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+--- Thread: 12.5" mid-length gas port size (/showthread.php?tid=16929)
12.5" mid-length gas port size - CVN2891 - 08-24-2018
I have an extra cheap 18" mid length Grendel barrel that I don't have a plan for and I've been wanted to try a Grendel sbr. I'm planning to get the 18" cut down to 12.5" or so and have the gas port opened up.
Now, I know this is not a recommended set-up, but I want to give it a shot anyway as something to mess around with. I'll be planning to shoot it suppressed, and plan on throwing an adjustable gas block on it. I'm pretty confident I can get it to run ok suppressed.
Even though the main point is to have it for shooting with a can, I figure I might as well try to get it to run unsuppressed with the gas block opened up more in case I want to burn up some Wolf from a short package . The issue is that I don't really know what size I should have gas port drilled to. I'd like to open up to a size that should definitely work, but I don't want to go totally crazy with the size. Right now I'm thinking 0.100 should work, but I'm wondering if anybody has tried this before, or maybe has insight as to how I could calculate the size more scientifically that my semi-educated guess about it.
Thanks.
12.5" mid-length gas port size - Sticks - 08-24-2018
IIRC the gas ports on shorter barrels are smaller than longer. .075 comes to mind. Maybe .070. Rifle length is in the .090s
12.5" mid-length gas port size - CVN2891 - 08-24-2018
Yeah, that's a good point, I know it needs to be oversized because of the short dwell time given by only having a couple inches of barrel after the port, but maybe 0.1 way oversized considering it's a midlength start with.
12.5" mid-length gas port size - LRRPF52 - 08-24-2018
For a MLGS 12.5", I would start at .0935" and take it from there.
It will run suppressed with a smaller port than that with 120gr+ ammo, but unsuppressed, it might not.
I think you're in the right mindset with .100", but test it out.
Also consider cold weather and reliability.
There's a guy who cut down one of his LaRue 18" to 12.5", but I don't remember what port he used.
12.5" mid-length gas port size - CVN2891 - 08-25-2018
Cool, thanks for the advice, I'll probably go with 0.935" unless somebody else chines in who has actually tested one.
12.5" mid-length gas port size - CVN2891 - 10-08-2018
In case anybody is interested for future reference, I went ahead with the 0.0935" gas port and runs like a champ with a non adjustable gas block. Locks back every time with Wolf and Hornady Black, and ejects at 4:00. I'm running an H1 buffer.
This was my first time with a short barrel Grendel and it's awesome, concussion isn't bad, recoil impulse is very mild with the midlength gas and it's sub-moa with the Hornady.
12.5" mid-length gas port size - biodsl - 10-08-2018
Amazing stuff! I didn't know a 12" middy would function well. Congrats!
12.5" mid-length gas port size - CVN2891 - 10-08-2018
Thanks! I wasn't sure about it either, but I'm glad I went ahead and tried it.
12.5" mid-length gas port size - LRRPF52 - 10-08-2018
Your next test should be in cold weather to see if it will still run.
Right now, you have a system that is really easy on the components and should be a smooth shooter.
Thanks for the data points on a 12.5" MLGS .0935" gas port with H1 buffer.
12.5" mid-length gas port size - CVN2891 - 10-09-2018
Sure thing, I'll report back when I get a chance to do some shooting in the cold. It's still hitting 90 here in Texas, so it might be a while.
12.5" mid-length gas port size - CVN2891 - 01-05-2019
In case anyone is interested, I shot it a bit with temps in the mid to high 30s and it still ran fine and locked back. Probably won't shoot it in anything much colder than anytime soon.
12.5" mid-length gas port size - Mhan7 - 11-24-2020
I'm happy you posted this. I have encountered NOTHING but naysayers, including my own damned gunsmith, on this very topic.
My own preference on almost any barrel, is a +1 gas system. But then again, I'm not slogging my gun off into battle. And I'm not the kind of keyboard warrior who views everything through that lens.
Let he who wants a Bronco, drive a Bronco, and he who wants a Ferrari, a Ferrari.
12.5" mid-length gas port size - montana - 11-25-2020
My 12.5" Lilja Barrel has a step gas port. The widest on the outside step is .083 and the inner step is .072 with a carbine gas system. I would gather the mid length gas system would have to be near the .093+. Best to start small and work up...
12.5" mid-length gas port size - Mhan7 - 11-25-2020
montana Wrote:My 12.5" Lilja Barrel has a step gas port. The widest on the outside step is .083 and the inner step is .072 with a carbine gas system. I would gather the mid length gas system would have to be near the .093+. Best to start small and work up...
Did you specifically step the port with the idea in mind to open it up, should it not function at the smaller diameter?
12.5" mid-length gas port size - montana - 11-25-2020
Mhan7 Wrote:Did you specifically step the port with the idea in mind to open it up, should it not function at the smaller diameter?
Lilja manufactured the barrel gas port this way. Bushmaster also did this with their 5.56 barrels back in the 90's. I'm not sure why this is done. Perhaps someone on this forum would have this answer, since I can not see any advantage for doing this.
12.5" mid-length gas port size - Mhan7 - 11-25-2020
Oh, I see. I thought you did it custom.
If I had to take an educated guess at it, I would say that it's probably just to create a "venturi", so as to get some pressure gradient that corresponds to a given hole size, but without drilling a hole that risks touching the high spots of the rifling. Small hole through bore, but venturi effect entering the gas block.
12.5" mid-length gas port size - Mhan7 - 11-25-2020
I realize my last statement sounds a little convoluted. So what I'm trying to say... Maybe they just tooled it that way, to get the proper pressure drop, based on physical testing, but without creating weird new drill sizes. A #49 drill measures .073", and .083" corresponds to 2.1mm. That might have been a very specific geometry, based on a very specific set of testing parameters.
Who knows. But it's fun to think about.
12.5" mid-length gas port size - montana - 11-25-2020
Mhan7 Wrote:I realize my last statement sounds a little convoluted. So what I'm trying to say... Maybe they just tooled it that way, to get the proper pressure drop, based on physical testing, but without creating weird new drill sizes. A #49 drill measures .073", and .083" corresponds to 2.1mm. That might have been a very specific geometry, based on a very specific set of testing parameters.
Who knows. But it's fun to think about.
A gas port with 2 different size steps, creating a different gas flow dynamic. Maybe, but it is a new one to me.
12.5" mid-length gas port size - Mhan7 - 11-25-2020
montana Wrote:A gas port with 2 different size steps, creating a different gas flow dynamic. Maybe, but it is a new one to me.
Not necessarily a *different* dynamic, just a *desired* dynamic. Based on the dates that you gave for manufacture (you mentioned the 90s), it would have been before adjustable gas blocks and adjustable mass were well known, if even a thing at all. Regardless of whether my guess is even in the ballpark, I'd bet dimes to dollars that it's a thermodynamic consideration of some sort, primarily.
But like I said... I could be (and probably am) wrong. Just taking a stab at it.