AR Gas Port Size

[WayneShaw]

Chambered a 12 twist 6MM barrel in 6x45, plan on shooting 65-70 bullets. Looking for a starting point on gas port size, 18" barrel.

 

[VA_XTC_Shooter]

Not sure about 6x45, but a 6 Arc 18" gas port size is around 0.097" on production barrels. Could always start small and work your way up...going the other way is a little more difficult. ha

 

[WayneShaw]

I certainly would err on the small side first! I've seen/read of holes of .075".

 

[VA_XTC_Shooter]

If you have read of guys with 6x45 and 0.075" gas port, start there, or just under, say 0.073". ASSUMING same length gas system, rifle, I presume? 18" could be mid-length. I don't know the answers, just trying to identify things to consider.

 

[Sandstorm]

An adjustable gas block makes all the difference here to get the perfect combination and cycling.

 

[Rocketvapor]

Gas length and port size?
That's what the crap 18" barrels did to the 22 Nosler.

 

[WayneShaw]

With the weight bullets I intend to shoot, is well within the same weight as a 5.56 uses. I don't think there will be much difference in function. I've been picking up parts for this for over a year, thought it would be an easy way to use up so many misc. 6MM BR bullets I have. You always end up with a handful of leftovers. I turned a used BR barrel down to make this thing.

By the way, anyone lighten the ejector spring on these? This feels like the brass will end up in a different zip code.

 

[jelenko]

With the weight bullets I intend to shoot, is well within the same weight as a 5.56 uses. I don't think there will be much difference in function. I've been picking up parts for this for over a year, thought it would be an easy way to use up so many misc. 6MM BR bullets I have. You always end up with a handful of leftovers. I turned a used BR barrel down to make this thing.

By the way, anyone lighten the ejector spring on these? This feels like the brass will end up in a different zip code.

Does the ejector have either the black rubber insert or the rubber o ring? Both of those increase the force needed.
On my rifle length gas system, I use the spring only. My understanding is the insert and o ring are to increase the force for shorter gas systems.

 

[VA_XTC_Shooter]

Does the ejector have either the black rubber insert or the rubber o ring? Both of those increase the force needed.
On my rifle length gas system, I use the spring only. My understanding is the insert and o ring are to increase the force for shorter gas systems.

Spring and insert is the best way to go. Not both insert and oring. If that is the case, its covering for a crappy spring. Sprinco extractor and ejector springs are the way to go.

 

[mac86951]

Question! A bit more important than the diameter of the bullet. What length gas system?

Carbine, mid-length, rifle length? That'll be a big change in gas port size.

 

[ShaunBrady]

With the weight bullets I intend to shoot, is well within the same weight as a 5.56 uses. I don't think there will be much difference in function.

If the same weight bullets are used and give the same case volume, the same charge weight of the same powder in the 6mm bore will drop pressure about 18% faster because of the area increase. The rest of the ifs won't line up, but the change in bore area is a good entry point for the process. QL is good for guessing at the rest.

I'm doing a similar project with a 6.5cm AR10. That's going the other way with port pressure, but it's the same set of problems. I used QL to rough out the port location and barrel length that gave a reasonable port pressure and post port dwell and then started with a deliberately small port. I used 0.072" to start. I was also doing ammo pressure ladders to rough out what the final load would be. It ejected cleanly but wouldn't lock the bolt back. I'm not putting a second round in the magazine until the bolt locks back. I increased the port size to 0.082"and it still didn't lock back, so on to 0.093".

If you overshoot, an adjustable gas block will fix it. If the gun is going to use a range of bullets, an adjustable block may not be a bad idea.

Ejection will be bolt speed and ejector spring strength. I'd see what you wind up for ejection before playing with the ejector spring. I've seen donuts around extractor springs, but never used anything but a regular spring for the ejector. The donuts are for short barrel guns starting extraction at high chamber pressures. I'm trying to build a gun that gives SAAMI velocities but doesn't maul brass so I removed the donut. It's extracting brass well so far without chewing up the brass, but I'm not up to full power on the bolt carrier yet.

 

[msalm]

barrel length has less to do with it than gas system length. Carbine, mid-length or rifle length. Rifle length w/ 5.56mm is .093”. Smaller as you get shorter on the gas systems.

 

[WayneShaw]

The ejector only has 3 parts, spring, plunger and pin to hold it in.