12FV 6.5CM - Unusually high velocity

[RBT321]

Newbie question:

I have a 12FV in 6.5mm Creedmoor and had an unusually high velocity with one round recently.

I was using Sierra Match King 150gr with 41.0gr of Reloder RL16. Velocities for this batch of 10 rounds varied between 2678 to 2700 ft/sec, but the very last one kicked off at 3,050 ft/sec! I stopped shooting at that point, until I could resolve the issue.

I've already traced the production issue down. I'm certain it must have been a bullet seated a bit further into the case than normal, and it was caused by a slip of a bearing on the press. I have already fixed the press in such a way that this problem can't happen again. It looks like this round got through my final inspection process - which I'm also changing in response to this!

Out of an abundance of caution I've retired the case (Starline), but before I take the rifle out again, a 13% higher velocity than expected means there must have been a considerably higher chamber pressure inside the rifle than normal, so I wanted to know if I should be concerned about the rifle itself?

Thanks for any advice.

Ross.

[Dave Hoback]

Yes sir. Bullet set back can raise pressure in a hurry!

I wouldn’t worry about it though. The Rifle is fine. The combination of a modern Action & good brass keeps the pressure at bay. In order for it to have damaged the Action in any way, it would have first had to damage the brass. You retired the brass, but not because it was cracked or anything, right? I’m sure the Rifle is just fine.

[RBT321]

Thanks Dave,
Yes, the brass looks perfect. I'll take that as the best indicator then.

I think I might still start off with a comparatively small charge and bullet though, and build back up to my normal levels, mostly just to rebuild my own confidence

Thanks again!

Ross.

[vulcan]

Where did that shot impact the target relative to the other shots? I would think 300 fps would put that shot noticeably out of the group. Maybe just a bad chrono reading on that shot?

[RBT321]

My group wasn't very good to start with (I don't think the Sierra MK 150gr work as well as Hornady ELD Match in this rifle, with this powder) about 1.5 inch spread at 100yds, but this one was definitely an outlier - a good 2 inches above any of the others. Given that physical difference I don't think it was a dodgy chrono reading this time - although I have had one or two of those too.

[charlie b]

To get that much vel change the difference in OAL would have been a lot, like 1/4". Very noticeable. One of the reasons I put my reloads in the plastic boxes. I can look and see if any are grossly out of whack. I'd also check my other processes, like charge weight consistency. When using some powder with charge dies I have had bridge issues where one round gets a partial charge and the next one gets an overage.

If the pressure was really bad the brass would have signs, like a huge primer pocket and extraction that would need a hammer (yes, BTDT). Milder over pressure shows completly flattened primers and imprint of the ejector on the case head.

[Dave Hoback]

I agree. Reason I’m so methodical with reloading. I charge by weight, using the Lee Powder Measure Kit & a Powder Trickler. I use an electronic scale, but I even test it against my beam scale every 25rds or so. And then yes, all rounds go in a plastic ammo boxes. Very, very SLOW! LOL! But, it eliminates any simple mistake. And luckily I enjoy reloading. So going slow doesn’t bother me. It’s just part of the hobby.

[RBT321]

Charlie B, agreed regarding the box - I do exactly the same and although I'm still new at this, I would have definitely noticed a difference of 1/4 inch - that's huge!

For a different hobby I design and print my own N gauge model trains, so I'm pretty good at noticing very small differences in things. At most, the difference here couldn't have been more than about 1/16 inch or I would have visually seen there was some inconsistency. I wish I had measured every round in the batch and know what the difference was, but it's obviously too late now. With my more recent batches I am now recording everything for every round, so I have data to help me trace any issues in the future.

As for charges, I literally measure everything to the final grain with a laboratory-grade precision digital scale - to get the exact quantity I'm after. I've been known to use a needle-point tweezer to swap a big grain for a smaller one to get my measures exactly right! That's probably more precise than I need to be, but I am *trying* to get every round as consistent as possible.

Thanks for describing the sort of things to look for on the brass if things go wrong. Hopefully I won't need it, but it's a useful reference should it happen down the road!

Ross.