Why pistol length gas for 16 inch barrels?

[metrebor]

Olympic Arms and Noveske do pistol length gas on their 16 inch uppers. Why?

I can see it being needed for 8-10 inch barrels to add more time between the bullet passing the gas port and when it exits the barrel, but 16 inch would seem to have plenty of that with a carbine gas.

Also, carbine gas should delay the unlocking and have less ejection-port noise and more bolt life (not sure if the differences are significant or not).

And if carbine-length gas was not making enough energy, why not just use a larger gas port?

What is the perceived issue with carbine length gas?

[mak91]

If you are shooting all or mostly subsonic then pistol position port is the way to go if not carbine position is what you want. It can be a challenge to get the carbine position port to cycle with subsonic loads. The gas tube opening is about .120 so opening up the barrel port bigger than that buys you nothing and its not enough for most powders with subsonic loads.

[metrebor]

I see. I agree one would not want a hole larger than 0.120 as that is what most gas blocks are anyway.

So correct me if I am incorrect, but the thought is, 0.120 is not enough for some subsonic loads for carbine gas. But if it were, then otherwise the carbine position is fine?

[mak91]

There are others here that have much more knowledge on the whisper than I do and I am sure they will chime in. I believe that even if you could go bigger on the gas port you would still have issues. Most guys that shoot subsonic do it suppressed and want the round as quiet as possible. Quiet = light charges with fast powder. I have a 10 inch and 16 inch upper and shoot the same load out of both. The 10 inch runs just over 1000 fps and the 16 inch about 950 fps. Why, the powder is long burned and the friction of the bullet in the barrel is slowing it down. The 16 inch is quieter too. Both run with a pistol position gas port. The 16 inch originally had a carbine position gas port and I found very few subsonic loads that would reliably cycle it. It has to do with where the pressure curve is in relation to where the gas port is and the distance between the gas port and the end of the barrel (dwell time)

[rsilvers]

Originally Posted by mak91

The 16 inch originally had a carbine position gas port and I found very few subsonic loads that would reliably cycle it. It has to do with where the pressure curve is in relation to where the gas port is and the distance between the gas port and the end of the barrel (dwell time)

When it was in carbine position, and few subsonic rounds would cycle it, was the gas port 0.120 or smaller?

Of the subsonic loads which would cycle it, was powder was that?

If you use these fast powders to make it quieter - and then open the gas port enough to make them work - does that not wreck havoc with supersonic rounds being severely over gassed?

[mak91]

I remember I tried Reloader #7, Accurate #9 these did not cycle, I did get a load with h335 that worked most of the time. These were all with 240 grain smk. This was a number of years back and I don't remember the loads on them but some were close to case capacity with those big bullets in there. The port was opened to the gas tube size and I had an adjustable gas tube for shooting regular loads. Without adjusting down the gas on the regular loads the gun would have extraction issues and cycle hard.

[rsilvers]

Ok, so I did a test - I just got a Model-1 Sales upper, which had a carbine gas opened all the way.

Originally Posted by mak91

The 16 inch originally had a carbine position gas port and I found very few subsonic loads that would reliably cycle it. It has to do with where the pressure curve is in relation to where the gas port is and the distance between the gas port and the end of the barrel (dwell time)

Originally Posted by mak91

If you are shooting all or mostly subsonic then pistol position port is the way to go if not carbine position is what you want. It can be a challenge to get the carbine position port to cycle with subsonic loads. The gas tube opening is about .120 so opening up the barrel port bigger than that buys you nothing and its not enough for most powders with subsonic loads.

I agree what you said is true (about most powders), but there is an alternate solution that retains better compatibility with supersonic ammo - just use the powders that do work for the subsonic ammo and a carbine-position gas port. AA1680 and RE-7 build lots of gas pressure behind a Sierra 220.

To have a good chance for both subsonic and supersonic to work without an adjustable gas block, you would want to get the subsonic ammo to make more gas so that the gun sees it more as normal ammo. Then you can use a small enough gas port such that it does not go nuts when it sees supersonic ammo. In other words, the goal should be to get the gas pressure and volume difference between subsonic and supersonic ammo as close as possible.

I loaded up subsonic ammo, and it worked well in the carbine-gas upper - the subsonic rounds threw the empties plenty far (like 6+ feet), and the action locked open on the last round. So a 0.120 or 0.125 gas port, in the carbine position with a 16 inch barrel, does have enough energy. I am not saying it does with Bullseye(R) powder, but it should with AA1680 and RE-7 - which I feel is ideal for the 220 subsonic rounds as they also fill the case so there is no fast pistol powder flopping around.

Now a bolt action person might want the fastest powder to be more quiet, but I feel in an AR, there are reliability considerations that are more important, as well as making the upper see the subsonic ammo as closer in gas pressure and volume to the supersonic ammo.

[mak91]

rsilvers, Have you tried any supersonic loads with the gas port opened up that much? I am wondering if you will see the extraction issues I had with the port that big. I'm guessing you will in some form or another. I don't remember what size port my m1s barrel came with but I know it was not that big and I opened it up to .120 to try to get it to work with subs. It all comes down to what do you want to shoot subs/supers or both. If its both you need to pick your poison. Carbine gas port wide open will most likely leave you over gassed for supers / under gassed for subs and limit your load / powder choices. Pistol position gas block good for subs not so good for supers. Either way I think an adjustable gas tube/block gives you the most flexibility in tuning the gun to a load.

[SgtCottle]

I spent the good part of 2 months experimenting on this issue. I had a 16" AR barrel from Montana Rifleman with the gas in the Carbine position. It would shoot supers all day long very accurately. When it came to subsonics - it was hit and miss. Unless I ran the heaviest bullets I could get with a fairly slow powder - it would not work. Even being subsonic; the heavier powders made quite a bit of noise.

I finally trashed the carbine system gas idea .... I want a gun that is reliable and doesn't have to be handled with kid gloves to run. I put together a pistol gas system with a selectable gas block and it made things very very interesting. Reliability is 100% and being able to use the faster powders has reduced noise signature significantly. Even with the reduced gas from the faster powders the gun cycles great. I even have some 168g bullets that I use when I shoot subsonic so it doesn't cost a fortune to play around. There is no way your going to get that on a carbine gas system.

Sarg

[BWE Firearms]

I have a couple of 300/221 barrels laying around the shop with the gas system in the carbine position that people have given me. They simply don't work reliably for what the 300 Whisper was designed for. When I was designing our 338 THUMPER I didn't care about supersonic, my main concern was could I get the 300 grain bullets going 1050 FPS quietly. Once that was accomplished we figured lets see how fast we can get the 160 grain bullets going which is 2100 FPS, but it is what it is and I wouldn't change the design to get more speed because that is not the point of either one of these cartridges.

To cycle subs reliably and as quiet as possible and use the full range of powders the gas system has to be in the pistol position You must then use a three position or an adjustable gas block to choke it down to cycle supersonic loads.

There are better supersonic cartridges out there if you want mostly supersonic. Its just an added bonus of the Thumper and the Whisper.