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which size drill bits do you use?

I believe he said a .093. That sounds like the perfect size of port. I'm betting it has to do with the port location or the gas block location.

kwg

I have reamers in stepped sizes but a drill works if you feed it slow to reduce the burr in the bore.

The mil spec for rifle gas on 20 inches is .093 but thats a bit small if you want PMC bronze to run when a bit dirty or cold.

Rifle gas is really forgiving and I would run a .100 at a starting place if OP can't get the block and port to line up.

I like guns that run so if it's going to run dirty and cold it's going to be a bit overgassed when clean and warm.

What Burns said.

All other things being eliminated as an obvious problem, the OP doesn't have enough gas.

I've even had a problem with PMC Bronze with a 16" KAC being undergassed in cold weather.

Changing to full power 5.56 NATO spec ammo will work under same conditions.

Ream the gas port to .103" & stop wasting time.

MM

I have an 18" barrel with the .093 and it's gassed perfectly. I would be reluctant to open up the port but a .093 would be perfect for making sure any burrs are gone.

kwg

.093' on 18" barrels usually works fine................but may be marginal with some low powered ammo & cold weather. I've seen it more than once.

It's easier to deal with a little extra gas than to deal with a shortage of gas, either via an AGB or different buffers & springs.

JMHO

MM

It's a rifle length gas system with 2 inches less barrel. I used the drill bit shank to measure the hole. I guess I could of mike'd the shank to confirm it's dimensions.

kwg

Crane mil spec is .105 for the 18 inch rifle gas and that's for the MK12 issued with a suppressor and shooting mil spec 5.56.

Unsuppressed a .105 port/18"/RG will choke in my world on PMC bronze unless very clean and it's warm. Suppressed is fine.

I have run .110port/18"/RG a lot and it works fine both suppressed and unsuppressed.

I think altitude may also have an impact on gassing.

Good stuff John.

Thanks.

Do you have a source for that?

MM

Yes.

Well, I'm too impatient, so, I went ahead and made up a jig so I could hold the front sight securely on the drill press, and worked the hole over in the direction it needed to go so that it easily catches the entirety of the gas port in the barrel. It's oversize now, but only just partway in, there's still the step down in the hole going towards the gas tube, and since it's just a passageway, I figured it can't hurt. Now, after firing a few shots, I have a gas mark on top of the barrel that is fairly concentric around the gas port hole in the barrel. All should be clear now.

But.... still weak. I only fired a few shots, and though they all cycled and functioned, ejection was weak, and the bolt didn't hold open when empty. Granted, this is not super powerful ammo, but it really ought to work with whatever I put in it. The gun shouldn't be that picky. Tomorrow when I have some daylight, I'll run some actual 5.56mm ammo through it and see what difference there is, if any. I expect that I will need to open up the gas port in the barrel, which I can do. It is currently a tight .093". Now, this has a .750" gas block journal diameter. Some of the charts show that for the original 5/8" barrels, the standard gas port would be .093", and for the 3/4" barrels, should be .096". I'll probably end up giving that a try tomorrow, after shooting it some more. We'll see.

BBQ skewers fits down the barrel perfectly to protect the offside while drilling.

Well, post it up, no need to keep it a secret...............I assume they have the correct specs for other length barrels & gas systems.

All I've ever really been able to find is various non-certified info, some of which seems right, some is questionable.

MM

You're not going to make it at .096". Man up & take it to .103 -.105".

MM

I did post it up. Crane spec is .105 for the MK12/SPR, has been for over a decade.

You're welcome to check on Snipers Hide.

You do realize the OPs barrel has 2 more inches of dwell than the MK12, right?

20" rifle gas runs a smaller port than 18" rifle gas.

Yep, sure 'nuff, John. I always attempt to verify info that I can't readily find a source for. But you are on your toes...................LOL.

And yes, I do realize that 20" may be smaller than 18" RG because of the added dwell time. But I also realize that more of the powder has burned in the longer barrel, which lowers pressure, requiring a bigger gas port. More that just one variable.

But, nonetheless, assuming the OP has no other issues that he hasn't found or told us about, he is simply short of gas with that ammo, at whatever temp he's shooting with a .093" port.

And also, as I said earlier, it's easier to deal with a little over-gassing.............compared to being under-gassed, especially if he wants to shoot lower powered ammo.

YMMV

MM

This discussion about increasing the flow to the gas system is interesting to follow along. I come from the other side of the coin. In the Service Rifle competition world, we tend to shoot loads that are on the warmer end of the spectrum (if you consider a 77 SMK at 2800 out of a 20" on the warm end). And we're trying to tone down the gas system. Mostly it's to preserve brass life and extend the life of our moving parts as well making recovery of our brass easier. The cam pin tends to take a beating as does the channel in the upper where the cam pin bears on our rifles.

We employ things like reduced size gas ports, heavier (H2 and H3) buffers or in a previous generation the Tubb CWS. I've always worried about increasing the mass of the reciprocating assembly beyond what the system was designed around. So what I'm left with is reduced gas port or more sane loads.

Considering that the OP is looking for the opposite, I'd suggest the opposite of our solutions; loads closer to milspec first then troubleshooting why it's not functioning properly. Lightening the recip assembly will probably cost you a chunk in parts and opening the gas port would be irreversible and narrow your ammo choices, so they'd be my last choices.

The AR is quite amazing when you consider the wide range of conditions and loads it's expected to operate in. However being an easy to DIY type rifle can make it prone to errors in parts fitment and assembly.

There is no “one way” when dealing with the general public. The MIL got as close as you can get by using a set pressured ammo and making sure it runs in the +120 to -20 degrees.

With folks like me burning as much powder as I can then you have folks using the cheapest (weakest) stuff they can find, it’s hard to set up a gun to run both equally well.

Over gassing one just to appease the guys running cheap ammo won’t work with guys that use good ammo and expect a smoother shooting gun. Can or not.

All true from the last 2 posters, but AGB or adjustable gas keys are made for fixing any over gassed issue allowing a wider range of ammo, assuming the weakest ammo will function at the size of the gas port & a full open AGB

Cold weather will wreak havoc on marginally gassed systems & weak ammo, & I mean temps in the 20's & 30's.

MM