How do you know the gas port size -vs- barrel dia.?

[Cannonman]

Dear smart people. I'm looking at gas blocks and I can't seem to find where they state the port size. I've got a barrel that's .875. I see this size block but other than an adjustable block, how do I know how to pick different sizes?

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[bfoosh006]

The gas block will have an oversized gas port ( helps ease port alignment. ) The various sizes... .625, .750, .875, and .936 (?) are the journal size on the barrel.
The barrel manufacturer will dictate the gas port size... and manufacturers gas port sizes can be all over the place.... Literally, from mild to wild.

Barrel manufacturers rarely share gas port sizes... so look for, folks like us sharing their info.

 

[Lennie]

Do a Google search using this phase

"guide for ar barrel gas port size"

As a rule of thumb, the gas hole in a barrel is very small for pistol length gas tubes and increases in size diameter from there as gas tubes increase in length IE a little bigger for carbine, bigger yet for mid length and still larger rifle length gas systems

bfoosh006 is correct about the hole cut for gas in the gas blocks

 

[titsonritz]

I've got a barrel that's .875.

How do you know that?

 

[titsonritz]

...

 

[Cannonman]

How do you know that?

BSF told me that's what it was. Thx. And your other info was a great read. I've now come to find that the only way to make a change is with an adjustable block. I think I'm going to start trying to make my own. The one I bought seems simple enough to copy. I've got 5-7 0.750's and my own mill and lathe. Why not?

 

[DirectDrive]

BSF told me that's what it was. Thx. And your other info was a great read. I've now come to find that the only way to make a change is with an adjustable block. I think I'm going to start trying to make my own. The one I bought seems simple enough to copy. I've got 5-7 0.750's and my own mill and lathe. Why not?

Adding an adjustable gas block to an AR means adding an additional maintenance item to your AR.
Better to buffer it and spring it correctly.

Stay far away from aluminum gas blocks.
They are junk.
If your barrel and your gas block are of similar materials, they will expand and contract at similar rates, thus maintaining a good gas seal.
Aluminum violates that rule and it is less resistant to erosion.

 

[eldbillbo]

How many Ar15 guys does it take to answer a gas port size question?

Four, one to answer it, one to question it, one to reaffirm it with more details, and one to make sure it goes to 11.

 

[Member 34319]

 

[eldbillbo]

Make that 5 one more to show the tools to get it right.

 

[Cannonman]

View attachment 845927

I'm a pretty lousy machinist but this is the wrong tool. Also, for what it's worth, for the size and difficulty of measuring a port, digital calipers of this grade are pretty notoriously inaccurate. I like Starrett and enjoy numerous measuring tools of theirs.

 

[DirectDrive]

I'm a pretty lousy machinist but this is the wrong tool. Also, for what it's worth, for the size and difficulty of measuring a port, digital calipers of this grade are pretty notoriously inaccurate. I like Starrett and numerous measuring tools from them.

Just use a drill bit shank and then caliper that.
done

Imma Kitchen Table Gun Plumber and can certify that the above method works.
Local 556

 

[Cannonman]

Adding an adjustable gas block to an AR means adding an additional maintenance item to your AR.
Better to buffer it and spring it correctly.

Stay far away from aluminum gas blocks.
They are junk.
If your barrel and your gas block are of similar materials, they will expand and contract at similar rates, thus maintaining a good gas seal.
Aluminum violates that rule and it is less resistant to erosion.

Nice! Expansion - contraction coefficients within metallurgy. Good stuff. Your the kind of guy that makes coming here a rewarding experience. I know what your talking about and just had not considered it until your post. Of course steel, with all of the variants out there, will do the same but nothing like dissimilar metals. Thanks for the heads up.

 

[DirectDrive]

Nice! Expansion - contraction coefficients within metallurgy. Good stuff. Your the kind of guy that makes coming here a rewarding experience. I know what your talking about and just had not considered it until your post. Of course steel, with all of the variants out there, will do the same but nothing like dissimilar metals. Thanks for the heads up.

Here's a company that always gets high marks over on AR15.com for being able to take on unusual AR work like gas port work. Pricing is reasonable.

Barrel Modification / Repair | Product categories | Adco Firearms LLC

adcofirearms.com

 

[Member 5605]

Calipers work.

 

[DirectDrive]

Calipers work.

If you were to ask a real machinist how he would measure a small bore, the answer would be "pins".
I've seen the set of precision-ground measuring pins that they use.

Myself, a lowly self-taught Gun Plumber, use the back (shank) of the best-fitting drill bit that I can find and then put the calipers on that. (ghetto pin method)
Trying to make the calipers directly work in a small bore would not be my cup o' tea for me, even at my Gun Plumber level.

 

[Member 5605]

If you were to ask a real machinist how he would measure a small bore, the answer would be "pins".
I've seen the set of precision-ground measuring pins that they use.

Myself, a lowly self-taught Gun Plumber, use the back (shank) of the best-fitting drill bit that I can find and then put the calipers on that. (ghetto pin method)
Trying to make the calipers directly work in a small bore would not be my cup o' tea for me, even at my Gun Plumber level.

More of a reference. If you measure .124" and you know standard sizes are .110", .125", .135" etc you can help narrow down. Not a fix all because not everyone uses proper and good calipers.

 

[powermad]

Myself, a lowly self-taught Gun Plumber, use the back (shank) of the best-fitting drill bit that I can find and then put the calipers on that. (ghetto pin method)

That's what I did with mine when I dimpled the barrel for the GB.
Being a lowly PSA barrel I grabbed a .250" bit at first after hearing how massive the port would be and it was waaay to big.
Came in at .067", not the most accurate way to measure but I'm not plotting the trajectory of an asteroid either so a +/- of .001" is acceptable to me for a quick measurement.
I do have .25" - 6" precision bore gauges but just grab a drill bit and measure that for the rare occasion I need something smaller than .25"

I didn't bother seeing what the block was when I had it off but it was quite a bit bigger and centered square over the port.
One small step more and it would have been a perfect installation.

I'm using a Vltor A5 action with an A5H2 buffer and it cycles much smoother than the carbine action with standard buffer did.
I'm using an A5H4 buffer in my AR9 that I'm going to pop in next time I'm out and see how it runs with that.

 

[Cannonman]

If you were to ask a real machinist how he would measure a small bore, the answer would be "pins".
I've seen the set of precision-ground measuring pins that they use.

Myself, a lowly self-taught Gun Plumber, use the back (shank) of the best-fitting drill bit that I can find and then put the calipers on that. (ghetto pin method)
Trying to make the calipers directly work in a small bore would not be my cup o' tea for me, even at my Gun Plumber level.

DD, in my humble and ignorant opinion, is spot on. When I was being taught how to thread a barrel I was told that often the bore is not the center of the barrel and to thread that would cause bad stuff to happen. Hence the pin gauges. These are a set of round little, over priced, little guys that you would insert into the barrel until you find the one that fits snug. Then you center for threading to the gauge. This mandated that the device you intend to put onto the barrel will have a co-witnessed alignment. He is also spot on about the use of drill bits. They do the same thing, as well as drill holes, for a heck of a lot less $$$. So why my post? I have found that many times what is, how it is and how it's done, is not the same as how a "student" machinist would. I was expecting something like, The XYZ of the ABC is the way to know the Dia. of a barrel. More like Djital's vernacular in his response. "gun talk" if you will. You both are awesome resources. Thanks guys!

 

[titsonritz]

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