapping the bolt will come when you actually install the barrel and do your build, this is the inspection stage.
If the bolt impacts on the barrel, but clears the receiver, I believe you will be fine. The barrel may have to be relieved once the USGI bbl is installed. If the front of the bolt lugs touch both the front & rear of the pocket, you may need to swap your “large bolt” for a slightly smaller one and re-do these tests.
IF you left the extractor in the USGI bolt, it is possible that the extractor could touch the barrel face near the top and cause the bolt to bind. If so you will have to strip the bolt to ensure no other issues are present. This is a likely scenario with an over indexed barrel.
If your rifle has passed these tests, then it will work for a build with the parts tested. Other parts may not be dimensionally the same, so you may want to check those parts as well. If you do these same tests with the OEM Chi-com parts, and they pass, the receiver is “inspec” and should be suitable for a build, but..... Unless you have tested fitment with the parts to be used, there could be a problem.
The next bit is “nice” but not necessary if you are planning a scoped rifle.
Remove the rear sight. Take a look at the rear sight pocket.
What you are looking for is a mark (deeper than a slight rub) across the the rear sight deck.
This one is the extreme of what is acceptable, if you want to install a USGI rear sight and actually be able to use it. If the holes in the rear sight ears are cut any deeper into the deck, then the rear sight may go into the pocket, but will not be “adjustable”, and if it isn’t adjustable, then why bother to upgrade the rear sight.
John