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.
https://www.ar15.com/media/mediaFile...ed-2358537.jpg
A 0.998” pin gauge easily slides into the front end of the upper receiver.
https://www.ar15.com/media/mediaFile...ed-2358532.jpg
The same pin gauge easily slides into the aft end of the upper receiver.
https://www.ar15.com/media/mediaFile...02-2358545.jpg
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.
https://www.ar15.com/media/mediaFile...03-2358533.jpg
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.
https://www.ar15.com/media/mediaFile...01-2358576.jpg
As before, when I tip the receiver to 90 degrees vertical, the pin gauge wedges in the mid-section of the upper receiver.
https://www.ar15.com/media/mediaFile...02-2358580.jpg
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