AR gas port question

[jelenko]

No, I use Eliminator and occasionally Iosso

Any chance Iosso was used just before taking the pics?
You might checkout the gas port after shooting it without first cleaning.

 

[Richard Jones]

The hole in the gas block is .125”

????

 

[dellet]

????

Generally drilled with the same bit as the minor diameter of the set screw. Tube ID and block ID is the same.

The port in the barrel does the tuning.

 

[fatelvis]

Wondering if you have checked it again since you probably have 4,000 rounds fired instead of 400 by now?

I have not checked it since then. I used it for the rest of the F-Class season, and it got me a high master classification. Since then, I removed the upper and am running 6ARC on that lower. I was a little worried that it would restrict the orifice too much, as I am running a heavier buffer, and a Tubb’s flatwire CS buffer spring.

 

[Richard Jones]

Generally drilled with the same bit as the minor diameter of the set screw. Tube ID and block ID is the same.

The port in the barrel does the tuning.

First 3 I checked from the parts box: .127, .157, .159. Just wanted to point out that they're not all the same and we shouldn't assume that they are.

"The port in the barrel does the tuning." Huh?

 

[fatelvis]

"The port in the barrel does the tuning." Huh?

I think he meant that the port acts as a chokepoint, and sets the gas flow.

 

[dellet]

First 3 I checked from the parts box: .127, .157, .159. Just wanted to point out that they're not all the same and we shouldn't assume that they are.

"The port in the barrel does the tuning." Huh?

Are you measuring the block or inside diameter of the tube?
If you find either smaller than .125” it would be the anomaly.

The point being that the port in the barrel will never be larger than the passage in the gas block. People trying to get AR’s to run subsonic found that gas ports over .125” needed modified blocks and tubes to deliver the gas. Also right around that size more leaks between the barrel and block developed because it was easier to force gas under the block than through a smaller diameter gas system. Example a .130” barrel port forcing gas through a .125” gas block and tube.

The cycling rate is determined mostly by the size of the gas port. It’s a matched system, but port size determines the weight needed to slow down the carrier.

In the case of the OP he states he’s using a heavier buffer and flat wire spring. Heavier buffer means one of two things, he’s over gassed, or never cleans his rifle.

The heavy buffers really aren’t for slowing down the extraction process, they are counter productive to accuracy and follow up shots. Too much weight flying around stopping and starting while your trying to aim.
A heavy buffer is there to drive the cartridge into the chamber when it’s full of mud, ice or the occasional banana leaf. If you need the forward assist and a hammer things have gotten really bad.

Now that you have a weight and spring rate to accomplish feeding in the worst conditions imaginable, you need to control the rate of fire. Cycling rate. The cycling rate is determined by the amount and pressure delivered to the carrier. Pressure is controlled by tube length and it’s a gross adjustment, based on a few inches of barrel length. The amount of gas delivered is based on the size of the port. So bolt speed is controlled by gas flow. If it’s the ever famous “MIL SPEC” it will be a window of 7-900 rounds per minute if I remember correctly. The system is over gassed and over buffered by design. It has to extract and chamber, or it’s a club not a rifle. Accuracy is less important than function.

This is why specs for a battle ready M16 for a target rifle doesn’t work out so well. You don’t need the bolt to close so harshly that it sets the shoulder back .003”. A lot of competition people single feed anyway. But for rapid fire less mass, moving slower means faster follow ups. It means the cross hairs never come off the target and seeing your own hits. Do you really need a rate of 900 rounds a minute to shoot paper? Can you get by with half that since the chamber isnt full of sand?

A lot goes into tuning an AR, if all starts and ends with gas flow. Before the popularity of adjustable blocks it was tougher to control. Kind of sucks to strip down a rifle enlarge the port, rinse and repeat. But in the long run it’s would arguably be better. Just don’t change loads after doing so.

This was the problem with 300 blackout shooting both subs and supers. So many people punched the ports out to .125” to get subs to even cycle with an off chart powder. Then they tried to shoot supers and the bolt out ran the magazines ability to feed.

An AR is over gassed by design. Reliability is number one. But if you can risk a little reliability shooting in average conditions, then reducing reciprocating mass and speed certainly won’t hurt most people’s scores.

 

[jelenko]

it sets the shoulder back .003”

You got my attention!
Mine will definitely resize any case with headspace greater than the chamber!

I'm running a 24" barrel, rifle length gas system; gas port .093; .936 gas tenon. 223 but with an AR10 buffer and buffer tube [I use a WOA adjustable buttstock]. Currently have a Tubb flatwire spring.

What should I do to tune the system so the bolt doesn't close so hard?

Thanks!

 

[dellet]

Have you ever chambered and removed a round that the shoulder was only set back .002”?
It’s enlightening to try. The shoulder contacts the chamber before the extractor pops over the case head. Exactly how hard do you need it to hit?

It’s a never ending circle. But worthwhile in my opinion. With an adjustable block it’s pretty simple, as long as the gas can be completely cut off. Not all will. Close the port until it won’t cycle, lighten the buffer. Repeat.
A lighter spring reduces forward carrier speed, less weight to push speeds it back up. So it gets into physics and math on ending inertia. What has more energy. Slow and fast or heavy and slow. It can be either way.

I tend to order barrels with undersized ports, so I can drill them to the size I like. But it really starts with intended use. The JP captured sling set up is nice in that you vary weight and spring tension, but a pile buffers works. Sadly it really does come down to trial and error.

Something I learned with subs, +/- .010” of seated depth can make the difference in whether a load cycles or not. That’s not a system I would want to count on for anything more than shooting tin cans. I wouldn’t want to rely on it for score. Not enough margin of error. Big enough temp change and time to go home. That’s where the balance of how much extra gas is enough for piece of mind.

The other plus to slowing down bolt speed is it’s so much easier on the brass. If you need a small base die, it’s cycling to fast. Too prove that point I dialed back the gas using an LMT enhanced bolt and carrier to the point that neck sizing only is possible in an AR.

The LMT has an extended cam path to delay extraction and the bolt face fully supports the case head. The extractor does not cut into the bolt face. High, probably over pressure loads with out swipes. Reliable cycling with a light buffer, a heavy buffer to me is an H2. Carbine can dip into unreliable to feed because you can get it going really slow. As in so slow your waiting for the round to chamber before completing a double tap.

Lots of party tricks to prove a point.

 

[Richard Jones]

Are you measuring the block or inside diameter of the tube?

Block, of course. Remove a set screw, use a pin gauge. Were you measuring the gas tube, because they're generally (oops, there's that word) .120 or so ID. I've never seen or heard of a .159 tube ID, but I guess some folks will try anything.

So, we're still at all gas blocks are not ported at .125.

Have put together a few 300 BO's, all with a Vltor A5 3.8 and a SA adj. block. All tuned easily for subs with little fuss.

 

[dellet]

Block, of course. Remove a set screw, use a pin gauge. Were you measuring the gas tube, because they're generally (oops, there's that word) .120 or so ID. I've never seen or heard of a .159 tube ID, but I guess some folks will try anything.

So, we're still at all gas blocks are not ported at .125.

Have put together a few 300 BO's, all with a Vltor A5 3.8 and a SA adj. block. All tuned easily for subs with little fuss.

You’re right. I shouldn’t have used the word all. That’s why I qualified it with minor diameter of the set screw. Same drill bit made it, just not tapped.

It’s incredibly simple to tune a 300 Blackout to shoot subs. The difficulty is getting it to not be over gassed shooting supers without being over gassed. An adjustable block makes that much easier.

If you go back to the original claims for the Blackout, shooting subs and supers with and with out a suppressor, all four configurations, it was basically impossible. only by limiting the powder choices could a pistol gas length barrel shoot unsuppressed subs and suppressed supers without cycling issuers on one extreme or the other. All
Without any changes of hardware or An adjustable block.

10.2” carbine gassed barrels are tricky also. But that’s probably one of the sweetest shooting setups I have for both sub and supers.