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9/22/2017 12:11:25 AM
Posted: 9/20/2005 6:18:09 PM EDT
[Last Edit: 9/20/2005 6:27:47 PM EDT by archad]
I put together a 6.8SPC upper I picked up the barrel in the EE from Lawman734 see his post(I think I got shanked). It is 18 " WOA barrel rifle length gas system. It will not pickup the next round in the magazine. It is like it is short stroking. I'm using a VLTOR Car Stock with 9mm buffer. I have used the standard buffer same result. I have changed gas blocks etc, changed B/C's, checked the gas keys, The ammo I'm
shooting is 115 OTM w/28.6 H322. It shoots fine in my buddys Model 1 6.8 16" upper. I have yet to try the Rifle length butt stock. I think this combo was designed around the Rifle Length Buttstock??? thanks ARCHAD
Link Posted: 9/20/2005 7:05:43 PM EDT
Idea, using only your upper, drop it on your buddies rifle,useing his Bolt and carrier and his ammo and see if the upper will run. If it does, then start then try one part added back until it chokes (read bolt, then carrier, then ammo, then the whole upper back on your lower.

If the upper doesn't run as a bare unit on his upper, then you know that the problem is totally in the new upper barrel assembly, and not elsewhere. Trust me, this will save us a lot of time since the probelm with be either in the gas system or chamber.
Link Posted: 9/21/2005 6:00:50 AM EDT
Dano523 I have already done that with my upper. Today I'm going to use a A2 length stock....
Link Posted: 9/21/2005 12:18:40 PM EDT

Originally Posted By archad:
Dano523 I have already done that with my upper. Today I'm going to use a A2 length stock....

So is the problem, the chamber, the ammo, the gas block alignment, or the gas port size on the upper?

As for rifle or carbine stock system, it shouldn't matter. Both systems create the same amount of tension against the carrier; just the carbine system has a slightly short stroke (read the carrier still has to make the buffer kiss off the back of the Buffy tube).

And so we are straight, you used your B/C in his rifle to weed out the possibility that the problem is not there?
Link Posted: 9/21/2005 6:37:44 PM EDT
Dano Yes I did. I tried a rifle length A2 tonight and it functions fine.......
Link Posted: 9/21/2005 10:52:21 PM EDT
Well, if you just swapped the receiver extension's alone, then you have narrowed the problem down to the buffer, and the buffer spring.

As for the carbine units, the extra mass off the 9mm buffer, or the buffer spring may have end burs (read end of coils scrapping down the tube) or just be too much tension (read may be too strong like a Extra power spring) until the upper/chamber breaks in.

I would say run the A-2 stock for a few hundred rounds to allow the rifle/upper/chamber to break in, then drop them back on to see if rifle runs carbinized. If your still having problems then, it's time for a standard carbine buffer, and maybe a standard strenght carbine buffer spring s well.
Link Posted: 9/24/2005 1:44:44 PM EDT
From Zack Smith in the 6.8spc FAQ. Will a 6.8 in a 18" w/rifle length gas system work?

My guess is no. I believe Wes & Paul did some experiments with regard to gas port location and size.

I did some studying with QuickLoad:


If I use QuickLoad to plot chamber pressure as function of bullet position in the barrel:

223, 61299 psi max, 3215fps from 18" bbl
approx 27500 psi at 9.5" (midlength port)
approx 20000 psi at 12.5 (rifle port)

223, 55123psi max, 3118fps from 18" bbl
approx 26000 psi at 9.5"
approx 19000 psi at 12.5"

6.8, 55069psi max, 2673 from 18" bbl
approx 19000 psi at 9.5"
approx 14000 psi at 12.5"

using different cartridge model datum
6.8, 55086psi max, 2640fps from 18" bbl
approx 18000 psi at 9.5"
approx 14000 psi at 12.5"

When I take 6.8 down to 26.5gr RL10X, the 9.5" pressure is more like 17500psi.

6.5 Grendel is closer to 223/556.

(This analysis assumes that the pressure is equalized throughout the combustion chamber, ie, pressure at bullet = pressure at breech, which is borne out within about 10% by Armalite's pressure graphs.)

So it looks like the 6.8 midlength port pressure is roughly equivalent to light 223 loads with a rifle-length gas port.

When you download 6.8, the port pressure is WAY less than even a rifle length 223 with moderate loads

Link Posted: 9/24/2005 5:29:13 PM EDT
[Last Edit: 9/24/2005 5:31:27 PM EDT by Dano523]
Archad, really hate to piss on anyones whole truth parade, but where there is a will, there is a way.

What is missing from the above data is the gunpowder being used. In regards to burn rate and port pressure, the slower the powder burns, the father the bullet is going to be down the barrel before the pressure tapers off. Hell, with the right powder, I can put the peak burn pressure at the port (read the full 55,000 plus PSI).

And since you have listed your load, you may want to take a look at the Hodgen charts. Start load for the 115 SIE HPBT bullet is 26 grains of H322 with a max of 28.2 compressed. Grant your bullet may have more surface tension than the listed bullet (read causes higher bore pressure), but still, my guess is that your reloads are under loaded even for a start point.
Note: I have no idea where you got the loading data, so if you want to share with the rest of the group, this may bring some light to the subject.

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