6.8 mm testing

[DocGKR]

http://www.lightfighter.net/forum/ba...le-performance

http://pistol-forum.com/forumdisplay.php?19-Ammunition

[Marcus L.]

Doc,

I noticed that the twist rate you used was 1/11. What is the ideal twist rate for the most common 6.8 SPC loads? I'm thinking of getting a XCR conversion and it comes with a 1/10 twist rate.

[DocGKR]

Per: https://www.m4carbine.net/showthread.php?t=19884

"Just like making sure your AR15 has a true 5.56 mm chamber and proper barrel twist, it is also critical in 6.8 mm to use the original AMU/"Murray" (also known as SPCII) chamber, as well as the superior 3 or 5 groove 1/11 or 1/12 barrel twists--the Remington SAAMI spec chamber is too tight (like a .223 chamber) and the 1/10 6 groove barrels needlessly increase pressures and reduce velocity."

However, with extremely short barrels, a faster twist can be necessary to ensure sufficient rotational velocity for normal upset of some projectile types.

[SHIVAN]

Doc: Have you shot the 85gr Barnes TSX in gel yet?

[Marcus L.]

Per: https://www.m4carbine.net/showthread.php?t=19884

Thanks Doc. I noticed that you mentioned "3 or 5" grooves as being superior......what's wrong with 4 grooves?

Here's how RA describes their XCR 6.8 barrels:
"We chambered our 6.8 barrels to the specifications recommended by Silver State Arms which produces 6.8 ammunition. It is the latest chamber design and can handle the faster loads. We use a 4 groove barrel with a 1/10 twist rate."

What problems do you see with this combo of 4 grooves in a 1/10 twist rate.....if any?

[worldskipper]

Marcus you need to head over to 68 forums and start reading over there, those guys have the 68 down to a science.

Shivian if you look at the recommended loads it's first on the list.

Doc a couple of questions:
Did you run these loads out of your cardnial upper? I assume you didn't get one with a bad chrome job?
The TNT looks a little slow, Shouldn't that round be stepping out at 28-2900 fps? Or did you y'all keep the velocity down to insure it stayed together?

Thanks,
Worldskipper

[DocGKR]

http://www.lightfighter.net/forum/ba...le-performance

http://pistol-forum.com/forumdisplay.php?19-Ammunition

[Marcus L.]

Thanks Doc, very insightful. However, it seems that a lot of manufacturers are using the 1/10 twist rate like Robinson Arms, LWRC, Bushmaster, LMT, and Noveske just to name a few. I read through the PDF, and I honestly don't see a measurable difference in peak pressure or velocity between the different twist rates and rifling grooves. I understand the the concepts of overspin and following trajectory, but I don't see it really demonstrated in the PDF. With so many high end companies using the 1/10 twist rate.......is there really that noticable of a problem here? Perhaps I'm not catching on, but I really don't see enough of a difference to be concerned.

[DRT]

The 6.8mm impresses in that the OTM loads still do well through glass - a noticable advantage over 5.56mm.

[SHIVAN]

Shivan, you mean like this:

What length barrel? Distance between muzzle and gel??