Gas port size

[pwalker]

I have a 28" barrel that I had cut down to 19.5 inches. The thing was over gassed before I had it cut. I was hoping that it would function without modifying the gas port after it was cut. I shot it for the first time today and it would eject the spent casing but would not consistently chamber a round when fired or lock the bolt back after firing the last round. I measured the gas port and it is .093 right now. Any idea what size it should be for a 19.5 barrel?

I was shooting some rounds that I was fire forming with a light charge. 26 grains of 2520 in Winchester 7.62x39 brass. Also tried some already fired IMI brass loaded with 28 grains of 2520 with the same results. Both of these loads would function before the barrel was cut. I will try some factory rounds before I open up the port.

Also does anyone know where to get a .906 gas block? This one has an adjustable Medesha gas block that I cannot seem to close all the way off. There seems to be some gas coming out of the escape hole on the top of the block even after I tighten it all of the way up. I looked around but haven't seen any .906 blocks.

[ss355]

I hope someone with port size on their 20" jumps in, but my 24" bbl has a .097" port. If we extrapolate and say .001" for each inch of barrel, that would put the size of a 20" port at .101". However, I'd certainly work up to that. A lot easier to make a port bigger than smaller.

[Rapidrob]

It sounds as though your gas block is gas cut/eroded as you cannot shut the gas all the way off and it shows signs of leakage. The pressure at the gas port is high. 15,000 psi is not uncommon.
As for your port size there are so many factors to try to compensate for it would make your head swim. Your best bet is to purchase a well made drill index set and go up one size and test fire. What will work in another's rifle may cause violent case ejection and case dents in your rifle unless they have the exact same barrel and chamber type. I learned that the hard way years ago.

[pwalker]

I never found a .906 gas block. I had the barrel re-profiled to .750 and put on a new gas block. Also opened up the gas port about .003. It runs great now.

[Drifter]

Along the same lines, I have an 18" midlength build that seems undergassed. Gas port measures 0.090". Is this the appropriate size? (If it matters, barrel is .750" as the gas block.)

Experiencing very weak ejection (with no brass marks on the shell deflector), and have eliminated the lower and BCG as culprits. I plan to try another gas block and gas tube in a few days, as disassembly / reassembly of existing system had no effect. Thus, I'm wondering about the gas port size, and whether it should be enlarged.

Any feedback would be appreciated.

Thanks

[WillD]

FWIW my 28" barrel, still in the box, has a .093" gas port.

[Drifter]

My 18" midlength has a .90 gas port. My issue turned out to be the way the ejected cases were catching the edge of the port preventing contact with the deflector.

[Malchira]

I never found a .906 gas block.

At the risk of repeating myself, Triton Arms makes .906 gas blocks. While I haven't used one myself, I've had work done by them and it was excellent.

[glane5910]

You can email Les Baer and ask what they are drilling their ports at. I did for a 6x45 build I just finished up. They were very helpful. I've got a list of port sizes for 5.56 which will be slightly smaller than the Grendel because of pressure differences. Smaller is always better, you can work up to what works the best and not over gas it. Now you can always go larger and put an adjustable gas block on it.

[martkin64]

Anybody have some new information for 2015 about the minimum and maximum gas port size?

[BluntForceTrauma]

M64, maybe specify which barrel length and gas system length you have in mind? Gas port size is going to vary depending on those, uh, variables.

[martkin64]

M64, maybe specify which barrel length and gas system length you have in mind? Gas port size is going to vary depending on those, uh, variables.

My bad..... I have a 20" barrel .... rifle length gas tube..