My new single shot AR upper

[BillB]

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]

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]

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]

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]

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]

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




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]

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"

[Red*Lion]

So the bcg cycles, but does not pick up another round from the mag and does not chamber?

[BillB]

Chambers, the first round perfectly.

[georgeib]

Can you blow through the gas tube?

[BillB]

I have mostly cheapo drills but a great Mitutoyo tool to measure them with so using the shanks as my pin gauges ( yes yes I know, they're not pin gauges) I found one drill that measures. 055" and it is a very near perfect slip fit. Believe me, that gasport is either .055" or extremely close to that.
Clint, I tried to send you a PM seeking your advice regarding the specifics you normally ask for when ordering one of your gasblocks or tubes. I wanted to get your suggestion because I'll just set up my press and give the barrel a properly sized gasport.
I bought this barrel as a beater build and I'm not wanting to go through all of the hassle and aggravation of an RMAthen waiting and then getting a barrel back with no explanation of what I waited on, I'll just do it myself.
Above you said the gp should be at least .062", is that for using only 5.56 pressure brass cased ammo and warm handloads, non suppressed? I have both the Vltor A5 setup I can use (A1 or 2 buffers and Sorinco green)or a Sprinco white spring and H1,2, or 3 available as well. Obviously I seek your advice because you know gas management and certainly steered me down the right road twice before and those worked beautifully.

[BillB]

Well, having neither the time nor the desire to send the barrel back to BA I set up the drill press and chucked in a 1/16" cobalt bit, set the speed to 3100 rpm, inserted a steel cleaning rod and put the barrel in my machinist vice, leveled it and plumbed everything as close as I could and lowered the bit. As soon as I saw movement of the cleaning rod I removed the bit
Checked the bore with my borescope and saw a beautiful, clean and crisp new, larger gasport . It looked great.

I reassembled the gun and rechecked gasblock/gasport alignment from inside with the borescope and finished the reassembly. I took my spring and buffer assortment, some 233 pressure ammo along with full 556 pressure loads and some hot handloads as well and headed to the range.

First 4 or 5 rounds of 224 went perfectly. Chamfering and ejection were as normal with the brass landing at a solid 4 Oclock position. Joy. Loaded 5 more and the second round wasnt pickedbup or chambered. I pulled the charging handle and loaded it manually and all subsequent round behaved normally and ejection again and still at 4 Oclock.
M855 cycled flawlessly with no issues at all and still the brass could be collected in a little neat pile at about 4 Oclock. Spring was a fresh Sprinco white and an H2 buffer. Recoil impulse was not what I'm used to or was expecting, it was really pleasant and I'll say soft. Really not what I was expecting. I think I pretty much got lucky and nailed the sizing with whatever size that bit is. My caliper said .061" at the shank.

I continued on with 90 more rounds of m855 and 180 rounds of a 62gr Trophy Bear Claw over 23.8gr of 8208xbr in LC brass and a CCI41 primer, not a single problem and I'm quite happy with my little build so far. I zeroed it in 6 rounds, just the optic not the irons but I'll get to that next trip.

[Disciple]

Nice work. :moil: