July 23, 2025

Adventures with a 6.8 Rifle

My rifle works, but acts weird. Something is not right with the gas system.

It uses a rifle length gas tube and has the usual rifle buffer. It seems undergassed.

I just pulled the gas block and used a set of pin gauges to check the size of the gas hole. It is 0.106, which is as big as it should be. I had contemplated drilling it bigger if it was undersized, but that will not be necessary.

When I tried the 0.106 pin in the hole, I had to tap gently and probably knocked off a burr. This may have been the problem all along. The hole in the gas block is about 0.130

The barrel is a Kotonics 20 inch, chrome lined, with a twist rate of 1 in 11.
As far as I can have been able to learn, with the newer 6.8 SPC II chamber.

The difference between the original chamber and the II is freebore. The original has 0.064 and the II has 0.105. Yes, the difference is 0.041 inches of freebore.

Ejection pattern

This is an indirect indication of bolt velocity. A slow bolt will give time for the ejector to throw out the case before the bolt reaches the case deflector. A fast bolt will slam the case into the case deflector, bouncing it forward.

An undergassed situation yields what is called "short stroking". Such a gun may have trouble locking back on empty. This may be due to gas leaks, too heavy of a buffer, too heavy of a BCG, even just a dirty system that needs some oil.

Somewhat related random facts

Rifle gas tube:  15.25 inches
Mid gas tube:  11.75 inches
Carbine gas tube:  9.75 inches