I’m going to the F2/F4 SBR build through Rainier Arms.
For this particular rifle, I want to cut down the existing barrel (Stag M4 14.5" with perm attached phantom) to 10.3".
RA can do the cut and thread work, and enlarge the gas port.
I need an idea of what size the gas port needs to be . . . ammo will be Speer 55 GD, AE .223, PMC Bronze, and occasionally some M193 or M855. I do not use Tula, Brown Bear, Wolf, etc.
I have read that the MK18 spec calls for .070" and a H2. My best guess, having looked at this on different boards for months, is to go with 0.73", so it’ll run .223 pressure ammo.
Suppressors I have are a AAC 556-SD and a 762 SDN-6. I can use whatever buffer and spring I want. I use Colt gold extractor springs and carbine (black) inserts.
Been looking through a lot of threads for months but not seeing an answer to my particular situation, hence I’m asking.
Yeah, read through that thread already.
Do not want to use a AGB, although if I did, it would be a Govnah. I may do that down the road for this gun.
I have a switchblock on non-NFA carbine and like it, but am not going to use a SB for this particular gun. I may buy a 10.5 SB barrel for my next SBR build, but I have yet to figure out all the rail issues that would incur.
Inferring from MicroMoa’s info that .003" is what they find to be the difference between 5.56 and .223, hence my inclination to enlarge the port from the ~.064" is it now, to .073" (.003" over the .070" M18 port size).
Funny how the DD MK18 is overgassed at .083" (or now, at .078"), but Colt’s 10.3" is .093" (for which SAW recommends a H3), if I’m reading correctly.
My 10.5 has a gas port of .082" and it’s just too large. It works well enough unsuppressed with a rifle RE assembly, but it works even better with the SLR AGB. I gave the Govnah a serious look but in the end, the lighter weight and ease of fine tuning of the SLR won out
The gas port on your Stag barrel is likely fairly large anyway so I doubt that you would need to have it opened up much (if at all) in order to function .223 pressure ammo. For a 10.3" barrel I would honestly start with no more than a .070" port and just see if it runs suppressed. If you’re going to be suppressed the majority of the time, then I would suggest starting closer to .060" and working up, if necessary.
You are trying to go cheap which usually ends up bad. Get a known quality Colt or BCM barrel with a carbine gas port of .063 and then have it opened up and cut down.
What do you mean “sbr build through rainier arms”, do you mean you are buying the parts through them and having them gunsmith/assemble it?
I have a chf bcm 11.5 and for this next upper want it about 10.7 (handguard+ can, was originally planning on 10.3), I bought parts through them, and got
their FN chf, rainier mountain, which is a 14.5 I was told had a .068 port so it might be just about right.
Not sure what Stag was using, want it to work suppressed and unsuppressed (therein lies the problem). I’m having them open it to .073" and see how that works, and am leaning toward using a Govnah.
No, it’s unverified. Have seen a lot of people claim this, though, for the Colt 694x 10.3" barrels.
Contrast this to people who claim to have spoken to sources at Colt who state their MK18 barrels use a .068" gas port. Which would make sense, since .mil uses 5.56 and nonmil and LE often uses .223 instead.
True, but the majority of what I use is .223, so I want to err on the side of the port being a little bit bigger. Amphibian of MicroMOA sizes the holes in his regulator plates .003" larger than what he recommends for 5.56 if one is using .223, so that is where I came up with how much larger I would want to go than the MK18 spec.
Another piece of the puzzle, it is widely reported that the newer DD Mk18 barrels use a .078" port, and they are still overgassed for .223, so I don’t want to go quite that large of a port.
As you are probably aware, in WA, individuals can no longer do their own F1 SBR builds at this time due to how BATF is interpreting I594.
So, until that changes (if ever), individuals to instead go through a manufacturer to make the SBR, do a F2 (or vice-versa), then transfer it back to the individual on a F4.
You supply RA with the gun and/or parts, they do the mods, and they submit the F2 and F4 to BATFE.
I want it to work with .223 ammo (though not the lower powered of the .223 loads), since I do not typically use 5.56 NATO ammo any more.
My understanding is that it would be beneficial to open the gas port slightly more than the Crane spec of .070, for unsuppressed use.
Is that not the case?
FYI, this particular barrel was from a special run that Stag did, has a 5.56 NATO chamber (though I haven’t checked it with a 5.56 chamber reamer) and the rifling is 1/7, so hoping it will stabilize bullets when cut to 10.3, and not have any problems with extraction due to the chamber being tight. I have shot it with M855 and M193 and not had any problems with popped primers.
Clint at BRT says the same thing, that .003" is about the port size eqivient to the difference between .223 and 556 chamber pressures for the overall pressure curve in maintaining an appropriate margin of reliability in function. Which is why for mixed use 223/556 unsupressed he recommends a .073" gas block insert when using 10.5" barrels over the crane spec which accounts for exclusively using 556 Nato pressure ammo.