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…
http://micromoa.com/12-5-midlength-5-56-nato-1-7-medium-contour-barrel-w-govnah/
I have no affiliation with the company.
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.
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.
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…
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
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!!!
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.
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?
All of the dissys I’ve seen were all carbine gas, but had a FSB installed at the rifle position to make the sight radius as long as possible.
I messed with an early warsport upper that was proprietary as all hell. I ended up cutting down an FN 14.5 middy to 12 and opening the port to .096 and it would cycle PMC on an H Buffer with an A2. That being said, I didnt feel any real advantage in recoil over a properly ported 12.5 carbine length upper. It was minimal.
Both styles are available, many people call the style you speak of mock dissipators. I suppose a Colt 605 would have been a better example.
BCM 12.5" Kino and be done with it 
THIS. If all you are after is more rail.
It also has to do with the spring rate and length. The A5 system utilizes a rifle action spring rather than a carbine spring. As I recall the spring tension is more consistent throughout the cycle making for a smoother recoil impulse and slightly slower carrier speed.
I’m not after more rail, but do like the recoil impulse of a middy better than a carbine length gas system.
In the balance of spring and reciprocating mass for a given system, the required mass is different when comparing the carbine type action spring and the rifle type action system. The rifle action system would need the higher mass to retain the balance compared to a carbine type. The higher mass means that the system will operate at a lower rate. That lower rate means reduced carrier velocities. The lower carrier velocities means it takes more time for the camming events to occur. The lengthened time of those events allows a quickly depressurizing barrel to depressurize even further. The reduction of pressure means many other things occur, reduced camming tension, reduced unlocking tension, etc.
When comparing rifle action types to carbine types, there is no standard mass, just spring type. The carbine system isn’t dedicated to a 3.0 ounce buffer, nor a rifle to any particular buffer weight. For either system, a balance has to be made for either to be proper for those systems. For the vast majority, a properly balanced rifle action system will have a broader range of function over the carbine type action system.
There is some more merit to the A5. The spring inside the buffer stacks the weights on the carrier side, and leaves a sprung space on the bumper side. It may not sound like much, but it improves the consistency of initial carrier movement very well. The variation, when compared to unsprung buffers is noticeable.
Forgot to add that carbine gassed 12.5" guns run really well. 12.5" carbine +1" gassed is about as far as I would go in my experience. Add a can to that for me could be a +1" gas gun for 50/50. Most 100% suppressed can run well 12.5" mid gas. Really, the end users goals determine the base needs of the platform.
I’ve come to believe that this “biasing” spring actually is more important than most people think.
Afaik, the original AR10 buffer and the new H6 buffer have no extra space for the weights to move, which is one way to keep the weights pressed against the carrier side.
All the other buffers (rifle,CAR) have extra space inside for the weights to move and their initial positions before firing are somewhat random, which causes inconsistent cycling.
It may be helpful to offer some comparisons.
The 12.5 MID has similar but a little worse timing than the 10.5" CAR.
IMO, Both of these have too little power time vs gas delay to be reliable in all conditions.
Notice how BCM does not even offer a 10.5" CAR barrel? This is a clue.
The 14.5 MID has near ideal timing and runs similar to the 11.5 CAR.
The reputation of these two configurations is very good.
The 12.5 CAR has good timing, just like the 16 MID.
The reputation of these two configurations is also very good.
Given the choice of any SBR barrel config, I’d select 11.5" CAR all day long.