Explain something to me about dwell time

[AR-10]

I pulled up an old thread on another forum that discussed dwell time and how the relationship of the gas port and the remainder of the barrel affect the cycling.

In the same thread a guy posted a pic of his CMMG 16" with a rifle gas system - I didn't even know such a thing existed. I have seen "cruiser" barrels, but CMMG sells a 16" barrel with an actual rifle-length gas system.

I got curious and found this on CMMG's site:

http://img146.imageshack.us/img146/3915/cmmg16inchmedconrifle55.jpg

Doesn't look like a whole lotta barrel left after that gas port, and now I understand why this config is not very popular.

Then, on the same page I saw this:

http://img89.imageshack.us/img89/2488/cmmg103inchmedcon556bar.jpg

This is their 10.3" barrel with a carbine-length gas system and it looks like there is even less barrel remaining after the gas port.

Why is it that the shorter of the two seems to cycle better, are there higher pressures in the carbine system?

Seems like 10.5" SBR's are pretty popular, but I don't see a lot of 16" uppers with rifle gas systems.

[todd.k]

Gas pressure decreases dramatically over the length of the barrel. There is much less pressure and very little time to tap enough energy to cycle the action on a 16" barrel with the gas port at the rifle position.

[AR-10]

Makes sense, and now I know why you don't see a lot of builds around these 16" rifle gas barrels.

I haven't found any other companies that are offering this setup.

[OutlawDon]

Check out this link...dwell time is too short with the rifle length 16" barrel, so it's a hit or miss depending on ammo, weather and setup. I was actually looking into something like that. Delton actually makes on too but they seem to be on backorder. I gave that idea up and just went with a 16" midlength. Now with a 17" custom or 18" rifle length barrel, it'll get closer to better functionality.

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

"Optimum dwell time is right around 0.200 ms when you use the two most common gas system/barrel length configurations of 20" rifles and 14.5" carbines."

http://ar15barrels.com/tech/pressure-time.gif

[AR-10]

Nice, those numbers say a lot.

No numbers for the 18" intermediate gas system, is that something only Noveske uses?

[todd.k]

As far as I know we are the only ones using the Intermediate gas system.

[AR-10]

Someone on another forum mentioned the KAC SR-15 has an oddball gas system but I'll never be buying one of those.

So if you take two middies, everything being equal but barrel length - one 14.5" and one 16", which one should be the more reliable setup?

That chart shows a slightly longer dwell time and less pressure in the 16" but these numbers are all Greek to me.

[Paulo_Santos]

Someone on another forum mentioned the KAC SR-15 has an oddball gas system but I'll never be buying one of those.

So if you take two middies, everything being equal but barrel length - one 14.5" and one 16", which one should be the more reliable setup?

That chart shows a slightly longer dwell time and less pressure in the 16" but these numbers are all Greek to me.

If both are set up the same, the 14.5" will have less recoil because there is less dwell time, but the 16" should be more reliable. You may get some short stroking with the 14.5" barrel with weak/underpowered ammo.

[Quentin]

Very helpful, paulosantos... Thanks! I had considered going with a 14.5' midlength but chose a LW 16" instead. Looks like that may be the better choice.

[AR-10]

Agreed, this made me feel better about the 16" upper I picked up a few weeks ago.

I'm not going to mess around with 14.5" barrels anymore unless I build another M4 clone.

Pinning the flash hider is a pain and I can't have any NFA items here so I'm sticking with 16" mid-length barrels from now on.

[TehLlama]

Anymore I'm agreeing that pinning muzzle devices only really makes sense if you're in a ban state where you need to pin a MB anyway (then a Battlecomp permed on makes perfect sense).

Another consideration to add into the mix is suppressors: while the initial SR15's were less than ideal with .223 pressure ammunition out of the box, they were ideal uppers to use suppressed, since a suppressor will add greatly to gas pressure and volume, so it's a different goal entirely - less gas means more short stroking with lower powered ammunition, but when running higher pressure loads through a can, that's less overgassing (lower parts wear, less felt recoil, etc.). The KAC SR15's oddball gas system is an SR25 gas tube shoehorned onto a 5.56 Diemaco barrel setup - I happen to really like how they set that up, but for typical use this setup does nothing that a properly assembled 16" carbine or middy wouldn't.

Dwell time can become a lot more important as carbon builds up inside the system of the rifle, leaving lower pressure and lower volume pulses to operate the action, combined with the slow fouling of everything else also slowing down the action - so a reduced dwell time setup to lower recoil is also more likely to start choking and short stroking if you start stacking variables against it (e.g. dirty, small gas port, low dwell time, tight chamber, low powered ammunition, heavier buffer).