So basically I've got a BCM 16" mid-length upper and CMR 12" rail that's not being used.
Is there any reason why it would not function if I chopped the barrel to 12.5?
Printable View
So basically I've got a BCM 16" mid-length upper and CMR 12" rail that's not being used.
Is there any reason why it would not function if I chopped the barrel to 12.5?
Dwell time. After your projectile passes by the gas port, until it leaves the barrel it is pushing gasses into the gas tube and BCG/piston. Really basically, if there isn't enough dwell time, then it doesn't push hard enough.
Best gas system for a 11.5 or 12.5 is carbine.
So a midlength 13.7" barrel has enough dwell time? 1.2" is the difference between working and not working?
Im just curious.
You may want to look up "dwell time". A midlength gas system on a 12.5 does not allow enough gas to get back to cycle the weapon reliably before the pressure drops as the bullet exits the barrel. Even some 14.5 middys can be finicky with lower powered ammo.
ETA I was beaten to the explanation.
If you are going to get your barrel chopped, why don't you just have adco do it and they can relocate the gas port too.
Is this going to be for a dedicated suppressor build? Just wondering. That changes things a bit.
Sent from my HTC One using Tapatalk
Robb Jensen is the man to ask. We were commenting on this the other day and apparently has has a project he has been working on with a 12.5" middy and it's working. I'm a little intrigued to say the least.
IINM, Constructor has had some luck doing 12.5 midlengths as well.
MicroMoa, who makes the Govnah gas blocks, also makes 12.5" mid lengths and says they run great with very little recoil. Maybe he will chime in.
minimum I would do for a middy gas system is 14.5.
Although this is interesting.
Quote:
Robb Jensen is the man to ask. We were commenting on this the other day and apparently has has a project he has been working on with a 12.5" middy and it's working. I'm a little intrigued to say the least.
Salient's 14.5" uses a rifle length gas system. If that works (and it apparently does, and very well), I don't see why a 12.5" mid-length wouldn't, assuming it's put together properly.
Isn't the LVOA SBR a 12.5" middy?
Sent from my HTC One using Tapatalk
Here's a bit of an older thread (2011), that has responses from Robb and Todd.k
https://www.m4carbine.net/showthread...-5in-Midlength
Did it a while back (few years actually). Its possible, but you may have to open up the gas port.
IMO not worth the trouble i went thru.
Here's mine. 16" BCM middy cut to exactly 13" from end of the bbl extension to the muzzle for a 12.5" bbl. I opened up the gas port .007" bigger and it runs 100% even with PMC Bronze .223 using a M16 carrier and a H buffer. Dwell time schmell time. This bbl I used to use on a dedicated suppressed upper with its original size gas port and a LMT enhanced carrier. I had it laying around so I said what the hell give it a try. I had my doubts too. Next I want to try an H2 buffer and also Wolf, Rem UMC and or Radway Green ammo to just see how what it'll do.
http://img.photobucket.com/albums/v4...agejpg1-13.jpg
http://img.photobucket.com/albums/v4...magejpg2-3.jpg
We have sold several hundred 12.5" barrels with a mid gas system. They run fine just port the barrel to run with the ammo you use.
Depending on the load it could be anywhere from .082-.093.
I agree that this can be the case for specific ammunition, but I don't believe that the 12.5" mid gas is preferable for a wider variety of ammunition over a wider variety of circumstances. The 12.5" mid runs a lot like a 16.5"-17" rifle gas, they can and do run, but can be finicky. They seem over gassed in some instances, while under gassed in others.
The carbine gas system works pretty well on 12.5" builds, It's one of my preferences for this barrel length that aren't suppressed or rarely suppressed. They can handle a variety of ammunition very well without premature signs of under/over gassing over a very wide set of variables.
The carbine plus 1" gas system on a 12.5" works almost exactly the same as a 11.5" carbine gas guns. Given purpose driven gas ports for both, the change is a projectile velocity and can erosion.
12.5" mid length works very well suppressed only, and is a preference of mine for 100% suppressed for this barrel length. It can run into issues without a can. If you run in a narrower set of circumstances they may be okay for you, for others they may not.
I would tend to recommend a setup that covers the widest range of instances that the end user would use. I would generally advise against 12.5" mid gas for general use.
What about a middie system on a 12.5 with an adjustable gas block? Is there such a beast that can theoretically be adjusted up to 0.93 as stated by constructor?
I'm glad to see some other opinions about the 12.5". It does seem to me that a 12.5" middy wouldn't run as smoothly as a 12.5" carbine gas across the board. I guess I would have to shoot one to see if the juice is worth the squeeze.
Seems like it would do what opening up a gas port would do. It just pulls in more air than is supplied, increasing the flow into the gas tube.
Its the same physics used on a gas stove or grill to mix the gas with the air, or on a pressure washer to suck the soap into the line (mine is setup like this anyways).
I checked my gen-1 SLR Sentry-7 set screw AGB (Todd at SLR Rifleworks upgraded me to a gen-3 melonited version) and the gas path is a straight cylinder from the gas port opening to the port that feeds the gas tube aperture.
I have heard some rumors via instagram comments that Salient had to modify the gas port dimension when transitioning from the BC to their new "Top Hat" muzzle device (due to reduced backpressure from their less restrictive device over the BC). They (Salient) seem intent on not having a technical discussion about any of these changes and they basically ignored all of the questions from potential customers about their gas system, other than it's a rifle length gas system.
I wonder if creating an expansion chamber between the gas port and the gas tube entryu aperture would have a) unintended harmonic effects and/or b) if unintended cooling of the gasses would happen with the rapid expansion from barrel gas port to expansion chamber leading to a venturi?
I'm not calling foul on their design, but I'm starting to wonder if there could be some unintended side effects that may not be positive.
EDIT: At the risk of getting over my own head, I read this in the article that Gunz linked to:
"Choked Flow
The limiting case of the Venturi effect is when a fluid reaches the state of choked flow, where the fluid velocity approaches the local speed of sound. In choked flow the mass flow rate will not increase with a further decrease in the downstream pressure environment. However, mass flow rate for a compressible fluid can increase with increased upstream pressure, which will increase the density of the fluid through the constriction (though the velocity will remain constant). This is the principle of operation of a de Laval nozzle. Increasing source temperature will also increase the local sonic velocity, thus allowing for increased mass flow rate."
http://en.wikipedia.org/wiki/Venturi_effect
I haven't had an issue with mine. It's the softest and smoothest shooting ar I've ever shot. Everyone who shoots it comments on how smooth and soft it is.
The gas pressure at the mid location is higher than the gas pressure at the rifle location in any rifle even a rifle length 20". The difference is dwell time. With a rifle length gas system you need more dwell time. Port the 12" to run with 5.56 Wolf or Tula or Remington then it will run with anything including M193.
As far as I'm concerned steel crap is a different caliber than 5.56x45mm so I don't damage my guns trying to convert it to work with that caliber.
I've found that, since I started using heavier buffers, the LMT enhanced carrier, and quality extractor springs, running steel cased ammo hasn't been an issue for me. Especially since converting all my guns to the A5 system. It's just a non issue anymore for me.
And, I totally agree that it is crap and is usually ultra weak. But i have been able to buy it off of some buddies who were basically giving it away. HELL, I even bartered some spare parts for some of it.
Is it bad for my barrel compared to brass cased stuff? Absolutely. But it's training that I wouldn't have otherwise been able to do. I just control my rate of fire and let the gun cool plenty between courses of fire.
Salient makes "game guns", in my opinion. If it runs for the game (and they do), then that's good enough. They are not worried about a marine in Afghanistan fighting in 110*F heat during the day and 15* weather in the mountains at night. They are worried about a 3-gunner or a hobbyist having the sweetest shooting setup available, which, arguably, they provide.
Same here.
Uppers with "optimum" gas system configurations, properly sized gas ports and A5 buffers will run everything.
Yesterday we went to the range. It was 25 and lightly snowing.
The 16" middy and 12.5" carbine ate Wolf and MFS like candy, ejecting to 4:30 in a neat pile.
Even the last round lock back test with a single round in the magazine worked with a very loose "floating" grip on the weapon.
On warmer days, these same uppers will run hot IMI M193, CBC 62FMJ and PRVI 75 all day long.
I'm not 100% sure we are getting our points across. Sure, shorter gas systems see higher port pressure than longer ones. What I've seen is a larger deviation of cyclic rate/carrier speed on the 12.5" mid vs the 12.5" carbine (or +1" carbine gas system) when a wide variety of ammo/circumstances come into play. I know that the 12.5" mid gas can run, but it's not as consistent as the shorter gas systems for this barrel length.
How many mid gas 12.5" barrels have you shot or tested?
You are trying to get your point across I just disagree. I know as soon as the carrier moves rearward 1/2" the remaining gases are exhausted out the side of the carrier. I've also seen enough pressure trace graphs to know it doesn't make that much difference. Who ever decides what size to drill the gas port in the barrel has the control to make it work or fail. You can't expect every load from Remington to M193 to land in the same pile if the Remington produces 42,000psi and M193 produces 58000psi no matter which rifle you shoot them from.
If we are talking about consistent the AR15 was designed with a rifle length gas system and a 20" barrel. It was designed to cycle the action with the port pressure much lower than what it is with a carbine system. Could be why so many carbines break bolts. It is the furthest from the original design except for a pistol length system.
It started out as a rifle length system with a fixed stock and rifle buffer. Now every single configuration has been modified to work. It's easier for everything to work perfectly if the military tells you exactly what ammo you will use and you will not deviate from it. If you change from a rifle buffer to a carbine buffer the gas port will need to be a different size, if you change to an A5 system the port will need to be a different size to get the same results. Of course if the barrel was over-gassed to begin with a heavier buffer and spring may improve the performance.