Arms Locker banner

1 - 12 of 12 Posts

·
Site Founder
Joined
·
4,814 Posts
Discussion Starter #1
All this talk about the 11" CAR got me thinking.

Doesn't the position of the gas port on the barrel have an influence on the amount of pressure that is bled back to the bolt? In other words, wouldn't a gas tube port closer to the chamber (short barrel) vent back more pressure than one that was further out on the barrel (long barrel)?

Seems to me I heard someone had some reliability problems with a very short barreled CAR using SS109s from Israel. I've used that ammo and it seems to be more potent in the recoil department then the standard run-of-the-mill ammo. So I wonder if this was a pressure problem?

Any thoughts?

Is there a point where the pressure would be much higher than the bolt was designed to handle reliably?
 

·
Registered
Joined
·
1,382 Posts
Somewhat, but the main thing is the diameter of the gas tap combined with barrel length.

On an AR-15, the standard gas tap is .075-.085 inch. Now, on a 14.5 inch barrel or a 16 inch barrel, that is fine. BUT, on the 11.5 and 10.5, you need a larger diameter tap to allow enough gas to impinge on the bolt before the pressure bleed off due to the bullet exiting the barrel. So, you actually have less pressure on the short barreled guns than you do on the longer barreled guns.

The bad part of all this is that on a full auto rifle, if the tap is opened up too much, you increase the cyclic rate of the gun to an alarming rate. This can cause the life of the gun to be shortened dramatically and it can cause injuries.

In short, the key to pressure is how much barrel is left after the tap...

Mike
 

·
Registered
Joined
·
5,696 Posts
What little I know of it, I learned when I owned an 11.5" Olympic that three gunsmiths and the olympic factory couldn't make work better than 90-95%.

There are two 'distances' to factor into the equation; the distance from chamber to gas port ("D1"), and the distance from gas port to barrel end ("D2").

Shortening d1 increases the pressure at the gas port, as you say, which increases the violence of the action.

On the other hand, shortening d2 decreases this back pressure. I.E., the bullet is what plugs the barrel, maintaining the pressure on the port; if the bullet's in the barrel (past the port) longer, the pressure is maintained longer.

That's why my olympic would never work reliably; according to three local gunsmiths. A combination of short d1 and short d2 made for a very narrow functional "window" for the gas action to do its thing, and a very tricky balancing act to tune. Even the olympic factory couldn't make it right. (They twice said they fixed it and test fired it, but miraculously, it wouldn't work when I got it back from them. Never buy another Olympic as long as I live.)

When bushmaster came out with the "Dissipator" version, this seemed to me like a very bad idea. They've basically eliminated d2, but you still only get CAR-length ballistics. (When talking to the bushmaster service departmen about an unrelated issue once, their own repairman said it decreased reliablilty somewhat, but there was a demand for carbine-length guns with full-length handguards, so they made it to meet customer demands.) I've heard of "pigtail" gas tubes, shaped like a corkscrew, to cushion the gas action of the CAR's, M4's, etc, but have no experience with them.

I'm tempted to get a cyclic rate reducer for my CAR, now that I shoot it suppressed a lot. It would likely help reduce wear as well as possibly somewhat reduce how quickly the gun gets dirty (from the increased gas backpressure a suppressor creates). Allegedly, they help increase AR reliability in cold weather, as well.

Anybody have experience with these cyclic reducers and pigtail tubes, and are they worth the expense?
 

·
Registered
Joined
·
9,117 Posts
If going with the 10" barrel, the pigtail is likely to be necessary. I know of several 11.5" CAR'S that function flawlessly, with or without the long flashhider.
 

·
Registered
Joined
·
1,382 Posts
That's a simplification of the problem John. It's not the distance between the tap and the chamber that is the issue. It's the gas volume and pressure in that distance that is the issue. The AR15/M16 requires a minimum amount of pressure to operate. There are several ways to measure the actual pressure but the best way of fixing the problem is stepping up the gas port size. Pig tails are okay but they do have some undesirable side effects such as changing the heat dissipation ability of the barrel as well as sometimes requiring a tap in the barrel that is larger than safety suggests should be used. The largest safe tap should be no bigger than .095 and that is pushing it.

The best rate reducers for the AR and M16 family is buffers. You can get rate reducing buffers that work wonderfully without screwing with the design.

Mike
 

·
Registered
Joined
·
5,696 Posts
I was assuming a new buffer was the only way to reduce cyclic rate, at least on a semi-auto. Some canadian outfit was praised in some suppressor article I read a while back, knocked the cyclic rate way down to something like 450 rpm, iirc; a major reduction, but still over 7 rounds a second; faster than a semi-auto is going to be fired. (Faster than mine is, anyway.) Canadian Tactical comes to mind, but I'm not sure if it was them or someone else.

Any recommendations on brands or suppliers? If I can reduce wear on my rifle (without negatively affecting reliability), I'd certainly want to do it.
 

·
Registered
Joined
·
1,382 Posts
Yeah, let me dig up the info and I'll post here in a bit... there is a super heavy hydrolic buffer that will reduce the cyclic rate from 950 to about 650...

Mike
 

·
Registered
Joined
·
9,586 Posts

·
Registered
Joined
·
1,277 Posts
If any of you ever heard of Lt. Col. Anthony Herbert , the most decorated soldier in the Korean War, he wrote about the 11" CAR in Vietnam.

He always used the standard M-16 and he said the CAR jammed, every one of them! He was a battalion commander and he saw many use the weapon. He said it was a POS.
 

·
Registered
Joined
·
779 Posts
DJetAce said:
If any of you ever heard of Lt. Col. Anthony Herbert , the most decorated soldier in the Korean War, he wrote about the 11" CAR in Vietnam.

He always used the standard M-16 and he said the CAR jammed, every one of them! He was a battalion commander and he saw many use the weapon. He said it was a POS.

Just by coincidence I'm reading his book SOLDIER and yes I noted that in reading it. :cool:
 

·
Registered
Joined
·
1,277 Posts
Todd, I read the book well over ten years ago. I still remember allot of what he wrote. Especially about how he conducted ambushes.

Really funny about the other guy going into the bunker with the .45 he borrowed from Herbert.
 
1 - 12 of 12 Posts
Top