16" Barrel, mid length or Carbine gas?)

[georgeib]

Dwell time is the time it takes the bullet to pass from the port to the end of the barrel, is it not? A quick google search seems to confirm that.

I think your definition might be closer to that article than mine. However, I guess I've always taken dwell time to have more to do with how early the bolt starts to unlock. The longer it takes, the longer the dwell time. Dwell time can be increased by lowering gas port pressure and the time it takes for that gas to hit the BCG (by extending the gas system length). It can also be increased by increasing reciprocating mass (BCG and buffer weight) and to lesser degree by increasing spring pressure from the action spring.

Hopefully one of our resident experts will be along to set us straight.

[lysander]

"DWELL TIME" by definition in all engineering texts on weapon design is: The time from primer ignition to the beginning of unlocking the bolt in an automatically operated weapon.

For some reason the AR community has chosen a different (and wrong) definition of the time from uncovering the gas port to bullet exit.

[georgeib]

"DWELL TIME" by definition in all engineering texts on weapon design is: The time from primer ignition to the beginning of unlocking the bolt in an automatically operated weapon.

For some reason the AR community has chosen a different (and wrong) definition of the time from uncovering the gas port to bullet exit.

Thanks man. Your input in this forum is invaluable.

[markm]

For some reason the AR community has chosen a different (and wrong) definition of the time from uncovering the gas port to bullet exit.

The AR community has been extra "special" here lately. Feels like the old AR15.com days.

[NYH1]

What is the deciding factor in which one to get?
Looking to start a build and a bit confused. I suppose they are both timed right, but why would I want one over the other gas length?

If I was starting from scratch, building my own 16" upper, I'd go with (and I am) the mid length system. That said, never had any issues at all with a 16" barrel and the carbine system (6920 and 6720). Ours all have Sprinco blue springs and H2 buffers.

Good luck, NYH1.

[jackblack73]

"DWELL TIME" by definition in all engineering texts on weapon design is: The time from primer ignition to the beginning of unlocking the bolt in an automatically operated weapon.

For some reason the AR community has chosen a different (and wrong) definition of the time from uncovering the gas port to bullet exit.

Very interesting. I've read the other definition a hundred times on this website.

[NYH1]

"DWELL TIME" by definition in all engineering texts on weapon design is: The time from primer ignition to the beginning of unlocking the bolt in an automatically operated weapon.

For some reason the AR community has chosen a different (and wrong) definition of the time from uncovering the gas port to bullet exit.

That's what I thought.

NYH1

[17K]

I think you get to a point of diminishing returns once you get past the top of the pressure curve which is right about where the carbine port is. The mid length port is a better idea than the carbine on a 16” barrel but only if it’s ported at about.071-.073. Bigger than that and I’d rather have a .0625” carbine. Seems like less barrel in front of the port makes them more temperature sensitive.

I’ve ran several Colt 16” carbines past 15K rounds and haven’t broken anything so I’m not real convinced on the parts life claim.

[Clint]

Don't forget the huge test NSWC-Crane did in 2018 comparing CAR vs MID length gas on 14.5" M4A1s.

The MID length has significantly higher reliability numbers.

https://www.m4carbine.net/showthread...-VS-MID-LENGTH
https://partner-mco-archive.s3.amazo...1527866983.pdf

[AndyLate]

Don't forget the huge test NSWC-Crane did in 2018 comparing CAR vs MID length gas on 14.5" M4A1s.

The MID length has significantly higher reliability numbers.

https://www.m4carbine.net/showthread...-VS-MID-LENGTH
https://partner-mco-archive.s3.amazo...1527866983.pdf

Wow, the "Background" slides in the linked PDF are a hot mess. If I was briefing a study comparing carbine and mid-length gas systems, I would maybe mention gas tube length for one or both or explain what a mid-length gas system is. Instead, a rifle gas system length is provided and the audience left to guess at the other two.

The slides also refer to a "dwell distance" of 7 and 6.7 inches, which Lysander has identified as incorrect usage of the term.

On a personal level, I vote for mid-length over carbine for 16" barrels, but would buy a Colt 16" carbine barrel before a PSA 16" mid-length.

Andy