Picked up a barrel for a pistol build, but I have concerns that it's not within spec.  Looking for the spec on the distance from the gas port to the front of the barrel extension flange, and/r from the shoulder of the gas block on the chamber side to the front of the barrel extension flange, and from the gas block shoulder to the gas port itself.

Putting together a piston-driven pistol and when the piston parts are installed the bolt can't go into battery.  Have to move the gas block froward on teh barrel about 3/16 off the gas block shoulder to get it to go into battery, but at that point I'm fairly certain the gas port and gas block aren't lining up as the gas block is almost certainly too far forward.

Thanks for the info when it comes.

Anyone?

I believe you've answered your own question when you state it will not chamber without the gas block being 3/16" off the shoulder.  There is a problem somewhere in your build...

Brand of barrel? Can you contact the manufacturer and ask them?

I had but had not received a response, until today.  Want to give them a chance to figure it out.  Hate to see anyone go on and flame a manufacturer without giving them a chance at "due process" so I'll see how it works out with them.  

They did give me specs, problem is they gave them to me for a carbine and not the pistol barrel, so just waiting to hear back on that.  I'd still be happy if someone have me something to work with though.

What specs are you looking for? Distance from seat to gas port and distance to what part of chamber side? To the extension, to pin, or to the seat ring or whatever it's called? I can go measure the 7.5 I have laying around to at least give you and idea.

You never said barrel length, is it carbine or pistol length gas? Meaning except for port diameter, carbine length gas specs would be same for any length barrel.

- OS

The  barrel is probably set up for an A2 style front sight and the same hand guard.

I have a low profile gas block on my pistol build and to get the gas port to line up there is a slight gap between the barrel shoulder and the gas block.  Just about the thickness of the hand guard cap.

Yeah it's an 11.5" pistol barrel in 300BO.  Ideally I'd like the front of the extension flange to the shoulder that the gas block is to butt against, and to the gas port itself, if someone is willing to measure one they know to be in spec.

OK so I heard from the manufacturer.  They are telling me specs are 3.660" from the front of the barrel extension flange to the back shoulder of the gas block mounting area, and 4.775" from the breech end of the barrel extension.  My measurements have them within 1/32 of both of those.  That doesn't account for 3/16.  My measurement is also from the front of the flange to the muzzle edge of the gas port is 4" exactly, if someone could get that as well.

I'm still hoping someone can break out a pistol barrel and measure theirs just to verify their numbers match the rest of the industry.  Soooooo, can someone help a brother out?

I can measure tomorrow, my calipers are at work.

Also the post above about the hand guard cap is right on. Some gas blocks are designed to be used without that gap for the cap, some need it.
Measure the distance from the gas block shoulder to the hole, then compare that to the gas block body to the hole.

I see what you are saying there.  Would only account for like 1-2/32" though right?  Isn't it just a piece of stamped sheet metal?  I freefloat everything so I've never ownd an AR with drop ins. I'm needing about 3/16ts to allow the thing to go into battery with the piston installed.  Thanks for any info you can give me though, it is truly appreciated.

OK, so a lack of "milspec" for Ar pistols has bitten me in the @$$.  The barrel I bought is a Franklin Armory piece.  It's a nice barrel.  Problem is it's made to the short side of the pistol-length systems.  For a DI gun this is likely a non-issue and should work just fine.  However, when it's mixed with the Adams Arms piston, it's just too short.

The spec dimensions I got from Franklin's tech folks (they were quite helpful, to their credit) are 3.660" from the front of teh barrel extension flange to the shoulder at the back of the gas block mounting area.  AA mesured several of their barrels they build pistols with and they were between 3.718 and 3.75" at the same measurement.  There's where my 1/8" went.

Soooo, found a Bergara on the EE and the guy measured it at 3.725" for me.  Just waiting for it to arrive.

The Franklin barrel just doesn't play nice with the AA system.  If you have plans to build a piston pistol be sure to get some measurements before committing like I did.  It can save you some heartburn along the way, and time and $$$ as well.

So, anyone need a nice 11.5" barrel with only 17 rounds through it?

with the shoulder being that much closer you might have issues with the gas tube bottoming out in the key with a DI system.  that's a pretty big difference...

You might have to conduct some "fitting" by shortening your piston drive rod.

Just make sure your gas block is in the perfect correct position first.