Technical AR question - optimal carrier velocity and dwell time

[John Hearne]

As best I understand the DI guns, the secret is getting the timing of the gun correct. You don't want the carrier to run too fast (dwell time issues, magazines can't feed fast enough, etc) and you don't want the carrier to be "energized" enough. We know that certain combinations of barrel length, gas port size, and gas port distance make for optimal performance. So, has anyone ever quantified an optimal carrier velocity or dwell time for the M-16 gas system?

[Wayne Dobbs]

A guy named Eugene Stoner did and it was a 20" barreled M16 with the FA bolt carrier assembly, rifle buffer, seven inches of dwell distance from gas port to muzzle and a gas system pressure of about 13.5K PSI. Everything else is something of an adaptation involving increased buffer mass to slow cycle time and M4 feed ramps on the upper's surface due to higher BCG velocities vs magazine spring efficiency issues. The carbine gas system pressures, being higher, simply aggravate the whole issue as does the much faster lock time of the carbine systems (the time the bolt is locked after cartridge ignition, not precision rifle lock time). Lots of issues here to keep in balance!

We do a good job of balancing that set of issues these days, but whenever you shorten a barrel under 14.5" or start running a can on a gun OR neglect standard PM routines, you can start having adventures.

[LittleLebowski]

This is a question for Sully.

[Suvorov]

And in a related question. Why do many manufacturers (good ones like Smith and Wesson, not only Bushman's) have a hard time with gas port sizing? Is it simply that the bigger the port and lighter the buffer, the "more likely to cycle" the gun is and long term effects be damned?

[ASH556]

And in a related question. Why do many manufacturers (good ones like Smith and Wesson, not only Bushman's) have a hard time with gas port sizing? Is it simply that the bigger the port and lighter the buffer, the "more likely to cycle" the gun is and long term effects be damned?

Sort of, but I think it's more because they get sick of hearing "Your gun won't cycle __________ ammo" (usually lower-end .223 pressure stuff like Tula, UMC, etc). Then, you add a can or even just shoot some actual 5.56 pressure ammo and you start having issues, nevermind the long-term effects of wear on the parts.

[Chuck Haggard]

What Wayne said.


Just an FYI, I'd definitely consider Wayne to be an SME on the AR/M16 system.

[Dagga Boy]

What Wayne said.


Just an FYI, I'd definitely consider Wayne to be an SME on the AR/M16 system.

I think he teaches a couple of classes for some company that makes the things...

To emphasize what Wayne posted, I'll add something from Monty LeClair at Centurion Arms who is considered to be an SME on this stuff by one of the top tier military units in this country. One of the things that Monty pointed out (and most with time on the system will understand) is that 20" Barreled full stock AR rifles just shoot really well. Soft, reliable, long lasting and easy on the system. One of the things Monty does when looking for optimal reliability is to mimic the 20" M-16 as closely as possible. He like to go to a mid-length gas system with a 16" barrel (I have a couple of Monty's guns in this configuration that run like sewing machines) to get a similar distance from the gas port to the end of the barrel as the M16, and the same with the shorter barrels with a carbine system. As soon as you get shorter dwell time, it has to get adapted elsewhere. I have run these 16" Mid-length guns with a rifle buffer system, and they are very functional soft shooting, utter reliable AR's.

[Doug]

John,

I have found the graph on the this web page very helpful in understanding the system operation and barrel length. YMMV

Doug

"See the Quickload plot below for more info on how gas port pressure and location relate to functioning"

http://ar15barrels.com/prod/gas-systems.shtml

More here

http://ar15barrels.com/prod/operation.shtml

[Doc_Glock]

Bumping this useful thread because I learned a lot. The second link in the post above was outstanding.