Bore diameter of stripped upper

[Molon]

It is not uncommon to encounter upper receivers in which the bore does not have a uniform diameter throughout the entire travel of the bolt carrier (note that I didn’t say “entire length”). This is easily demonstrated with pin gauges.

The first demonstration uses a Larue Stealth upper receiver, which is an expensive billet upper receiver. This particular upper receiver has had 2,317 flawlessly cycled rounds fired through it using a Young Manufacturing National Match bolt carrier. The receiver was thoroughly cleaned prior to this demonstration.





A 0.998” pin gauge easily slides into the front end of the upper receiver.




The same pin gauge easily slides into the aft end of the upper receiver.





However, when I tip the receiver to 90 degrees vertical, instead of the pin gauge sliding all the way through the bolt carrier travel section, it wedges in the mid-section of the upper receiver.





The next example uses a brand new SOLG forged upper receiver. A 0.999” pin gauge slides easily into the aft section of the upper receiver.





As before, when I tip the receiver to 90 degrees vertical, the pin gauge wedges in the mid-section of the upper receiver.





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[JediGuy]

Is there a good tool to bring uniformity if it is lacking?

[lysander]

Is there a good tool to bring uniformity if it is lacking?

There really is no need to do anything, so long as a bolt carrier passes freely.

[StainlessSlide]

There really is no need to do anything, so long as a bolt carrier passes freely.

Passes freely under what conditions? Clean, dirty, external debris? In a windy desert with fine sand occasionally blowing, and you've been there for a few days? I do want uppers that pass the .9981 gauge at least, all the way through (or at least to the forward most BCG contact).

Of course the dry lube and some anodizing will wear at the BCG rail contact arcs, but may remain on the rest of the bore. The gauge will measure the bore diameter at the unworn points. So the effective BCG clearance may be greater than the gauge indicates, for a broken-in upper.

I like your explanation that the area under the ejection port is flexing outward when the bore is machined, and so is not cut as deeply. Finger pressure often gets the gauge through, which may mean this area is flexing outward to accommodate.

Thank you Molon, it's good to know that this is widespread. You've probably noticed that this doesn't happen at random, it's usually in the ejection port area. I've also been detecting it on some brands. As I mentioned, some won't pass the .997 GO gauge, it hanging up in this area.

[lysander]

The gauge is cylindrical, the bolt carrier rides on four rails, so if a bolt carrier passes through the diameter at the important points (where the rails ride), then the minimum clearance criteria has been met, with that bolt carrier.

For normal reliability, the largest bolt carrier and smallest bore only have to have a 0.0001" clearance.

[StainlessSlide]

The gauge is cylindrical, the bolt carrier rides on four rails, so if a bolt carrier passes through the diameter at the important points (where the rails ride), then the minimum clearance criteria has been met, with that bolt carrier.

For normal reliability, the largest bolt carrier and smallest bore only have to have a 0.0001" clearance.

Sure, that's the geometry, but do you really think that a tenth clearance is enough? I don't know of any other sliding part intended for field use where a tenth would be enough, e.g. 1911 slide rails. True, the force on the BCG is coaxial with the rails, unlike the 1911 case, but still...

Or maybe you're allowing for the dry lube wear in the contact arcs, which would fairly quickly add .0006 to the minimum clearance. But I don't think this is enough, either.

And I don't see the BCG phosphate thickness in your calculations. I'm not finding a spec for this.

[StainlessSlide]

After a merry chase through the specs I found an expected bolt carrier phosphate thickness of 5 to 10 microns, or about .0002" to .0004". The dimensions we have been using are before finish, so when we factor this in, the BCG could have an interference fit even after the upper passes the .9981 gauge.

The drawing gives MIL-STD-171 Finish 5.3.1.2, which refers to MIL-DTL-16232G for the phosphate layer.