Anyone played with a 12.5" Mid-length Gas System?

**Jmacken37** · 07-07-15, 23:02

I like the recoil impulse on a 14.5" mid-length gas system better than the recoil impulse on a 14.5" carbine-length gas system.

Would I like the recoil impulse on a 12.5" mid-length gas system better than a 12.5" carbine-length setup?

The reason I'm even considering this is this barrel/gas block combo I ran across that looks intriguing...

govnah_12_5_mid.jpg

http://micromoa.com/12-5-midlength-5...rrel-w-govnah/

I have no affiliation with the company.

**Junkie** · 07-08-15, 08:45

I would expect it to be a picky eater. 16" rifle gassed "dissipators" tend to be hard to make run well, and this is a similar concept.

**Jwknutson17** · 07-08-15, 08:56

Depends on the gas port size. And that Govnah?? gas block. Properly ported 12.5 carbine barrels can have very little impulse. I know a few guys on here that run 12.5 mids and have great results with the barrel ported properly (aka large) to run reliably. If running a suppressor I would say go for it. If not, I would rather spend my money on a properly ported barrel and run an A5 system to achieve same results with increased reliability.

I have no experience with Govnah so I don't know what if any part that plays in the system your looking at.

**Rayrevolver** · 07-08-15, 10:07

I was going to build a suppressed only 12.5" Mid using that Micromoa barrel. I decided that was not crazy enough and am trying an 11.5 Mid + SLR sentry. It has a huge port .095" to run unsuppressed I suppose. I bought a cheap Trek TM and will see how it all works out.

The guy from AR Performance sells a 12.5 Mid also and if you search you can find the last thread where they discussed 12.5" Mids.

It wasn't that long ago that people poo-poo'd the 14.5 Mid as being too finicky, but they seem to be accepted today. A proper 12.5" Mid should be ok too (looks like the same dwell time as a 10.5" carbine, no?). I think my 11.5 Mid might be a nightmare...

**Junkie** · 07-08-15, 11:50

The farther from the breech you are, the lower the pressure gets and the higher the velocity gets, so I would expect you'd need more barrel length past the gas port for a longer gas system. Dwell time for the midlength will be shorter than for the carbine with the same distance from gas port to muzzle, and pressure will be lower as well. That's not to say it can't be made to work though.

**Jwknutson17** · 07-08-15, 12:09

Originally Posted by Junkie

The farther from the breech you are, the lower the pressure gets and the higher the velocity gets, so I would expect you'd need more barrel length past the gas port for a longer gas system. Dwell time for the midlength will be shorter than for the carbine with the same distance from gas port to muzzle, and pressure will be lower as well. That's not to say it can't be made to work though.

Ding ding ding. This is why you can not say that a rifle gas with two inches of dwell time at set port size is the same as a middy with two inches dwell and same port size and carbine with two inches of dwell and same port size. All there own animals in themselves. While I do agree you can get a 12.5 middy to work properly. I personally do not shorten mid length barrels less then 13 inches. I think a 13 inch barrel with a .073 gas port is an excellent shooter. IMO

**Rayrevolver** · 07-08-15, 15:23

Thread Jack!

Good catch guys. But we are really talking about Mass Flow anyhow. I am hoping the general idea that a suppressor adds 30% more "dwell time" means the Mid system will work like Carbine system and I can close down the gas block. It will be my first adjustable gas block, so I am looking forward to tuning the system with buffers/springs etc.

Go for it Jmacken37!!!

**tom12.7** · 07-08-15, 16:42

Dwell time for the gas port is simply just that, a measurement of time. It only applies for a distance measurement when comparing barrels of the same length and gas system with the same loadings. The term "dwell" can be accurately used on many functions on an AR type of firearm.
As for a 12.5" mid gas, I would recommend that on suppressed only setups. You can set it up for non suppressed, but the operating band becomes narrower. Changes in carrier velocities happen quick in non suppressed versions, generally more consistency in function helps.
Not all suppressors share the same traits. There is a difference in operation in the amount carrier velocity from having more pressure operating the action the amount of lower pressure blow back into the action. That is another subject altogether, but remember different cans operate differently. You can't put a number on it, there's too many factors.
13.5" mid gas is about as short as I would go without a can, you can make a wider band of operation. It can be used well on a 50/50 suppressed gun well with a buffer swap.
I would highly recommend the A5/rifle like action system any day over the carbine action system. The reduced carrier velocities of the rifle like system help pressures to reduce better than carbine type systems. That broadens the range in operation itself, and components see much less wear/longer life.

**Junkie** · 07-08-15, 23:49

How does the A5 or rifle system reduce the carrier velocity, is it just the heavier buffer? From what I'm seeing a rifle buffer is 5oz, while an H3 is a little heavier at 5.6oz. For a given buffer weight, how does the longer body of the buffer (the only significant difference I can find) change how it fires, does it have to do with the weights having more wiggle room in the buffer?