I need some help on a custom build.  I have a 6x45 in the works with a tentative 24" barrel.  I am trying to figure out the gas port location.  I plan on a adjustable gas block, and the location is up to me.  I am trying to figure out if there is a way to determine the minimum length location where I would not get bolt unlocking and possible accuracy issue with the bullet still in the barrel.  I really want to go carbine length, as my QCB is set up this way with all of my other barrels.  If I go rfle length, then I have to use the QCB handguard adaptor, or have lots of gas tube exposed to possible abuse.  I do understand that I have a lot of variables at work here, with port size, buffer weights, powder choices and such, just trying to get some thoughts.  

Thanks,

Craig

Dano,

That is exactly what I wanted to know.  I thought that with pressure regulation at the gas port I might be able to mitigate that somewhat.  I supose that would just lead to other problems with recoil feeding and such.

Thank you very much, and you are doing a wonderful job at filling those pretty big shoes.

Craig

11.5" barrel = 3.850" from muzzle

14.5/16" barrel = 8.375"  from muzzle

20" barrel = 6.875" from muzzle

Quoted: 11.5" barrel = 3.850" from muzzle 14.5/16" barrel = 8.375"  from muzzle 20" barrel = 6.875" from muzzle

Does that apply for a 6x45mm?

OK, here's what I know about 5.56mm.  Since I don't know anything about 6x45 I won't comment on that, but I can't imagine that it would be much different.

In an ideal world the gas port should be located where the pressure has dropped to about 19,500 psi.  The rifle length gas system uses a gas port of about .090", but can be larger.  A carbine (M4 or thereabouts) uses a gas port of about .062", the reason being that it's taped at a point along the barrel where the gas pressure is still quite high.  If is was drilled to the standard rifle size it would throw the bolt to the rear with such great speed that it would move to the rear too early and cause malfunctions due to the bolt returning too early and at too high a speed to pick up the next round out of the magazine.  Just in case you're interested, the SPR uses a mid-length gas tube with a port in the barrel is somewhere between .080" and .089".

If you'd like to build a custom length gas tube you can tubing that's quite suitable from Wick's Aircraft.  The size that you'll need is 3/16" tubing.  

What I would do is use a rifle length gas tube and drill the port small in size.  You've got to drill it in steps or you'll wind up burrs on the inside of your barrel.  Then test fire it and if it short strokes then go up on drill bit size (using a 115 piece drill bit set).  It'll take some time, but it beats trying to undo what you've already done.  

You might also try a JP gas block that you can adjust.  That's what I'm putting on an SPR that I'm building.  Good gas block and I can adjust it for different loads.

So that's the sum total of my knowledge or at least all that I can think of right now.  I'd love to hear how the 6x45 build goes.  I used to shoot a pistol (Thompon Center Contender) in 6mm TCU in handgun silhouette matches.  Very accurate and the round is very similiar to the 6x45.

Thanks again for everyones help.  I am now probably going to request a rifle length gas system, and adjust my QCB accordingly.  I will also ensure that the gas port is large enough to not need future drilling.  I already had planned on the adjustable gas system, as the upper has to see duty with various barrels, it is much easier to regulate gas on the front end.  This way I am not fiddling with buffers and springs.  I am running the MGI buffer, and it works like a champ for all of my calibers, so far.

As this will be my longer range PD and antelope barrel, anything that might adversely affect accuracy is critical.  I was concerned that bolt activity before the bullet cleared the barrel would be an issue.  My 458 runs great with the carbine port and 20 inch barrel.  It is an easy sub-moa gun.  Perhaps the lower chamber pressure of the 458 allows for bullet clearance before bolt unlocking.

The plan is to run the barrel at 24 inches without a brake, to try and maximize weight, velocity, and the chance to see bullet impact.  I'm gonna compare my ballistics with others who are running 20" barrels, and then decide on the final barrel length/comp/brake configuration.

Thanks again,

Craig