View Full Version : Rifle length gas system with a carbine tube and buffer?
A month or so back I put together a new rifle length system. I had a 20 inch White Oak Arms DMR barrel cut down to 18 inch. I was going to build kind of a SPR style rifle with a PRS stock and rifle length tube and buffer.
But after feeling how light and nimble the gun handles (that WOA barrel is supper light) I was thinking it would make a great battle style rifle. So I canned the PRS stock idea and went with a Magpul ACS and carbine H2 buffer and carbine tube. I figured it should be a very smooth and light recoiling gun with the rifle length system and it really does handle like a 14.5 inch gun its so well balanced.
So I take it out tomorrow to shoot it for the first time. Here's my question......I want to tune the system to run as smooth as possible, what exactly should I be looking for in brass ejection and other signs to tell me if the gun is over or under gassed....aside from it not cycling or locking back on the last round?
Do you guys typically just shoot it and if it runs it runs?
I only ask because I've never built a rifle length system that didn't use a rifle length buffer tube and buffer.
Again, my goal is to have it shoot as light as possible but obviously still be 100% reliable.
Any pointers?
Heres the rifle BTW
http://images55.fotki.com/v608/photos/2/25033/1861530/P6252110-vi.jpg
Dionysusigma
11-25-11, 10:04
I would've used the A5 extension and buffer, personally. Aside from that,
Do you guys typically just shoot it and if it runs it runs?
You got it. :)
I would've used the A5 extension and buffer, personally. Aside from that,
You got it. :)
I was actually thinking an A5 would be the way to go. I went with the carbine just because thats what I had laying around.
Im thinking youre right though.
What size is you gas port? Was it opened when the barrel was cut down?
No, it wasnt. I asked my smith if it would required to modify the port and he told me he didnt think so but to just shoot it and see. Should it have been?
I dont know the gas port size, I just checked WOA website and they dont list it.
http://www.whiteoakarmament.com/xcart/product.php?productid=17565&cat=250&page=2
You don't know until you shoot it. I used a carbine length extension on my precision AR because I wanted the UBR stock and the A5 wasn't readily available at the time. Mine uses the Lothar-Walther barrel. I didn't think to size the gas port before installing the gas block, but I understand rifle ports are about .090" or so
http://i115.photobucket.com/albums/n289/SgtSongDog/Liberty/DSC_0157.jpg
This AR uses a standard carbine spring & buffer and runs with no problems
My 18" BCM Mk12 has a rifle length gas system, and I run it with a carbine receiver extension (Magpul UBR) with a standard carbine spring and an H2 or H3 buffer as recommended by BCM. The difference is that this upper would be spec'd by BCM to have the correct size gas port.
http://farm3.staticflickr.com/2507/5798104532_b66f6b6a5c_b.jpg
It runs 100% with a carbine, H, H2 or H3 buffer. I keep an H3 in it.
Cameron
underachiever
11-28-11, 22:33
i built a rifle length system with a carbine stock, re, spring, a d buffer.
it shot everything great. no malfunctions at all. as far as the ejection pattern i wouldnt be concerned about that.
I asked my smith if it would required to modify the port and he told me he didnt think so but to just shoot it and see. Should it have been?
For reliability, it probably does need to be opened up. I don't care for 18" barrels with rifle gas, I think they are too sensitive to temperature and powder burn rate to begin with.
The problem you have is that you can't really determine the reliability of a rifle by shooting it some.
The problem you have is that you can't really determine the reliability of a rifle by shooting it some.
So how would one determine that?
Cyclic rate or hi speed camera.
What are the target numbers?
My only camera is an iPhone 4...so I may be shit out of luck.
For reliability, it probably does need to be opened up. I don't care for 18" barrels with rifle gas, I think they are too sensitive to temperature and powder burn rate to begin with.
The problem you have is that you can't really determine the reliability of a rifle by shooting it some.
Can you suggest a port size?
Also, I don't get the shooting a rifle doesn't tell you if it's reliable thing.
I'm obviously going to bow to you better knowledge, I'm sure anyone from Noveske knows their stuff.....but the whole cyclic rate or high speed camera thing has me intrigued.
I'm assuming cyclic rate shows if you're getting proper gas amounts through the port before the bullet leaves the muzzle.....so I guess running it fully auto, problems will be shown through an increased or decreased cyclic rate?
Am I warm?
What's up with the high speed camera?
GeorgiaBoy
11-29-11, 18:37
I think what he is trying to say is that without a high speed camera, it's very difficult to see things such as near-short stroking or bolt bounce.
Search "Tula ammo .223 2 minute review" on YouTube. Vuurwapen blog has some excellent high speed footage of these things.
You can use a hi speed camera to see the cyclic rate.
I can easily make a gas port that will run great when it's clean, summer, and running M193 ammo that will choke hard on 223 when dirty or in the winter.
It "ran" great in the summer, but with the cyclic rate I would have known it was too low for less ideal conditions. So if you can shoot multiple thousands of rounds in less than ideal conditions you can say it's reliable.
The acceptable cyclic rate for an M4 is in MIL-C-70599A(AR)
You can use a hi speed camera to see the cyclic rate.
I can easily make a gas port that will run great when it's clean, summer, and running M193 ammo that will choke hard on 223 when dirty or in the winter.
It "ran" great in the summer, but with the cyclic rate I would have known it was too low for less ideal conditions. So if you can shoot multiple thousands of rounds in less than ideal conditions you can say it's reliable.
The acceptable cyclic rate for an M4 is in MIL-C-70599A(AR)
From the document you listed...
3.4.5 Functioning. Each carbine shall operate without
malfunctions or unserviceable parts. The cyclic rate of fire for
a 3 round burst using a 30 round magazine shall be within 700 to
970 rounds per minute when firing Government standard M655,
5.56mm ball cartridges conforming to MIL-C-63989.
Of course this test obviously uses 5.56mm pressure ammo, and a select fire lower. The later presenting a problem for about 99%. Kind of a big range too, 700 to 970 RPM.
So obviously not really a test most people can carry out at all. Even using .223 pressure ammo changes the parameter(s) of the test.
Is there a fesible way one would test this?
Interesting question.
Outside of just buying a rifle built to those specs, how could John Q Public test their rifles to that standard?
That question is why most people are better off buying a barrel or carbine from a company they trust to test things like gas port size.
Full auto is a good way to guage the performance of an AR...
Here's my video of an undergassed gun.... then shot with the silencer mounted to bring the cyclic rate closer to normal.
http://www.youtube.com/watch?v=dOeWNKhSKwE
Yeah, it's easy to see in retrospect how easy it is to spot an under gassing issue when fired in full auto.
I can clearly see how much brisker it was suppressed which I imagine will cause it to function better in adverse conditions.
I don't know.
There were quite a few people singing the praises of the A5 for how it slowed down the cyclic rate not too long ago.
The mil spec was for a 14.5 barrel with a carbine gas system, firing 5.56mm ammo.
This doesn't apply to a large portion of AR15 owners.
Its hard to even say fire a few thousand rounds and see. If you change ammunition, you changed a parameter. Ammunition is as much of a factor as anything else. I've felt larger difference in how my gun shot with 5.56mm vs .223 than any buffer combination.
There is little change when going from a carbine to a H. Buffer changes, in my case anyway, don't really offer any real difference until you get up to an H2.
My 18" BCM Mk12 has a rifle length gas system, and I run it with a carbine receiver extension (Magpul UBR) with a standard carbine spring and an H2 or H3 buffer as recommended by BCM. The difference is that this upper would be spec'd by BCM to have the correct size gas port.
http://farm3.staticflickr.com/2507/5798104532_b66f6b6a5c_b.jpg
It runs 100% with a carbine, H, H2 or H3 buffer. I keep an H3 in it.
Cameron
the original rifle buffer is like 5 ounces right? carbine is 3.1 or so?
It would seem like you'd want a lighter buffer as the gas port got further out.... Yet the opposite seems to be true...
most of the SBR guys run H3's too right?
any explanations? gas port size must be part of it... but? :confused::confused:
The difference is in the spring.
The spring used with a rifle buffer is longer and softer.
And the fact the buffer weights are not just weight, they move inside the buffer body for a reason.
the original rifle buffer is like 5 ounces right? carbine is 3.1 or so?
You can't compare buffer weights on two different springs.
You can't compare buffer weights on two different springs.
fair enough. i don't think i've ever taken apart a 20" gun before come to think of it.
I am using a 20" FN barreled PSA upper half on a lower with a carbine stock and CAR buffer.
Functions fine with M855, SS109, Tula, and some various commercial brass .223Rem.
May not be applicable to your situation as my WOA barrel started out as an 18" SPR contour barrel, so the port may be different than your 20" DMR. Mine has run anything I have fed it, hot, cold, rain...whatever reliably with either the rifle buffer and spring or a standard carbine buffer and spring. Haven't tried any heavier buffers yet. I've run 20 YO surplus SA ammo to Wolf to Fed GMM without issues.
Glad I came across this thread. I am thinking of using an 18" barrel with rifle length gas port. I have a carbine spring and buffer and was wondering if I could use it or have to go to a different spring and buffer. Seems from the above answers, I really don't have to. Am I correct in that assumption?
I believe that's your assumption is a good starting point. Just cause the combo runs fine in my rifle it doesn't mean it will run reliably in yours. It's always prudent to test the crap out of it before betting your life on it. I've run lots of different brands and bullet weights through mine in different conditions and haven't seen any issues.
If you have a regular H buffer, I would start with that. I had a PSA 20" FN upper and before finishing an A2 lower, mounted it on one of my existing lowers using an H buffer. It worked fine for the time I had it on that lower.
I did a budget build for a friend last week. It's a 20 bushy upper and we put a carbine h3 buffer and carbine spring in it....like I said its a budget build for his first AR. I used mostly stuff I had just laying around. The gun was short stroking with the H3 buffer so we swapped it out for a Reg. Buffer and it ran fine. He was using Tula ammo, its all he had so I did not get to check it yet with good ammo. I will test some more and report back.
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