ARP 6.8SPC build -- short stroking and dinging brass up? Bad magazine?

[thormx538]

I just got back from the range with my newest build -- a 6.8SPC, with a 16" AR Performance scout barrel (midlength) on a carbine lower (standard spring, standard buffer). PSA bolt in a NiB Ares carrier.


I only ran about 15 rounds through it (8 S&B FMJ, the rest are my 110gr VMax/8208XBR loads). Both types of ammo function in my 14.5" PSA 6.8SPC without issue.


I'm getting some serious case mouth dings from the brass deflector -- they're semicircle, so it's from the case mouth. Most case mouths have pretty good dings in them -- usually two of them, about 90deg each and perpendicular to eachother. Ejection is pretty much exactly at 3:00, and only landing about 3' off the bench. All 15 shots failed to chamber the second round. I had one instance where the rifle partially stripped the next round off and jammed up a bit, but every other time the bolt seems like it skimmed right over the top.


I am using ASC mags -- I only have two of them, and both performed the same. I tried an H2 buffer from another lower I had with me, and it behaved exactly the same.


I am confident I aligned the gas block properly, at least as best as possible visually. Is there a better way to check this? Do you think it just needs more rounds to break everything in? I read on another thread somewhere about trimming the spring by a few coils, but I'm hesitant to WECSOG anything right off the bat. Should I try it again on a rifle lower?

 

[xtry51]

If you can smoothly pull back the charging handle and lock the bolt back by hand, its either not enough gas or too much buffer/spring weight.

I'd try an even lighter buffer. It sound like the bolt is barely making it back far enough to eject at all. What stock do you have on? Did you put a rifle spring into a carbine buffer tube?

 

[maleante]

I have an ARP barrel/bolt from 2009. I would say double check your alignment on the port hole/gasblock.

The best way I found to do it is with the port hole at 12', take a brass punch and make a line from the port hole along the top of the barrel at 12' back about 1/2" toward the chamber. Then take your brass punch and make a mark on the dead center of the gasblock going vertical on the side nearest the chamber (when installed). Then align the two.

 

[thormx538]

I have an ARP barrel/bolt from 2009. I would say double check your alignment on the port hole/gasblock.

I think I may have set the gas block too far, based on some replies to my thread on ARF. I'm going to pull the gas block off, measure the distance from shoulder to gas port, make the same measurement on the block itself, and use feeler gauges to set the distance from the shoulder so it's centered perfectly.

What stock do you have on? Did you put a rifle spring into a carbine buffer tube?

Carbine spring/tube/buffer.

 

[thormx538]

Update.


I realigned the gas block using the dimple. I inspected the lockup between the gas tube and key, and everything looks OK visually (doesn't look different than any of my other ARs), but I didn't measure anything. Manually cycled the rifle about 150 times.Took it to the range with some MSR Fusion ammo.


20/20 failure to strip the next round. Does not lock back on empty mag. Had 4 FTE's where the action stopped with the brass about 3/4 out of the chamber. Added soft velcro to the ejection port bump, and that stopped all brass dings.


Had extractor/case head swipe on the 7th, 8th, 9th, and 10th round I fired. That seems odd to me on a rifle exhibiting undergassed symptoms, especially with factory ammo?


There doesn't appear to be much blowby around the gas block, so I didn't do the loctite bedding yet.


I think I'll try another carrier next time to rule that and the gas key out.

 

[quietman]

Check those mags.

There's a DIY mag fix on 68forums.com
Look in Accessories for a Sticky: Six8 Magazine Mod

Drove me nuts until I used a different magazine.

 

[thormx538]

Check those mags.

There's a DIY mag fix on 68forums.com
Look in Accessories for a Sticky: Six8 Magazine Mod

Drove me nuts until I used a different magazine.

Thanks -- I read through that, but I don't think it applies. I'm not loading long (yet) or using PMAGs.

I made it back to the range the other day, brought my other 6.8 along so I could mix/match bolts, carriers, buffers, etc...


First 5 rounds S&B 110gr -- no change


Swapped BCGs -- no change. Other rifle cycled fine with the ARP gun's BCG though.


Popped the BCGs back into their original rifles, fired a round. Success! It locked back on the empty mag.


Ran a few more, cycled fine and locked back on the empty mag.


Overall, I ran about 70rds of S&B through it. I'd say there was an 85% success rate on feeding the next round, and about a 60% success rate on locking back on an empty mag.


Tried 3 MSR Fusions -- continued to short stroke, would not lock on empty.


Tried 3 hand loads (115gr VMAX on top of 8208XBR) -- short stroked, wouldn't lock back.


So overall it seems to be breaking in. Carbon residue around the gas tube/gas block junction continues to increase. I'm tempted to clean it up without removing the gas tube, and put a drop or two of purple Loctite on the seam to sure things up there.


Obligatory pic --

 

[beaker]

I had similar issues with a recent 308 build and it turned out to an out of spec bolt. The extractor pin hole was way oversized allowing the extractor to extend outsode of the bolt body resulting in drag inside of the BCG. The extractor pin would also walk out causing the same problem. It caused the bolt to have difficulty unlocking causing short stroking. I would also check to see if the bolt release is dragging on the BCG which can also cause these issues. Do you have a heavy duty buffer spring or a regular spring?

Sent from my Galaxy S4 using Tapatalk Pro - typos are from the GD auto correct unless they are funny substitutions those I'll take credit for.