Short-stroking with 90gr TNT, fixed with gas port work

[Harpoon1]

90gr TNT was one of the original Alexander Arms factory loads, but was discontinued for this reason.

You really have to push it to get the gas system to run in many of the common gas system configurations on the market.

It runs perfectly in my 12” suppressed, but unsuppressed, I had 2 FTFeed and Failure to Lock back on last round. That’s CLGS even, but not a lot of dwell time with 12” unsuppressed.

That was all with Federal factory 90gr TNT, which is a screaming little round at the muzzle.

I’m averaging 2698fps with it at the muzzle from the 12” suppressed.

16” CLGS will run it fine with most port sizes .076” and larger, but then you often get excessive cyclic rate with 120gr and higher.

31.7gr of H335 is one of the best-performing max loads in all of the industry tested and published data under 90gr TNT.

Are you backed-up on that bullet? If 95gr V-MAXs were available, I would point you there. They pressure-up better and don’t "punt out" as fast.

You could try a low-mass carrier to see if that helps.

Another thing would be to seal your gas system at the block and the gas tube to make sure there isn’t any leakage.

Also, check your bolt-to-carrier gas seal by seeing if there is any soot in the fire control group pocket.

I fixed it by opening up the gas port to .078. Runs like a top now with the 90 TNT’s and H322 and 8208XBR. Even the starting loads a grain or two below max ran great.

I’m sure the H335 loads will be just as reliable now. It’s gonna be a hog/coyote rig, so the 90 TNTs are a perfect fit. Leaning towards H335 because it meters great on Lee Auto-Drum powder measure and their little 4 stage progressive press and the shots will be close, so temperature sensitivity won’t be an issue.

Thanks for the tips.