I'm stumped doing load development for a Seekins SP10 6.5 creedmoor

[giannid]

About a month ago I purchased a Seekins SP10 6.5 creedmoor to be my long range semi auto. I choose three different bullets to try from it, berger 130 OTM, Hornady 143 Eldx, and 142 SMK. Out of the three I decided to go with the Sierras because the best load from them was 41.5 grains of H 4350 powder versus the 37.5 in the Hornady's. I figured i could use the extra velocity for long range. On the chrono, the sierras were 2592 FPS, ES was 26 and SD was 10.4. I was pretty happy with it and I was seeing .6 to .7 MOA accuracy at 200 yards which is where all my load development was done.

The one thing I noticed about this gun was it was banging up the brass pretty bad. On occasion it was banging up the shoulder pretty bad on some shots and most shots had a pretty bad whack in the body. I didn't think about it much as I figured I'd mess with the gas block and tune things down a bit when load development was done. I put a piece of velcro on the deflector and it seemed to keep the damage of the brass to a minimum.

So at this point I've been to the range three times with this rifle and have shot this load with the sierra 142 SMK and 41.5 grains of H4350 a few times. These were all loaded to magazine length 2.81" BTW. I felt pretty confident with this load and purchased a box of 500 bullets. After each range trip I cleaned the barrel to get the copper out during the break in using Butches with a copper brush followed with patches. The barrel probably had around 300 rounds through it at this point.

So today I went to the range with my load and figured I'd do my final sight in and adjust my gas block. That's when everything went to shit. Adjusted the gas block and now the gun is shooting like shit. It's averaging a 3.5 to 4 inch group at 200 yards. I have no idea what happened. After adjusting the gas block, my fps went about 25 FPS and ES and SD were about the same. I actually tried to take the gas block setting back to where it was with no luck. I brought my 6.5 grendel to the range and it shot normally so I don't think it was me or the wind.

I'm kind of confused with this gun. The only thing that really changed was a new lot of bullets and the gas block setting. Mount is a Seekins and scope is a trusty Nightforce NXS. I really haven't messed with adjustable gas blocks before. Can this be the problem? I've also never had an issue with changing lot numbers especially with the SMK's. I don't know where to go from here. Is it possible my barrel broke in to a certain point and no longer likes the load of 41.5 grains?

I'm lost guys, please help.

 

[hafejd30]

Did your neck tension change with new bullet lot? My RR LAR8 worked like a bullet puller and matter how much I crimped the case neck the bullet would slide forward in the neck when the bolt slammed shut

 

[giannid]

I didn't check the neck tension nor do I ever. I'm FL sizing my bullets with a Redding die with the expander removed. After that, I run a 21st century mandrel to do the inside of the necks with a little bit of graphite lube every second or third case.
I never thought of that. You're thought is the bullets are moving in the case as the gun chambers the next round? Makes sense as it would change the OAL. I'm not putting a crimp on my bullets.

 

[hafejd30]

Correct that is what I’m saying. Mine moved the bullet up to .010”. Managed to crimp down and they fired fine when I single fed them closing the bolt slowly. But opened up when I let them feed semi auto from the mag.

Best I could get is about .002” movement on a bullet with a cannelure. Then had a bullet contact the feed ramp and shear the polymer tip off the bullet. I gave up then lol

Measure 2 rounds. Load both in mag and fire the first. Remove the 2nd and remeasure it. I’ll bet it moves a decent amount.

Not saying this is you issue. Could have dinged the crown, scope ring/base screws loosened etc etc

My bullet that contacted pushed the bullet back into the case. Luckily I was checking for bullet movement and took out to check it. She was beat up pretty good. I run suppressed and didn’t want to chance jacking my Ultra 9 up. I run factory rounds in it now. As it’s just a hunting rifle and doesn’t see a lot of rounds. That way when I destroy the gun or suppressor at least my warranty might work. If I fire reloads I know they wouldn’t help me fix shit

 

[giannid]

I'll check that. Not going to be able to go to the range again till next weekend. It does make sense. If you change the OAL length, it's going to shoot a little different. Going to try and get to the bottom of this. When I did my original load development, all loads were fired from the magazine, not single fed. So if that's going on here, it's from the bullets being slightly different.

Was never a fan of polymer tip bullets in an AR as it seemed a few would always get messed up in the feed ramps. Another reason I wanted to use the SMK's over the Hornady's.

 

[hafejd30]

In the meantime check over the rifle for loose parts etc. If you were running a cheap scope I’d suggest that but I’m thinking the scope should hold fine but don’t count that out. I’m guessing the gas block probably won’t change it much. But that’s just a guess. I swapped my RR gas block out for a Superlative Arms and never noted any accuracy loss. Mine was very overgassed with the factory gas block and the can on

 

[giannid]

I checked the scope mount torque tonight and all checks out.

 

[hafejd30]

If you have a SAFE spot to test the bullet hammer theory just measure a round, load in the mag and hit the bolt release on the rifle to chamber it. If it’s moving it will move doing this just as well as it will firing the rifle.

You do this at your own risk tho. If you put a hole in your neighbors garage I’m denying I suggested this

 

[giannid]

After some thought, something came to my mind. The brass I’m using is hornady from my bolt gun I had for years. I’m not sure how many times it’s been fired either and I didn’t keep track. I also never had the brass anelled. When I purchased the new rifle, I did some load development with 100 new rounds of brass I purchased. I’m wondering if the brass is getting old and doesn’t have the proper neck tension. Most of the brass I’m shooting now at the range is from my old batch and that’s where I’m having the problems.

 

[giannid]

I've got some new brass ordered and I'm going to load up some ammo and try it again. I can't imagine it's the rifle. The biggest variable is the handload. I'm going to try that first.

 

[TriggerHappy0125]

I am curious, where did you end up with this issue?

 

[Mmfujita]

I’m curious also. I tried to switch a few parts around on my 6.5cm AR. And had hell of a time getting everything back to shooting good. Now it’s smooth as sewing machine. Getting my gas block, buffer spring and the buffer weight was something of a chore.

 

[garandman]

https://bergerbullets.com/product/6-5-mm-140-gr-hybrid-target/