What is needed to make an upper with a Rifle length gas system function 100% on a lower with a collapsible stock receiver extension?
Thnaks
Has to do with the gas port sizing. I just assembled a Mk12 with a rifle length gas, and built it using carbine stock setup. Runs fine so far but the spec gas port on the Mk12 barrel is pretty large.
You should contact the barrel maker, and ask them if their gas port sizing will run with a carbine stock setup. I know LaRue has you specify when ordering a 20" Stealth upper which stock setup you are going to run.
This is a COLT Factory 20" H-bar, the user wants to get rid of the fixed stock and put a collapsible stock on. I looked at my armorer manuals, but did not see any reference.
anyone with this set up?
Jack
you may want to put a heavier carbine buffer in. h2 or maybe the 9mm buffer. the 9mm buffer is close to the rifle buffer weight
i put a 20" BCM on a BCM lower [collapsible stock] with a sopmod and spikes ST-T2 buffer and it feeds everything i’ve shot through it. no burps at all, with apporx. 6 types of ammo. this is also one heck of an accurate rifle also, i’ve got a scope ordered for it, and will find out exactly how accurate it is. i’m suspecting bragging groups. at any rate, it’s a fine set-up.
Im running two Armalite 20" M15A4 SPR uppers on LMT Defender SOPMOD’D lowers.Barrels are advertised as 1:9,but rod out closer to 1:8.
In general I have found the CAR buffer to be the most widely acceptable of varying ammo in my rifles.Particularly .223 Rem spec 50-69gr.,which I use the majority of the time.
The H and H2 run without issue with true 5.56 NATO pressure ammo(and some heavy OTM’s).
Overall I’d recommend getting the CAR,H and H2 buffers,run a selection of ammo and function test with the rifle to see which buffer is the best for you.
As for gas port size on my barrels…I have no idea.I simply went through the buffers until the rifles choked with various ammo and made my notes.
For more info do a search for C7A2.Several of the SME’S and IP’s have posted a ton on info on this configuration.
Jack, see if you can get in touch with KevinB from KAC. He’s former Canadian military and they have issued 20" guns with collapsible stocks, and I’ve seen him post about them before.
As mentioned, gas port size is going to matter somewhat, and from there you can adjust the buffer weight. It’s not as simple as matching the weight of the standard rifle buffer because the tube is shorter and the spring is stiffer in the carbine.
If you don’t get the answer you need you could try ordering one each of the red and blue from tactical springs and a full complement of buffers C, H, H2, H3, and 9mm and see what combination works. Would take a little trial and error of course. I have the buffers if you want to borrow them but I think I’m out of the springs.
On a side note, that gun is going to balance really, really, funky with a 20" HBAR and M4 stock.
Here is one of the posts that KevinB,Nick and others discuss the buffer weight for the C7A2.
https://www.m4carbine.net/showthread.php?t=15254&highlight=C7A2
What I gather from all the info provided in the past on the subject,the C7A2 buffer selection was based on overall performance as a do all plug and play with various barrel length uppers from 10"-20" and was tested and setup to run 5.56 NATO spec type ammo.
As always I’d appreciate correction in my assumptions.
In my experience with my weapons(sample of two non-govt standard) if lower pressure .223 Rem(particularly underpowered ammo like UMC or Wolf) is to be used the buffer selection will have to be explored as I noted above.
HTH
ETA
On the balance of the weapon configuration.My barrels are what I call between HBAR and medium contour.With the SOPMOD,standard A2 handguards,ADM with Burris FFII Tactical 3x9x40 mounted foward the balance point is at the Delta.With a loaded 30rnd mag the balance point moves slightly rearward.
YMMV