Help: under gassed or over gassed

[Dt219]

New 6.5 grendel build. 18in mlgs non adjustable gas block. To begin with I ran an A5 system H buffer had zero issues cycling factory hornady 123sst. I read where the oring under extractor was not a good thing so I removed it. Yesterday I ran across some factory 90gr nosler varmageddon loads they would not cycle at all. The spent cases stayed in the chamber. I reloaded some test loads with a 95gr vmax with 28.5-30gr of benchmark. The lower dose cycled fine but the bolt would not lock back, and the next charges wouldn’t cycle. Today I put the oring back on extractor and went with a standard carbine buffer. Ran the same test with reloads this time they all cycled and the bolt locked back on the last rounds except for the factory noslers they cycled but bolt does not lock back. Ejection pattern is between 3 and 4 with all rounds factory and reloads. Is my rifle over gassed or under ? What do I need to do ? Would love for a genius to guide me through this thanks

[VASCAR2]

I have no personal experience with the Vltor A5 buffer recoil system. I did a little reading and it sounds like they use an intermediate length buffer tube with rifle length buffer spring with the option of various weights for the buffer. Since the lighter weight bullets are having the functioning issue your rifle sounds under gassed.

If I were in your situation I’d put my upper on a different lower with a carbine buffer tube, carbine buffer spring and carbine buffer. Another option would be to put your upper on a lower with rifle buffer tube, rifle spring with rifle buffer. If the upper functions 100% with either the carbine lower or rifle lower then you know your A5 buffer system needs the buffers tweaked (maybe lighter weight).

Without knowing the specific gas port size on the barrel and having no first hand experience with the A5 buffer system I can’t offer any more advise.


Determining whether a rifle is over gassed or under gassed is difficult because the symptoms can appear to be the same. You might be able to tell if the bolt is traveling to fast by filming the action as your shooting. If the impulse is to light the BCG may not be traveling far enough to the rear to lock the bolt back. In your situation I think this is the cause since your rifle functions with ammo with heavier bullets and more recoil energy.

[Dt219]

I did try it on my other lowers and it was the same result but they had heavier buffers in them including one rifle length which is a good bit heavier. I went with a lighter standard carbine buffer and it cycled all my reloads I thought maybe it was under gassed also, I have 1 semi auto bolt carrier I swapped it out thinking it will lighten it up even more hoping it would cycle and lock back the factory 90gr noslers and it did. Fired 1 locked back, fired 1 locked back then I put 2 in the magazine it cycled the first and locked back on the second. So I’m pretty sure it’s under gassed, I do not want to open the port in the barrel so I ordered a reduced power spring from sprinco to try or I might just leave it alone. Do you think it was under gassed now also ?

[VASCAR2]

Yes I think it is under gassed. My 18” mid length gas Faxon Group Buy barrel port is .076”. Changing to a lighter commercial bolt carrier and improving function really points to an under gas system. As the rifle and magazines wear in reliability might improve. With the ejection being 3-4 o clock sounds more like under gassed. I have seen three or four over gassed 6.5 Grendel rifles and the brass almost always went to 1 or 2 o clock.


Shooting fully loaded higher capacity mags can add drag to the bottom of the bolt carrier which requires more momentum to over come. One concern for an under gassed firearm is in cold weather. Temperature can affect pressure. In hot weather your firearm might cycle the 90 grain load but it might short stroke at 0 degrees. I like how my Faxon 18” mid length shoots and it seems like the gas is well regulated with a .076” port. (No Suppressor)

If you decide to measure the gas port you can use drill bits to help get an idea of the port size. It wouldn’t be the first barrel to leave the factory with an under size gas port.

[Dt219]

Next time I have it apart I will take it to work we have a nice new set of pin gauges there so the measurement will be exact. It’s between .070 and .076 Hard to tell with calipers. And what you said about the magazines and temperature makes sense. This is the first under gassed gun I have had

[knitepoet]

It worked

To begin with I ran an A5 system H buffer had zero issues cycling factory hornady 123sst

You removed a part

I read where the oring under extractor was not a good thing so I removed it.

Then is stopped working

Yesterday I ran across some factory 90gr nosler varmageddon loads they would not cycle at all.

First off, I'm curious where you read/heard that the extractor o-ring was a "bad" thing? And secondly, why you didn't replace it when a previously working upper quit working?

Doesn't sound like a gassing issue to me in the least. You've greatly reduced the tension the extractor has on the case head and now a "normally" gassed load is jumping the "rim"
The loads that function, but don't lock back aren't accelerating the BCG enough for the extractor to jump out of the groove.

Something tells me that replacing the o-ring will fix most of your problems


ETA, for the record, MOST factory barreled ARs, regardless of the caliber, are over gassed, so they'll still work with some of the weaker factory loads. With my "cans" I've gone, ALMOST exclusives with adjustable gas blocks, to help with the increased back-pressure related to running a silencer

[High_n_Dry]

https://www.65grendel.com/forum/showthre...extractors

o-ring thread....

I read the same thing and wondered how legit this was

[Dt219]

When I put the oring back on it did pull my case out and kick out of the chamber. I believe it was under gassed, when using the lighter bullets. The lighter I went with my buffer it improved and when i lightened up the bcg it functioned flawless so far. Just what I came to the conclusion of, well me and a couple others on here.

[LRRPF52]

90gr TNT is pretty notorious in a lot of MLGS rifles to not cycle fully.

I experienced that with my 16” MLGS Grendel, while everything else runs fine.

In CLGS with the correct port for the barrel length, 90gr TNT will run reliably for me, like on my 12” suppressed. Runs like a raped ape.

The issue is that they use a fast powder for 90gr TNT since it’s a short little bullet that likes to be punted out quickly.

If you load slower-burning powder, it doesn’t build start pressure ideally.

This can play games with MLGS and longer.

AA used to make a 90gr TNT factory load and discontinued it.

This has nothing to do with extractor tension, and everything to do with the propellant burn rate, start pressure with a tiny, short bullet, and how the mid-length gas system isn’t left with much port pressure.

[LRRPF52]

It worked


You removed a part


Then is stopped working


First off, I'm curious where you read/heard that the extractor o-ring was a "bad" thing? And secondly, why you didn't replace it when a previously working upper quit working?

Doesn't sound like a gassing issue to me in the least. You've greatly reduced the tension the extractor has on the case head and now a "normally" gassed load is jumping the "rim"
The loads that function, but don't lock back aren't accelerating the BCG enough for the extractor to jump out of the groove.

Something tells me that replacing the o-ring will fix most of your problems


ETA, for the record, MOST factory barreled ARs, regardless of the caliber, are over gassed, so they'll still work with some of the weaker factory loads. With my "cans" I've gone, ALMOST exclusives with adjustable gas blocks, to help with the increased back-pressure related to running a silencer

Funny how we never had these problems with early M4s shot in high volume, and then the KAC QD suppressor was introduced with the first SOPMOD kit, then you started seeing FTExtract issues quite often.

That’s where the O ring and D Fender came from. To put more extractor tension on the extractor as a band-aid for the cyclic rate being driven out of the ideal window of operation for reliable primary extraction.

There was at least one day where I ran 1100rds of M855 through my M4, no malfs at all. This was pre SOPMOD heavy barrel profiles, pre H and H2 buffers, pre M4 PIP.

Same with my Grendels. None of them have the O ring on the extractor. If I see the added O ring on a bolt, I remove it immediately.

[knitepoet]

However, enough people in the military DID have a problem that they felt the need to look for a solution for it.

Feel free to run/build your guns/bolts however you see fit. No worry to me. Just keep in mind the old axiom, "There's more than one way to skin a cat". Your way works for you, however that doesn't mean it's the only way.

You and I both know each firearm is a completely different animal unto itself, and what works in one, has a distinct possible of NOT working in "the same" gun for someone else.

[Dt219]

So far with a reduced power spring and the semi bcg the factory noslers has cycled and bolt has locked back. I’d say the port is just a touch too small for the lighter bullets

[VASCAR2]

Thanks for the update Dt219.

[knitepoet]

Glad you got it sorted out Dt219

[LRRPF52]

However, enough people in the military DID have a problem that they felt the need to look for a solution for it.

Feel free to run/build your guns/bolts however you see fit. No worry to me. Just keep in mind the old axiom, "There's more than one way to skin a cat". Your way works for you, however that doesn't mean it's the only way.

You and I both know each firearm is a completely different animal unto itself, and what works in one, has a distinct possible of NOT working in "the same" gun for someone else.

It wasn’t military-wide, it was M4A1s with KAC QD can attached. Cyclic rate exceeded the optimum rate for reliable extraction.

AR15s, M16s, and M4s had run without an O Ring for decades prior to that.

People with no understanding of SOCOM’s unique challenges with going suppressed and unsuppressed said, “SOCOM made their M4s better with O rings around the extractors. They know what’s best, so we should put those in too.”

By installing an O ring around the extractor spring and increasing tension that high, it increases the wear on the extractor itself during chambering because the extractor has to snap over the cartridge rim with more tension on it now.

It’s a bad band-aid that works temporarily on an improperly-configured carbine, as long as the extractor is built like a tank.

For going from unsuppressed to suppressed and vice-versa, it’s better to be able to adjust the gas settings, or use a flow-through/low back pressure design.

My personal approach is to just use a Bootleg Adjustable gas carrier.

I have had zero Failure To Extract malfunctions with that set-up, and I put the gas setting on fully suppressed.

Cyclic rate is tame but more than sufficient for reliable operation.

[knitepoet]

Funny, the ONLY extractor I ever had any trouble with broke on the VERY first shot in an 8" 300BO

Worn out 2 barrels and 3 sets of rings on my most used AR bolt I'm well north of 30k with it (223 .920" barrels. Got a lilja with 168rnds through it currently) and haven't TOUCHED the extractor yet. I'm pretty sure there's an O-ring in there, but honestly haven't looked, and REALLY don't care. It ain't broke, I ain't "fixin' " it

[Dt219]

That’s the route I’m running with too. Don’t try and fix something that’s not broke like I did

[LRRPF52]

Funny, the ONLY extractor I ever had any trouble with broke on the VERY first shot in an 8" 300BO

Worn out 2 barrels and 3 sets of rings on my most used AR bolt I'm well north of 30k with it (223 .920" barrels. Got a lilja with 168rnds through it currently) and haven't TOUCHED the extractor yet. I'm pretty sure there's an O-ring in there, but honestly haven't looked, and REALLY don't care. It ain't broke, I ain't "fixin' " it

Several guys have had their O ring-powered extractors break at about 1000rds.