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Discussion Starter · #1 ·
First post.

My build includes an Anderson upper and lower, a 18" mid length gas ARP barrel, carbine buffer and spring, AIM full auto style bcg, non adjustable gas block, reduced power JP trigger spring kit with stock Anderson trigger parts.

I have 70 rounds thru the rifle and about 200 hand cycles. I run the gun 'wet' with a mixture of gun oil and a dab or two of grease that forms a thin slurry.

No evidence of gas leakage at the gas block or around the gas key.

The second round from new using Hornady American Gunner 110gr bthp stove piped.

I cleared the jam and the rest of the 50 rounds fired without incident. The Hornady cases eject at 2:30 to 3:30.

But, in twenty rounds of 110gr S&B PTS, about 1 in 4 rounds would stovepipe. The S&B cases eject about 1:30 to 2:30. The S&B has a point of impact about 1/2" lower than the Hornady at 100 yds.

Bolt locks back on empty mag every time.

I have a 5.56 Bushmaster that I can swap uppers on to check function with a different lower if need be.

I have used the search function and found several posts with a similar issue and most replies point to overgassed, but haven't found a follow up where the OP reported back on the fix.

Question/s:

Is the S&B ammo known to exhibit higher gas pressure and should I leave the gas system as is and avoid the S&B ammo? (I plan to use this rifle to hunt with and use handloads. The Hornady and S&B was purchased mainly for break in and brass.)

Is it a good idea to swap the non adjustable gas block with an adjustable to have the flexibility down the road even if a pet hunting load can be found that is 100% reliable?


My intent is to use 95gr Barnes bullets and 120 gr SST for hunting.

Comments appreciated.
 

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When I built my 6.8 everything I shot ejected at 1 to 2 O'clock. I swapped out my non-adjustable gas block with an adjustable one. Problem solved! That being said, I would definitely run a few hundred more rounds thru your gun just to make sure everything is broke-in an running smoothly before I went to the time a expense of swapping out gas blocks. Just my two cents. As a hand-loader an adjustable gas block will allow you to set up your gun to allow you to run many different types of loads with a more "perfect" rejection pattern. Good luck to you, the 6.8 is a fun and wonderful cartridge.

Mort123
 

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Discussion Starter · #3 ·
When I built my 6.8 everything I shot ejected at 1 to 2 O'clock. I swapped out my non-adjustable gas block with an adjustable one. Problem solved! That being said, I would definitely run a few hundred more rounds thru your gun just to make sure everything is broke-in an running smoothly before I went to the time a expense of swapping out gas blocks. Just my two cents. As a hand-loader an adjustable gas block will allow you to set up your gun to allow you to run many different types of loads with a more "perfect" rejection pattern. Good luck to you, the 6.8 is a fun and wonder cartridge.

Mort123
Sounds like a good plan, thanks.
 

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Generally the same for me. My ARP Scout was a little over gassed out of the gate. Functioned fine, but a little overgassed which, when coupled with questionable magazines, was giving me the occasional issue. I haven't had a problem since switching to PRI magazines and an SLR Sentry adjustable gas block.
 

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Two questions. First, what Mags are you running? If your rounds are hanging up in your mag or they are having a hard time stripping from the mag this could cause issues. Two, does your carrier lock back after the last shot? If you are a touch under gassed this can sometimes cause stove piping. Also, how close is your GB located to the shoulder on your barrel?
 

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If the brass is coming out cloudy instead of shiny with the 110 PTS, your chamber pressure is still high when the gas is trying to open the bolt - brass is getting held against the chamber wall while bolt tries to cycle which turns brass cloudy. An adjustable gas block will turn down the gas and delay extraction which can help with an overgassed situation. However, if you are in an over pressure situation, then an adj gas block does not remedy that. There are recent reports of overpressure loads of 110 PTS and blown primers. I'll try and find the link so you can cross-check your lot number.
 

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Discussion Starter · #8 ·
Generally the same for me. My ARP Scout was a little over gassed out of the gate. Functioned fine, but a little overgassed which, when coupled with questionable magazines, was giving me the occasional issue. I haven't had a problem since switching to PRI magazines and an SLR Sentry adjustable gas block.
Thanks for the input.
 

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Discussion Starter · #9 · (Edited)
Two questions. First, what Mags are you running? If your rounds are hanging up in your mag or they are having a hard time stripping from the mag this could cause issues. Two, does your carrier lock back after the last shot? If you are a touch under gassed this can sometimes cause stove piping. Also, how close is your GB located to the shoulder on your barrel?
The carrier locks back on an empty mag every time.

In hand cycling loaded rounds and watching them feed, I did notice a slight hesitation as the rounds were being stripped from the mags. The rounds would load if the slide/carrier release was used, but when letting the carrier down slowly, the edge of the neck would occasionally bump the forward edge of the mag. I found a discussion on 68.com about tweaking the mag lips and followed the advice. The rounds now feed smooth without any hesitation when feeding slowly by hand.

When the stovepipe occurs, the next round has been stripped from the mag and is started into the chamber.
 

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Discussion Starter · #10 ·
If the brass is coming out cloudy instead of shiny with the 110 PTS, your chamber pressure is still high when the gas is trying to open the bolt - brass is getting held against the chamber wall while bolt tries to cycle which turns brass cloudy. An adjustable gas block will turn down the gas and delay extraction which can help with an overgassed situation. However, if you are in an over pressure situation, then an adj gas block does not remedy that. There are recent reports of overpressure loads of 110 PTS and blown primers. I'll try and find the link so you can cross-check your lot number.
Thanks for the link. I had ran across that thread already. None of the 110 PTS rounds show signs of pressure.

The surface of the cases on both the 110 PTS and the 110 hornady are essentially unchanged from new.
 

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Discussion Starter · #11 ·
I shot another 30 rounds today with about the same percentage of stove pipes.

After thinking things thru, I decided that the cost of ammo in trying to find two accurate bullet weight/load combinations that were accurate would easily outweigh the cost of an adjustable gas block. It made more sense to me to replace the gas block with an adjustable and if needed tune the block to cycle reliably when an accurate load was found rather than to try different powder and load combinations that would satisfy reliability and accuracy with the non adjustable block.

I replaced the block this afternoon. Hopefully time will permit a trip to the range this weekend to tune the block with a ladder series of 120SST'S.

Results of the next outing to follow........

Thanks again for all the advice.
 

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Any luck with the adjustable block? I'm running into similar issues and have a PRI mag coming tomorrow so I can weed out the magazines and move on to other possible issues IE the gas block etc.
 

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Between myself and my Sons, we have 4 AR style rifles (2-6.8s); and all are running adjustable gas blocks. Given the various factory loads plus the options for handloading and/or adding a can, I would never own another without using an adjustable block.

IMO, these have become just as beneficial/important as a great barrel, bolt, and carrier!
 

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First post.

My build includes an Anderson upper and lower, a 18" mid length gas ARP barrel, carbine buffer and spring, AIM full auto style bcg, non adjustable gas block, reduced power JP trigger spring kit with stock Anderson trigger parts.

I have 70 rounds thru the rifle and about 200 hand cycles. I run the gun 'wet' with a mixture of gun oil and a dab or two of grease that forms a thin slurry.

No evidence of gas leakage at the gas block or around the gas key.

The second round from new using Hornady American Gunner 110gr bthp stove piped.

I cleared the jam and the rest of the 50 rounds fired without incident. The Hornady cases eject at 2:30 to 3:30.

But, in twenty rounds of 110gr S&B PTS, about 1 in 4 rounds would stovepipe. The S&B cases eject about 1:30 to 2:30. The S&B has a point of impact about 1/2" lower than the Hornady at 100 yds.

Bolt locks back on empty mag every time.

I have a 5.56 Bushmaster that I can swap uppers on to check function with a different lower if need be.

I have used the search function and found several posts with a similar issue and most replies point to overgassed, but haven't found a follow up where the OP reported back on the fix.

Question/s:

Is the S&B ammo known to exhibit higher gas pressure and should I leave the gas system as is and avoid the S&B ammo? (I plan to use this rifle to hunt with and use handloads. The Hornady and S&B was purchased mainly for break in and brass.)

Is it a good idea to swap the non adjustable gas block with an adjustable to have the flexibility down the road even if a pet hunting load can be found that is 100% reliable?

My intent is to use 95gr Barnes bullets and 120 gr SST for hunting.

Comments appreciated.
This is the second problem with S&B ammo in 2 weeks. I wonder if the rim is thicker and hanging up in the bolt...extractor hanging onto the rim? Can you give me a call at the shop when you have the upper half or maybe just the carrier in your hand? 423-353-1107 The best time is 9-4 est
 

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Discussion Starter · #15 ·
This is the second problem with S&B ammo in 2 weeks. I wonder if the rim is thicker and hanging up in the bolt...extractor hanging onto the rim? Can you give me a call at the shop when you have the upper half or maybe just the carrier in your hand? 423-353-1107 The best time is 9-4 est
Consructor,

Tried your number, but it was busy.

To clarify, the BCG is from AIM and not from ARP.

But your thoughts prompted me to investigate the possibility of a rim dimension issue.

Bolt face counterbore dia .422 inch

Hornady rim thickness .043 to .048 inch.

S&B rim thickness .044 to .045 inch

Hornady rim major dia. .417 to .419 inch (twice fired)

S&B rim major dia. .417 to .418 inch (once fired)

Hornady rim minor dia. .353 to .356 inch (twice fired)

S&B rim minor dia. .354 to .355 inch (once fired)

My experience with bolt action rifles is fairly deep, but with AR's is limited to two with the 6.8 being the first one assembled/built by me. The above dimensions do not indicate a problem as far as I can tell. With an empty case rocked into the bolt face under the extractor and against the ejector, there is enough clearance between the extractor and bolt face to manipulate the case and feel some clearance between the rim and the extractor. Visually, I would guess somewhere between .010" to .020" clearance, but haven't taken the time to get proper measuring tools to get the exact number.

When cleaning the rifle there are a few small flakes (no shavings) of brass on the bolt face, but nothing that spells problem to me. It seems that the amount of flakes are becoming less as the round count increases.

I will try to ring you again some time.

A follow up to the adjustable gas block install in a separate post.
 

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Discussion Starter · #16 ·
Overgassed? Update

After installing the adj gas block it took a while to find time to get to the range.

Using the S&B ammo with the adjustment about 2 turns open the ammo stove piped.* The adjustment was closed to about 1/2 turn open and the carrier would not lock back on an empty mag.* Approximately 1/8 turn adjustments were made until the carrier locked open on an empty mag.* At that point some of the empty cases would eject, and some would remain in the reciever on top of the empty mag.* A couple more 1/8 turns and the empty cases ejected to about 3:00 without any stovepipes.* ?

On to the reloads:

AA2200 is not available in any of the local haunts, although the search is continuing.* I have worked up some accurate loads* in 5.56 with Benchmark and H335, and loaded a ladder sequence with 120gr SST using both of those powders.*

All loads were at 2.275" c.o.a.l.

I do not have a chronograph (yet?).*

The intended purpose of the 6.8 is for hogs with likely shots occurring from 50 to 150 yds.

Range day from a benchrest with the handloads: 100 yds, 4-12x50 Crossfire II, 88 degrees, 3-7 mph crosswind, sunny, 18" ARP Barrel.

Results with H335

Charge weight, group size, # of rounds

27.1 gr**** .875"***** 3
27.6 gr**** 1.55"***** 4
28.0 gr**** 2.187"** 4
28.5 gr**** 1.062"** 4

Results with Benchmark

Charge weight, group size, # of rounds

27.2 gr**** 1.562"*** 5
27.6 gr**** 1.625"**** 5
28.0 gr**** 0.900"**** 5
28.4 gr**** --------****** 1* this charge weight was loaded to check for pressure signs
*********************************** (All looked normal)

I am only giving the H335 27.1 gr load a minimal nod since it was only a 3 rnd group and is likely the lowest fps load.* The .900" Benchmark group with 5 rnds at 28.0 gr is the winner in my book so far.

From the results above, 5 rounds have been loaded with 28.0 gr Benchmark @2.275" and 5 rounds @2.295".* Also 5 rounds with 28.4 gr @2.275 and 5 rounds with 28.4 gr @2.295"

Also since there were no signs of high pressure with the loadings of H335 with 28.5 gr @ 2.275", and it appeared the 28.5 gr load was approaching a sweet spot, 5 rounds were loaded with 28.7 gr of H335 @2.285".

I have not had a chance to get to the range and try the latest batch of reloads.

Obviously more velocity is better for hunting, but it seems there is not much data available for Benchmark in 120gr SST, and only sketchy data for H335 under the 120SST bullets. Caution is the guiding influence with charge weights.

Any input regarding 120SST bullets and the above powders is most welcome.
 

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How is your ejector? Good tension, no dragging?
 

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Sounds better?

When you think something may be out of whack pull the BCG out and hold it straight up. Slide the rim under the extractor and straighten up the cartridge depressing the ejector. Slowly let the cartridge lean from the ejector pushing it. When the ejector stops pushing let go of the cartridge. Does it fall out? It should. If it hangs up in the bolt face it may be sharp corners on the extractor or a few other things but it gives you an area to check out.
 

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Discussion Starter · #20 ·
Sounds better?

When you think something may be out of whack pull the BCG out and hold it straight up. Slide the rim under the extractor and straighten up the cartridge depressing the ejector. Slowly let the cartridge lean from the ejector pushing it. When the ejector stops pushing let go of the cartridge. Does it fall out? It should. If it hangs up in the bolt face it may be sharp corners on the extractor or a few other things but it gives you an area to check out.
I will have to check, but from the handling done earlier, I am pretty sure that it will fall out on its own.
 
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