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

[Harpoon1]

Been reloading the Grendel for 7-8 years now but almost exclusively for bolt guns. Never been a huge AR guy but I’m trying to assemble and tune up a lightweight hunting AR. So I’m willing sacrifice some reliability for lightweight and the smoothest shot cycle/felt recoil.

Cut to the chase. I have a 16” Fluted BA barrel with a mid length gas system that shoots lights out. Over a PSA lower with a stock carbine 3 oz buffer and spring. The gun will cycle Hornady 123 Am Gunner but short strokes my 90 tnt handloads. That’s with 31.7 of H335 averaging 2666 FPS out of a 16” barrel. Going any higher in charge weight and I start to see faint ejector marks. I put a SUperlative gas block on and tried it wide open, 4.5 turns out from closed, no restriction, no bleed off. And it still won’t cycle with the TNT’s. Pulled the gas block to check it was centered up over the gas port and it was perfect.

So before I start drilling out the gas port, I have a low mass BCG for another project and was wondering if that might remedy my problem in lieu of opening up the gas port? Will the adjustable gas block be enough to keep the low mass BCG from unlocking too soon? Or am I missing something?

[lazyengineer]

Odd. There is nothing obvious to me at first glance to be causing this, aside from a gas bleed somewhere?

[Harpoon1]

Could be, I don’t see any obvious exhaust leaks, other than possibly around the gas key/gas tube connection. That’s a good thought though, I’ll swap BCG and see if that makes any difference.

American Eagle claims 3000 fps with the TNT, suppose that with a 24” barrel. Using 25-30 fps per inch, that still leaves me considerably on the low side. IIRC DNS was getting 2800+ fps out of his 18” AR’s with TNT’s. Maybe I just have a tight chamber and slow barrel?

[Klem]

That sounds like a reasonable load.

What weight buffer and spring are you using?

[lazyengineer]

Could be, I don’t see any obvious exhaust leaks, other than possibly around the gas key/gas tube connection. That’s a good thought though, I’ll swap BCG and see if that makes any difference.

American Eagle claims 3000 fps with the TNT, suppose that with a 24” barrel. Using 25-30 fps per inch, that still leaves me considerably on the low side. IIRC DNS was getting 2800+ fps out of his 18” AR’s with TNT’s. Maybe I just have a tight chamber and slow barrel?

American Eagle is smoking drugs on that velocity claim. Don't use that velocity as a benchmark for anything. No reloading manual anywhere reports that, even in 24" test barrels. There is some serious rounding up going on there.

[Harpoon1]

That sounds like a reasonable load.

What weight buffer and spring are you using?

From my OP “ ……over a PSA lower with a stock carbine 3 oz buffer and spring.” !

[Harpoon1]

American Eagle is smoking drugs on that velocity claim. Don't use that velocity as a benchmark for anything. No reloading manual anywhere reports that, even in 24" test barrels. There is some serious rounding up going on there.

I agree it’s a little “hot” but maybe not as much as you might think at first blush! At least with a 24” barrel. Look, they all pad their numbers but, DNS was getting 2800 PLUS in his videos, out of an 18” barrel! Add 150-180 fps and you’re right there! Anyway, I swapped out the BCG with a low mass Faxon BCG and drilled out the gas port to the next size drill bit. Thinks it’s now 5/64’s, which is just slightly over what LRRF52 said Bill Alexander recommended. .076 vs .078, I think? Anyway, have the Superlative gas block installed, we’ll see tomorrow! Plus, I just watched a bunch of JPRifles tuning videos! :roll-eyes:

I’m pretty sure according to the JP videos, I should have gone the “mass” tuning route with the 6.5 Grendel, versus the “gas” tuning route with the 5.56/300 BO. Of course I didn’t have the hardware to do that. Anyway, will know more tomorrow!

[myrifle]

I agree it’s a little “hot” but maybe not as much as you might think at first blush! At least with a 24” barrel. Look, they all pad their numbers but, DNS was getting 2800 PLUS in his videos, out of an 18” barrel! Add 150-180 fps and you’re right there! Anyway, I swapped out the BCG with a low mass Faxon BCG and drilled out the gas port to the next size drill bit. Thinks it’s now 5/64’s, which is just slightly over what LRRF52 said Bill Alexander recommended. .076 vs .078, I think? Anyway, have the SLR gas block installed, we’ll see tomorrow! Plus, I just watched a bunch of JPRifles tuning videos! :roll-eyes:

I’m pretty sure according to the JP videos, I should have gone the “mass” tuning route with the 6.5 Grendel, versus the “gas” tuning route with the 5.56/300 BO. Of course I didn’t have the hardware to that. Anyway, will know more tomorrow!

No you did it the right way. Always fix the route cause of the issue. In your case it was a small gas port.

Now going the lowmass bcg and adjustable gas will make your gun run very much better.

So your on the right track for sure. A good tip is too set your gas too the weakest ammo you have. I set my gas with the wolf steel case ammo.

Also if you want to really make your rifle feel butter smooth. Add a JP scs to it.

[ragrendel]

FWIW, an adjustable gas block does nothing for under gassed problems. They only help to reduce gas not increase it.

[montana]

Adj gas blocks leak more gas than a standard block which could be the difference.I would be sure to check every other option before drilling out the gas port..
There could be many other reasons why your rifle is short stroking besides a small gas port..

Drilling the gas port should be the last option.

ragrendel is correct about adj gas blocks...

[Harpoon1]

Adj gas blocks leak more gas than a standard block which could be the difference.I would be sure to check every other option before drilling out the gas port..
There could be many other reasons why your rifle is short stroking besides a small gas port..

Drilling the gas port should be the last option.

ragrendel is correct about adj gas blocks...

I didn’t install the AGB to try and solve the under gassing problem! In fact, I never knew I had an under gassing problem until I put the AGB on the gun and tried to run my 90 TNT handloads!

[montana]

I didn’t install the AGB to try and solve the under gassing problem! In fact, I never knew I had an under gassing problem until I put the AGB on the gun and tried to run my 90 TNT handloads!

I never said you did, I was confirming ragrendel's statement about adj gas blocks..

Were you having over gassed issues before you installed an adj gas block?

[Harpoon1]

Gun ran great today after opening up the gas port. Problem solved. Could not slow the low mass BCG down slow enough bleeding off the gas, and didn’t want to use the restriction side of the adjustments. So, went back to the standard BCG and tuned to 1.5 turns of bleed off adjustment, now it runs perfect.

[Harpoon1]

American Eagle is smoking drugs on that velocity claim. Don't use that velocity as a benchmark for anything. No reloading manual anywhere reports that, even in 24" test barrels. There is some serious rounding up going on there.

Data points

16” BA Fluted Barrel

LabRadar

84f/Sea level

Speer 90 TNT/ 31.7 grns 8202XBR/Hornady brass/ CCI 450/ 2.250” COL/ 2728 fps

Federal American Eagle 90 TNT w/cannelure and crimp, 5 shots/ 2786/22 Es/ 8.3 sd. The brass looked fine, for the most part. 2 pieces had just the faintest hint of the ejector just starting to leave a little shiny spot.

Interpolating 8 more inches at 25 fps per, is getting pretty close to 3000 fps at 24”.

[LRRPF52]

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.

[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.

[ragrendel]

Good to hear and thanks for the updates! Learn a lot from this forum.

[LRRPF52]

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.

Great news. What was the original port diameter?

It should have been at that for MLGS 16”.

[Harpoon1]

Great news. What was the original port diameter?

It should have been at that for MLGS 16”.

I don’t know? Not much smaller, 5/64th was barely larger. I measured the 5/64ths drill bit shank and it was .078”. Called BA ,multiple times yesterday to see what the “factory” size was and got the answering machine every time. :o

IIRC, I’ve heard BA drills their gas ports on the small side, assuming most of their customers run “cans”! I’ve got 6-7 BA barrels, they all seem, perform like they’re undergassed! Have restricted gas ports. Opening them up to the next size drill bit always seems to work better!

[grayfox]

Harpoon I can roger that on the BA... except for me it was for 2 18" BA 223 wylde barrels. Went up your 1-size drill bit (I got a wire gage set of them in order to do a more finely tuned 1-up), that fixed their short stroking just like you saw here with your barrel.