New 6.8spc build and some primer issues at the range

[RetardedTiming]

Planning to go hog hunting for the first time so in anticipation I just finished (re)building my 68 with all new components as listed.

Mega megalithic keymod upper
Mega ambi lower
Proof carbon 16" 6.8spc barrel
USO MR-10 1.8-10x37
AAdmount 20MOA
JP offset BUIS/RMR mounts
Troy BUIS
Trijicon RMR09 1 MOA
JP LMOS BCG with titanium firing pin
JP Silent Captured spring (lightest spring)
Noveske/Radian CH
Seekins adjustable GB
Timney 3lb skeletonized drop-in
Elftmann speed safety
KNS anti rotation pins
Strike Industries bolt catch
Strike Industries dust cover
Strike Industries LINK rail covers
PRI/MSTN Quiet brake
Two tone UBR Gen 1
FAB pistol grip
PRI 10-rd mag with Magpul ranger plate
Fenix TK30 LEP laser flashlight
AR mag lock/king pin (CA compliance)


Took it to the range yesterday and after getting the gas adjusted I wanted to zero at 100yds and log different factory ammo. Initial gas adjustments were done with S&B 110gr hunting ammo I bought years ago as "water damaged" packaging bulk bag. Which seems to be the only ammo I used that had two primers blow out. Inexperience and excessive smoke didn't clue me into (which I learned from a quick search) the primers blowing out until I noticed one of the primers luckily falling out of the receiver on a FTF. So I stopped using that ammo, I was about 60 rounds into it. I then wanted to see if it would happen with the Barrett and Hornady ammo, which it didn't after 100 rounds of each. What I did notice was it looks like the ejector is scraping headstamp on most (but not all) the brass regardless of ammo MFR. Is this excessive or normal? Sometimes it seems more excessive than other times. I haven't checked headspace because I used Proof's recommended (JP) bolt. But I realize that shouldn't be assumed it will be good and should have it checked.

Another thing I noticed (that might be a clue for the more experienced) is that the Barrett ammo would "stick" in the chamber when chambering a magazine of 10 rounds at home prior to the range trip. It took a pretty significant two-handed pull of the CH to remove the round. The S&B and Hornady ammo didn't do this.

Are there specific areas on the bolt or receiver I should be looking for damage from the primer blow out? Planning to strip the rifle down completely, I just don't want to miss anything.

Another glitch is that the bolt catch wasn't holding the bolt open indefinitely. It will eventually slip and go into battery all on it's own. Kinda freaky if you're not expecting it. Not sure what to look for to fix this issue.

 

[Tanker10]

I would try the 85lb spring and even add a tungsten weight to the SCS. I run the 85lb spring with two tungsten weights on the SCS on my 6.8 setups. Thousands of rounds without any issues.

 

[RetardedTiming]

Thanks for the suggestion. I'm trying to understand what that change is going to accomplish. Are you suggesting that the softer spring and lighter weight could cause the primer to blow out?

 

[supercorndogs]

Put some sharpie on a bullet and chamber it. Look for marks where the bullet engaged the rifling. It kind of sounds like the OAL is too long, and the ogive is jammed into the lands. This will spike pressure.

 

[RetardedTiming]

Thanks, I will try that and report back.

BTW I forgot to mention bullet drop. I zero'd with the S&B 110gr plastic tip ammo. When I switched to the Barrett 115gr custom comp and Hornady 110gr BTHP ammo POI was 1.5-2.0" lower at 100 yards. In my mind that means higher pressures with the S&B? Hard to know for sure without a chrono I guess.

 

[supercorndogs]

The primers falling out is pretty much proof the pressure is high.

 

[RetardedTiming]

I'm assuming can only be the effect of the cartridge, correct? Nothing else, rifle related can increase the pressure?

 

[RetardedTiming]

Put some sharpie on a bullet and chamber it. Look for marks where the bullet engaged the rifling. It kind of sounds like the OAL is too long, and the ogive is jammed into the lands. This will spike pressure.

Faint mark on 2 of the 3 pictured. I loaded 10, these were the only ones with marks on the bullet.

 

[blackblue]

Looks like a short throat to me. Not sure what chamber thrybuse but it would be worth having it checked and reamed to SPC II or just return it and get a ARP.

 

[supercorndogs]

How do you know its s short throat and not long ammo? We seem to have two kinds of ammo that work fine. I think I would try seating the bullets .02 deeper on a few and shoot them.

 

[RetardedTiming]

Looks like a short throat to me. Not sure what chamber thrybuse but it would be worth having it checked and reamed to SPC II or just return it and get a ARP.

I will contact Proof to find out and have it checked.

How do you know its s short throat and not long ammo? We seem to have two kinds of ammo that work fine. I think I would try seating the bullets .02 deeper on a few and shoot them.

Given the random nature of the two that had failed primers. What would I look for to determine if it made a difference? Would the POI indeed drop if there was high pressure due to ammo being too long?

 

[supercorndogs]

Your POI at 100y is not really connected to velocity. You would look for pressure signs on the brass to go away. Also a chronograph would show a drop in velocity. You clearly have some jammed into the lands. ITs possible either the the throat is short or the ammo is long. You have some swipes and pressure signs on the Hornady brass too, looking at the picture. It may be more likely a short throat than long ammo, if you are getting pressure signs with all ammo. I don't know how bad the SPC is at beating up brass in an AR. A lot of grendaal's beat up brass with slight ejector and extractor marks.

 

[RetardedTiming]

Thanks for that explanation. I'll pull the barrel and have it checked out and reamed to SPC II if necessary unless Proof swaps a new one. Need to do some more reading on high pressure signs with brass.

 

[hlee]

Semi autos can be hard on brass. Some can be really hard. Violent unlocking can cause “pressure signs” in cartridges that are perfectly safe. It can be a bit like reading tea leaves at times…

 

[supercorndogs]

I would give proof a call before you pull it down. Do you have any measuring tools comparators or calipers. An OAL measurement and base to ogive would be a good starting point. It would be the shits to ship it back just to find out it was some wonkey ammo. I would expect if it was made recently, it probably is the 6.8SPCII chamber. SPC is .05 freebore SPCII is .114 freebore

 

[RetardedTiming]

I would give proof a call before you pull it down. Do you have any measuring tools comparators or calipers. An OAL measurement and base to ogive would be a good starting point. It would be the shits to ship it back just to find out it was some wonkey ammo. I would expect if it was made recently, it probably is the 6.8SPCII chamber. SPC is .05 freebore SPCII is .114 freebore

Will do, agreed about shipping for no reason. I have some Mitutoyo calipers, that's about it unfortunately. Yeah, I would think it's SPC II or at least I would hope so. I'll see if I can figure out a way to measure freebore with the caliper plunger.

That makes me think though...like I said I bought the ammo years ago (like 2009 I think). Could it have been manufactured to accommodate SPC spec? Wait, I guess that would be more clearance in a SPC II chamber.

 

[supercorndogs]

Call proof, they will ask you what they want to know. The OAL of the ammo might be something they ask. You aren't going to be able to measure your freebore with your calipers. Do you know what bullet, Hornady 110 Nosler 110, is the OAL of the ammo over 2.26? I see 2.26 max listed for the v max anyway.

 

[RetardedTiming]

Sounds good.

Looks like the OAL of the two with marks are 2.252".

Here's the spec...

6,8 mm REM. SPC – PTS 110 GRS SB68B – Sellier & Bellot

www.sellierbellot.us