6.8 mm testing

[DocGKR]

http://www.lightfighter.net/forum/ballistics-and-projectile-performance

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

http://img.tapatalk.com/d/13/07/07/ju3u5ybu.jpg

[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/ballistics-and-projectile-performance

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

http://img.tapatalk.com/d/13/07/07/ju3u5ybu.jpg

[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:

http://i459.photobucket.com/albums/qq319/DocGKR/85grTSX10ft.jpg


What length barrel? Distance between muzzle and gel??

[DocGKR]

As per the original post: http://www.ar15.com/forums/topic.html?b=3&f=121&t=408438&page=2


16" 1/10 barrel, 3099 fps MV, block at 10 feet. FWIW, the results are nearly identical when tested at 100 yards...

[SHIVAN]

That's pretty impressive. I will be using it for hunting actual game animals, but that's pretty impressive if used for hunting other things too.

[Zhukov]

Doctor Roberts: Is the testing you've done on these rounds reflected in your overall 6.8SPC ammo recommendations in this post: https://www.m4carbine.net/showthread.php?t=19884 ?

[ccoker]

I have shot 3 deer and 2 hogs with my 6.8, 1 / 11 twist, DMR chamber, 18" stainless barrel from AR15Performance.com

30g of H322
Barnes tipped TSX 110g
2.30 COL
1/2" groups at 100 yards

in all cases near instant terminal results with similar wounds

this deer was shot just in front of the shoulder quartering towards me at about 125 yards, caliber sized entrance wound, fist sized cavity and quarter sized exit wound behind far off shoulder

exit wound
http://img233.imageshack.us/img233/5636/exitwq4.jpg

entrance wound (peeled back hide)
http://img389.imageshack.us/img389/1308/exitxr0.jpg

[DocGKR]

Zhukov--Yes.