My new single shot AR upper

**BillB** · 08-24-21, 11:33

Yes, I built a single shot upper, just for the aggravation of having one.

Kidding. I've built all of my ARs save for my BCM and my Mk12 and have never had a single issue with any of the 9. No, I am not boasting just stating how lucky I've been. I just finished a new upper though with a BA Hanson 13.7" carbine gas system, something new to me because all of mine are middys and a few rifle length guns. This upper will not pick up the next round and chamber it. Let me run it down. BCM upper, BCM bcg, CMMG gas tube, BA 13.7" Hanson ( barrel has their GB and its pinned in a pre drilled slot) and a B E Meyers 249f. The lower is a known- to- function- 100% lower that I've been using for years.
I finished it and took it to the range to sight the optic in and started with some 223 ammo to see if it was on paper, it was but it didnt chamber the next round. Switched to m855 and again, no joy. Switched to some hot handloads and no luck either.
The lower has a fresh Sprinco white spring and an H2( what I was told should be used) buffer so I switched the buffer out for an H1( the lightest I had with me) and still got the same result.
I went back home and pulled out the borescope and the GP is not occluded in any way, buffer and spring travel their full range freely, measured vases with a comparator and measurements look spot on (tight chamber though). What I knew to check looked fine to me. What else is there for me to check?
Gas port too small (undergassed)??? Eouldnt that be something? Everyone else seems to complain about overgassed guns.
Thanks for any help.

Oh yeah, gas rings are in great shape and the gastube fits and makes up with the gasket just about the best I've ever been able to accomplish, no clinking sound and no perceptible movement of the tube. I saw no carbon around the gasblock/gastube joint or around the base of the gasket or any other sigh of any gas leakage anywhere.

**Clint** · 08-24-21, 12:01

If the BCG is new, swap it out for a known good unit, just to eliminate that as a variable and it's easy.

Next would be to pull the gas block and pin gauge the gas port.

**georgeib** · 08-24-21, 14:10

These seems like dumb questions, but other than what Clint said, I can't think of anything else. Are you sure the gas block is aligned correctly, and do you have another gas tube you can swap in? In case the gas tube is occluding the flow.

**BillB** · 08-24-21, 15:54

Thanks Clint. BCG is a good unit, I know this for sure. While anyone can let a bad anything slip by now and then I e used this BCM unit in several new builds and it has done fine in other builds. I'll take your advice tho and try another one just to eliminate that or maybe find out that is what it is.

As I detailed in my OP, I got out my borescope and observed the gasport from inside the barrel and there is no misalignment of the gasblock.

I too suspect that the gasport is undersized but I was looking for any other possibility before I go tearing it apart, plus I have no pins.
What should a 13.7" carbine gas port be anyway? Equivalent drill size maybe? I have alot of those, even some very small ones

**Disciple** · 08-24-21, 16:04

Originally Posted by BillB

Equivalent drill size maybe? I have alot of those, even some very small ones

The shanks on drill bits aren't always the same size as the cutting end, even when there is no apparent difference, at least with the cheap bits I have. Measure the shank with calipers or a micrometer if you are using it as a gauge.

**Clint** · 08-24-21, 16:43

The port should be at least .062", which is 1/16"...

Originally Posted by BillB

I too suspect that the gasport is undersized but I was looking for any other possibility before I go tearing it apart, plus I have no pins.
What should a 13.7" carbine gas port be anyway? Equivalent drill size maybe? I have alot of those, even some very small ones

**BillB** · 08-25-21, 06:00

Yep, already measured one, always do if on the very rare occasion that I use one for this type of purpose, my probably even cheaper 1/16" bit measures .61"