SBR Gas port question

[Hogshooter]

In addition to my 300 BLK SBR I have a 10.5" 5.56 SBR. The carbine gas port presently is at .089". The rifle runs well with the suppressor attached but short strokes and double feeds without the suppressor. I am running a stock carbine spring and H buffer (3.8 oz.). Definitely a under gassed issue as gas key is tight and no leaks around the block or tube. I intend on opening up the gas port a bit. My question is how much should I open the port up at a time and then check for good operation. I have a set of numbered gauge drill bits and could go a couple thousandths with each drilling. If I go too large I may use a AGB but I would rather use the standard low profile block (non adj) ....thanks

[dellet]

When I have enlarged ports I generally go one or two sizes. Either by hand or using a pin vice. If it takes a drill, it's too much change.

.002 does not sound like a lot until you figure the actual surface area or volume change. It takes surprising little increase to make a difference.

The other way is just go big and get an adjustable block.

How easy it is to test fire might make a difference. If you can only get to the range once a month vs being able spend an hour one day drilling five times.

[Hogshooter]

When I have enlarged ports I generally go one or two sizes. Either by hand or using a pin vice. If it takes a drill, it's too much change.

.002 does not sound like a lot until you figure the actual surface area or volume change. It takes surprising little increase to make a difference.

The other way is just go big and get an adjustable block.

How easy it is to test fire might make a difference. If you can only get to the range once a month vs being able spend an hour one day drilling five times.

Thanks Dellet ! I have a range in the back yard so it's no problem test firing after each enlargement. I don't have access to a pin vise but with the numbered drill set the next size up from .089 is .0935 then .0938, .096, .098, .0995 and up a couple of thou. each step. I see a lot of people run at .100 or .110. From what I gather you are saying is to go no more than a couple of thousandths at a time until I get bolt hold open. I would have to start .0045 larger for my first test.

[dellet]

When I have enlarged ports I generally go one or two sizes. Either by hand or using a pin vice. If it takes a drill, it's too much change.

.002 does not sound like a lot until you figure the actual surface area or volume change. It takes surprising little increase to make a difference.

The other way is just go big and get an adjustable block.

How easy it is to test fire might make a difference. If you can only get to the range once a month vs being able spend an hour one day drilling five times.

Thanks Dellet ! I have a range in the back yard so it's no problem test firing after each enlargement. I don't have access to a pin vise but with the numbered drill set the next size up from .089 is .0935 then .0938, .096, .098, .0995 and up a couple of thou. each step. I see a lot of people run at .100 or .110. From what I gather you are saying is to go no more than a couple of thousandths at a time until I get bolt hold open. I would have to start .0045 larger for my first test.

Go with what you have. As far as drilling goes you might have trouble with a melonite treated barrel, but you can probably still drill that with your fingers.

I do have a question tho, Is this a carbine gassed short barrel in 300 Blackout? if so how short is the barrel and are you planning on running subs or just supers?

[Hogshooter]

When I have enlarged ports I generally go one or two sizes. Either by hand or using a pin vice. If it takes a drill, it's too much change.

.002 does not sound like a lot until you figure the actual surface area or volume change. It takes surprising little increase to make a difference.

The other way is just go big and get an adjustable block.

How easy it is to test fire might make a difference. If you can only get to the range once a month vs being able spend an hour one day drilling five times.

Thanks Dellet ! I have a range in the back yard so it's no problem test firing after each enlargement. I don't have access to a pin vise but with the numbered drill set the next size up from .089 is .0935 then .0938, .096, .098, .0995 and up a couple of thou. each step. I see a lot of people run at .100 or .110. From what I gather you are saying is to go no more than a couple of thousandths at a time until I get bolt hold open. I would have to start .0045 larger for my first test.

Go with what you have. As far as drilling goes you might have trouble with a melonite treated barrel, but you can probably still drill that with your fingers.

I do have a question tho, Is this a carbine gassed short barrel in 300 Blackout? if so how short is the barrel and are you planning on running subs or just supers?

No, it is a carbine gassed 5.56 10.5" SBR. My 300 BLK is a 10.5" SBR which has a pistol gas port. I run mainly supers in it unsupressed. Also have a Liberty Leonidas integral that I run subs only. I have a small table top drill press that I plan on using to drill the port.

[dellet]

I doubt you'll need to go to .110, that's more of a max needed for 300 Blackout.

I would not be surprised if it runs at the first try if it's .093+

[bangbangping]

.002 does not sound like a lot until you figure the actual surface area or volume change. It takes surprising little increase to make a difference.

Yup. Area goes up with the square of the difference.

.0935 will give you about 10% increase over .089.
.0938 will give you about 11% increase over .089.
.096 => 16% increase

[Hogshooter]

.002 does not sound like a lot until you figure the actual surface area or volume change. It takes surprising little increase to make a difference.

Yup. Area goes up with the square of the difference.

.0935 will give you about 10% increase over .089.
.0938 will give you about 11% increase over .089.
.096 => 16% increase

What is bothering me is that I hear of many others running smoothly with .070 and similar sized ports. Crane spec is .070 for M855. I am shooting 50 gr. Vmax handloads running at 2700 fps out of that barrel so pressure should be sufficient.

[dellet]

.002 does not sound like a lot until you figure the actual surface area or volume change. It takes surprising little increase to make a difference.

Yup. Area goes up with the square of the difference.

.0935 will give you about 10% increase over .089.
.0938 will give you about 11% increase over .089.
.096 => 16% increase

What is bothering me is that I hear of many others running smoothly with .070 and similar sized ports. Crane spec is .070 for M855. I am shooting 50 gr. Vmax handloads running at 2700 fps out of that barrel so pressure should be sufficient.

I have very little experience with 5.56 SBR's, as in only one that I helped with. The port sizes were more confusing than Blackout to me.

The barrel length did not matter as mush as the distance form port to muzzle and also barrel diameter came into play.

The closer to the port, and the larger diameter the barrel, the larger to port needed to be.

We pulled this chart off the M4 forum, it might help you out some, original source unknown but the same numbers showed up multiple places:



If those numbers are reasonable, it would make sense a 10.5 might need a larger port.

[Hogshooter]

Thanks all, still not there yet. Barrel at gas port is .750. Also I need to find a set of decent bits that are not tapered as I can't drill deep enough without going into the opposite side of barrel. Possibly a small end mill needed. The cheapo bits I have just will not cut worth a damn either.