New Uppers - Page 2 - Home of the 6.8 SPC

carnaby · 05-09-2009, 09:09 PM

No I have not tried those SSA loads. I'll see if I can get my hands on some next week. OAL with the 85 gr. TSX and 30 gr of H322 is 2.29", which I picked simply to fit comfortably in my PRI magazines.

Renegade360 · 05-10-2009, 12:08 AM

Great honest Testing. I'd say there would be few people that would be that honest in testing with hiccups occuring. Looking forward to getting the barrel. All sounds good.

carnaby · 05-10-2009, 03:01 AM

Went to the range this morning and here's the scoop: still having short stroking issues with both intermediate and rifle length gas systems. For the intermediate gas, the second round never strips off the magazine except with the 29gn of H335 and a 110 gn bullet. For the rifle length gas, the spent case won't even eject most of the time. I checked and double checked alignment of the gas port this time, and that is not the problem. The rifle length gas is working *better* now that alignment is correct between the gas port and gas block, but it still isn't working.

Does anyone know the gas port diameter used on the 20" Cardinal Armory/Ko-tonics barrels? I don't want to just assume that opening up the gas port will help, although I can't see what else the problem would be. For a recap, I'm currently at a #43 drill, or 0.089" for the intermediate gas port, and #42 or 0.0935 for the rifle length system.

sodapop · 05-10-2009, 03:46 AM

Interesting thread - thanks for sharing the play by play. What would the advantage be with a mid-length gas port on a 20" barrel? Why not just use rifle length as designed?

paulosantos · 05-10-2009, 04:23 AM

Quote:

Originally Posted by sodapop

Interesting thread - thanks for sharing the play by play. What would the advantage be with a mid-length gas port on a 20" barrel? Why not just use rifle length as designed?

The 6.8SPC uses faster powders than the 556, so the Midlength gas system is the best for the 20" and 18" barrels.

carnaby · 05-10-2009, 05:29 AM

That may be true, paulosantos, but my barrels cannot take a mid-length gas system without machining the profile, which I am loathe to do. IIRC, the Cardinal Armory/Ko-tonics 20" barrels had a rifle-length gas port, and weren't short stroking as bad as mine. Do you know the port diameter on the Cardinal 20" barrels?

sodapop · 05-10-2009, 06:54 AM

Quote:

Originally Posted by paulosantos

The 6.8SPC uses faster powders than the 556, so the Midlength gas system is the best for the 20" and 18" barrels.

OK, I think I follow, thanks. Since combustion occurs more quickly, less gas pressure is available at rifle-length gas ports on the 6.8 versus the 5.56?

I don't have a preference one way or the other, but I wonder: why not just use a larger gas port orifice to compensate? I thought one of the points of using a mid-length gas port on a 16" 5.56 was to tap the gas at a "less peaky" point and soften the recoil impulse. Wouldn't you get same "benefit" on a 20" 6.8 by using a larger orifice at rifle length?

Maybe I have it all wrong. It's been a while since I looked at this stuff.

carnaby · 05-10-2009, 07:31 AM

Sodapop, that is my hope, but I don't want to rush into enlarging the port diameter just yet. There may be something I'm overlooking and I need to be patient. I'm already up to a 0.0935" diameter hole for the rifle length system, and 0.0890" for the intermediate length system. The rifle system is working less well, but neither are working acceptabley.

carnaby · 05-10-2009, 07:42 AM

Here's the scoop: a 20" .223 barrel with rifle gas apparently uses a 0.093" diameter gas hole. Given the larger bore diameter of the 6.8, combined with powders that are as fast or faster than used with .223, it seems cleaer that I need a larger gas port for these barrels. I'm going to go to the 0.093" with the intermediate gas system, and 0.0102 for the rifle (#42 and #38 respectively) and see how that goes. I won't get to this for a week, but this coming saturday I'm heading to the range to figure this out.

For now, the 18" barrels are shooting great. I just shipped my first order today. I plan to get a whole bunch more of the 18" orders out in the next two weeks.

sharpshooter33 · 05-10-2009, 05:34 PM

I have a 300 whisper and I do remember something about opening up the gas port up, but there is a limit. The gas tube it's self has only so much of an opening. At a certain point opening the port is useless and also starts to cause leaks. I wish I could remember the thread on AR15.com that talked about opening the gas port. It maybe more of a gas leak off somewhere in the system. You might try sprinkling baby powder or something around the port and block and when you fire if your getting poofs of white smoke from there you can be pretty sure it's leaking somewhere. I had a port leak on the 300 Whisper when I first assembled it because of some type misalignment.

Anyway my$.02
GL
SS33

Oh another after thought. With my Whisper I was short stroking and JD Jones suggested a different powder, quicker...Worked great and no more short strokes.

Have fun!