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Old May 19, 2020, 10:34 PM   #1
rickyrick
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6.8spc short stroking???

Ok, I should have checked this a long time ago, but I’ve had a 6.8 upper in the safe for a few years now. I’m not sure how I over looked this for so long. So today I finally mated it to my reliable rifle lower and took it out out and fired it.

It’s a green mountain, 16” carbine gassed, government profile. I believe the bolt and carrier is YHM.

To my surprise the upper was short stroking on a variety of factory ammunition.

(I forgot the types already, but some was nosler and hornaday and something in a government looking white box.)

Some of it ejected the cases and failed to strip a new round- last round bolt locked back on the carrier and not the bolt face.

The nosler would just cycle back a little and put the spent cartridge back in.

The gas port looks tiny compared to a 5.56 carbine. To confirm this I had a brass rod of unknown diameter that would fit in the gas port of a spare 5.56 carbine barrel that I had laying around. It doesn’t fit in the 6.8 gas port. I am sure that the sizes would logically be of a different size. I’m not a gunsmith, so I don’t know if the hole is the right size or not.

The gas block was properly aligned.
Nothing mechanical was noted, manually cycled with ease.
Gas key is ok.
Cases extracted easily.
I didn’t see any signs of gas leaking, but it could be. It’s a snug fit on the gas journal.
Could this upper just need to be used on a carbine lower?
Could this be due to using the rifle lower?
Should I try it it on a carbine lower?
I’m surprised that it wasn’t over gassed, I was expecting it to be beforehand.
When I initially put this together, I had considered putting an adjustable gas block on this but changed my mind, so I’m glad I didn’t. I probably would have blamed that, lol.
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Old May 20, 2020, 12:08 AM   #2
stagpanther
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My guess is that you're probably right, small port is being used to alleviate big hit in pressure. Personally, I wouldn't even bother with a carbine gassed 6.8 spc barrel these days.

If you REALLLY want an expert's opinion, my advice is go over to the 6.8 spc forum and pose this question to "constructor" (aka Harrison of ARP fame); he is a technical expert on the 6.8 spc and it's wildcats (and just so happens to make great 6.8 barrels). Again, just a guess, but probably a simple widening of the port and using an adjustable block might be the prescription.
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Old May 20, 2020, 12:35 AM   #3
rickyrick
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Well at the time, that was the only 6.8 barrel I could find, plus had all the other parts laying around except for the bolt.
It was touted as a direct m4 replacement barrel, so maybe it’s “tuned” for a standard carbine buffer and tube, but I could probably try it on a carbine lower.
Purchased so long ago, lol. I’m not sure how much more a rifle buffer is compared to the carbine. I guess I could try in a few days, just hate carbine stocks.
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Old May 20, 2020, 01:18 AM   #4
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My guess is a little widening of the port will do the trick, though I suspect it will be hard on your brass over time.
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Old May 20, 2020, 02:05 PM   #5
ed308
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Good advice from Stag. I'd add if a new build, make sure its broken in. Lube the bolt well and manually cycle the bolt couple hundred times to speed up break in.

Direct from AR15 Performances site:

Short stroke? Align gas block/ port location and the dimple.
Most gas blocks have a hole .298-.300" from the rear of the gas block. That matches the port in our barrels +- allowing for a little manufacturing tolerance of holes in gas blocks. If you use a handguard with a cap the cap forces the gas block forward another .033" unless you use a FSB with a hole at .265" from the rear. The rear set screw in the gas block is where the gas port in the top of the gas block is drilled. To align the holes place the rear set screw in the dimple on the bottom of the barrel. Remove the rear set screw, align the hole in the dimple and tighten the front set screw then replace the rear. http://68forums.com/forums/showthrea...EMS-build-tips ***FYI- The hole in the top of most gas blocks is .1495" If the port size in the barrel is .082 you can slide the gas block as much as .034" toward the shoulder on the barrel to close the gap if need be. ARs cycle from pressure created by the ammo. Different ammo and different loads create different amounts of pressure at the gas port which cause the carrier to move at different speeds. If everyone used the same ammo porting a barrel would be easy. We try to port the barrels with the combination most people use(carbine stock, standard springs and buffers with Hornady, S&B and Federal xm68 ammo). Any ammo or handload that produces 4000psi less or using heavy buffers and springs may cause the rifle to not cycle or strip cartridges from the mag. Remington and Federal Eagle ammo and many starting book loads do not produce as much pressure as Hornady, S&B or Fed XM68 ammo.
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Old May 20, 2020, 03:43 PM   #6
brasscollector
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Quote:
If you use a handguard with a cap the cap forces the gas block forward another .033" unless you use a FSB with a hole at .265" from the rear.
This would also apply if the barrel was made for using an M4 style HG and you went free float. I've had a similar issue on a LR308 barrel that was made for an M4 style HG and I installed it into a FF. I had to space the gas block forward of the barrels shoulder to fully align the ports. It went from short stroking to way too much and I had to get an adj gas block...
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Old May 20, 2020, 04:05 PM   #7
rickyrick
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The port is .073.
I now have the gas block off, and will realign, but I think it was ok.

It’s an old build, just got pushed to the back of the safe and forgotten until now.
I’m using a 6.8 specific mag with it.

I have a drill press and a good x-y vise, but I’m not ready to drill it out yet, as it is a chrome lined barrel. It is a free float guard over an M4-cut barrel. I’m aware of the spacing for the M4 handguard bracket thing, I’ve done 5.56 barrels the same way.
I’ll recheck everything and try with a carbine lower. I’m absorbing all suggestions though.
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Old May 20, 2020, 04:25 PM   #8
bfoosh006
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Don't drill the gas port yet....

How many rounds through the upper ? Is it broke in yet, is where I am going.

And it could be that the rifle lower, with its 5.4oz'ish buffer weight.. is to much ( especially if the gas port is small ) , and it hasn't had a lot of rounds through it yet.

Let us know if your carbine lower ( and typically 3.0oz'ish ) carbine buffer weight works.

Also double check gas rings and the gas key... it sure does sound like a gas issue.
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Old May 20, 2020, 04:25 PM   #9
stagpanther
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Or just save time and buy one of ARP's, probably one of the best 6.8 spc barrels on the planet and very inexpensive for what you get.

How does your spent brass look? does it show signs of being "torn" from the chamber before it has had time to relax? Your gas port will almost certainly show a "carbon plume" if it wasn't precisely aligned with the block (mine are rarely absolutely "perfect"--but every barrel I've had has always been overgassed to some degree).
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Old May 20, 2020, 05:19 PM   #10
rickyrick
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The gas port is in the center of the circle of carbon around it. I have a steel gas block, I’m installing it in place of the aluminum block I had used. It is adjustable, but I have set it to full open.
I also have a complete spare bolt and carrier.

I won’t put too much effort into this, I will probably take your advice on buying a different barrel since they are more readily available since the time I purchased this one.


No abnormal marks on the brass. No marks on the deflector.
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Old May 20, 2020, 09:24 PM   #11
stagpanther
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I only asked because sometimes overgassed barrels can have similar "symptoms" to under gassed ones. Another way I've seen carriers lose pressure is when there is possibly something wrong with the gas tube. Anyway you're probably better off ditching it and getting an ARP--Harrison's design squeezes a bit more velocity out of the cartridge than most--and is very accurate for a 6.8.
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Old May 21, 2020, 06:57 AM   #12
wild willy
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.073 is small for a 6.8 SPC .I had the exact same problem.After asking at .68 forums I drilled mine out with a #44 bit .086 using a AGB mines working fine now.Easy to do just put a wood dowel or cleaning rod down the barrel.If your are thinking about changing barrels its worth a try.
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Old May 21, 2020, 09:11 AM   #13
stagpanther
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Quote:
.073 is small for a 6.8 SPC .I had the exact same problem.After asking at .68 forums I drilled mine out with a #44 bit .086 using a AGB mines working fine now.Easy to do just put a wood dowel or cleaning rod down the barrel.If your are thinking about changing barrels its worth a try.
excellent suggestion. What rpm and bit type did you use--and did you use an accurate drill press?
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Old May 21, 2020, 12:45 PM   #14
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Quote:
Originally Posted by wild willy
.073 is small for a 6.8 SPC .I had the exact same problem.After asking at .68 forums I drilled mine out with a #44 bit .086 using a AGB mines working fine now.Easy to do just put a wood dowel or cleaning rod down the barrel.If your are thinking about changing barrels its worth a try.
.082-.086 is just about right for a midlength gas system. He has a carbine gas system, and will require a smaller port. If everything checks out and is assembled right, I'd open it up one size at a time. It might need a .075 or .078 port

49 0.0730
48 0.0760
5/64 0.0781
47 0.0785
2mm 0.0787
46 0.0810
45 0.0820
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Old May 21, 2020, 01:44 PM   #15
rickyrick
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I’ve carefully and meticulously aligned a steel gas block, and took off the aluminum one. I am pretty sure the first was aligned... but I realize that I am probably not the first to think it was. lol.
I’ll give it one more try with the rifle lower and a carbine lower, if it still has issues, I’ll open up the port a little.
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Old May 21, 2020, 06:59 PM   #16
CarJunkieLS1
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If it's a melonite barrel you'll need to give the chamber a really good scrubbing to get the chamber really clean before shooting it. My melonited ARP barrel had the same short stroking issues you have, after a really good chamber scrubbing the rifle works perfectly and I've made no changes to the buffer/gas system.
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Old May 22, 2020, 05:57 AM   #17
wild willy
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Quote:
Originally Posted by stagpanther View Post
excellent suggestion. What rpm and bit type did you use--and did you use an accurate drill press?
My gun is a midlength a 5/64 bit would fit.I used a drill press and barrel block to line up the 5/64 bit then changed to #44 to drill.I was a little leary of doing it asked a lot of questions at 68forums.Theres realy nothing to it.Like like bedlamite said maybe try a little smaller with a carbine.Before I did it I tried all the other suggestion given to me at 68forums.
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Old May 22, 2020, 07:40 AM   #18
stagpanther
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Quote:
My gun is a midlength a 5/64 bit would fit.I used a drill press and barrel block to line up the 5/64 bit then changed to #44 to drill.I was a little leary of doing it asked a lot of questions at 68forums.Theres realy nothing to it.Like like bedlamite said maybe try a little smaller with a carbine.Before I did it I tried all the other suggestion given to me at 68forums.
Thanks for that. I was wondering if there was a certain speed that reduced the likelihood of internal flashing when the bit comes through.
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Old May 22, 2020, 08:48 AM   #19
rickyrick
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It’s a Green Mountain Barrel Company and it’s chrome lined and phosphated. Never used them before. Seems like a decent enough barrel, I had bought it because the barrels had good reviews and the company isn’t a one-trick-pony like some of the other companies; they make barrels for other firearms as well.
Anyway it’s new, but been in the back of the safe for at least two years, maybe three.

I kinda like this situation, I will use it as a learning experience because I haven’t had anything that I’ve assembled with problems before.
Short-stroking was the last thing I expected with this assembly due to the carbine length gas system on a 16” barrel. I expected it to be over-gassed if anything.

I believe based on comments here and reading other forums that the gas port is near minimal size for the intended application and may be undersized for the rifle lower in which I attempted to use it. I’m betting that this barrel was intended for carbine use and the manufacturer sized the gas port appropriately.
I would also add that I am not ruling out misalignment of the gas block, as that is really the only other possibility.
I should have tested this long ago, lol. But life gets in the way sometimes and I feel fortunate to be in a position that I could forget about an upper for so long.
Hopefully I can test this out in a short time. Started a new job this week, so schedule is unknown at this point.
If it comes to needing to drill it out, I will go one step at a time, I don’t have much to loose.
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Old May 22, 2020, 11:23 AM   #20
stagpanther
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I have a couple of barrels made from green mountain blanks and they shoot very well, they make good stuff in general though I can't speak for your barrel. my guess is a little port widening as suggested will do the trick, even if you get a bit too much pressure you can use the "tricks of the trade" to throttle it back.
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