AR Barrel Bore Pin Guage

**markm** · 03-01-24, 08:52

Thanks, Ned. So a .220 pin passing through would be a problematic situation? (nothing to do with my situation, but still interesting)

**Clint** · 03-01-24, 08:53

With simple pin gauges, you can only measure the bore diameter of .2190+.0010

This 5.55mm precision ground rod works nicely after you chamfer the ends.

It has a .2185-.0002 diameter, which gives a nominal clearance of .0005"

It is almost 4" long, so it doubles as a straightness gauge too that checks a better straightness than the mil spec straightness gauge at 6" x .2175", which seems a little sloppy.

https://www.mcmaster.com/catalog/130/4278/2900A323

Originally Posted by markm

I just ordered 4 pin gauges from .221 to .224. That should cover it well no?

**markm** · 03-01-24, 09:12

Nice. Thanks, Clint.

**ViniVidivici** · 03-01-24, 11:29

Originally Posted by joedirt199

There is that metal stuff that you can melt into the chamber and bore to measure what is going on. Can't remember what it is called but it is low temp soft then hardens when cooled. I have an ingot of it but never used it. It is for making chamber casts.

Cerrosafe.

**joedirt199** · 03-02-24, 09:38

Yup that's it. Thanks Vini.

**markm** · 03-04-24, 19:05

Got the McMaster pin Clint posted above, and WOW! This barrel has a tight spot about the first inch of rifling in front of the chamber. Explains everything. I mean the gun cycles like a perfectly gassed 14.5, but still pops primers. The initial spike/overpressure has dropped by the time the gas system is actuated.

This pin is perfect for gauging bores. It passes perfectly through a Colt barrel and fits like a glove.

**titsonritz** · 03-04-24, 20:31

Originally Posted by markm

Got the McMaster pin Clint posted above, and WOW! This barrel has a tight spot about the first inch of rifling in front of the chamber. Explains everything. I mean the gun cycles like a perfectly gassed 14.5, but still pops primers. The initial spike/overpressure has dropped by the time the gas system is actuated.

This pin is perfect for gauging bores. It passes perfectly through a Colt barrel and fits like a glove.

I’m going to add one to next order from them.

**markm** · 03-04-24, 20:46

Just pinned all my AR barrels. Only the Colts pass the gauge all the way through with no resistance. A Sabre 14.5 middy will pass it with some force.

Most of the barrels stick about 1" into the rifling. That's curious. I'm wondering if that part of the rifling gets roughed up with wear. Both Colts are relatively new barrels.