Gained 75-100 FPS With My Standard 4064 Load!

[Grump]

And accuracy slipped down again at the same time.

22-inch barrel, been using this load for years and changing powder lots had stabilized a downward trend from 2580 down to 2540 fps at about the same temperatures on various shooting sessions.

Now I'm getting 2650 and 1.3 to 1.6 inch groups at 100. Don't laugh too hard, it's a semiauto that only rarely gives me a 3/4-inch group. It had been .9-1.1 inches for a good while though. Probably not more than 1,000 rounds through the new barrel, too.

Everything else is the same, even the carton of primers. New battery in the Chrono (it drops 1,000 or so fps when low anyway).

Had to DRASTICALLY alter the Ba burn rate in QuickLOAD to make it match the results.

So, anyone other than Varget shooters get this big difference with a full-power load when using a different lot of IMR powder? I've been using this stuff for decades in two calibers and it has been as consistent as a McDonald's cheeseburger.

Oh, it screwed with my 100-yard zero, too.

???

 

[pinsandpitons]

I think we can ignore the group size change. 100 fps more will spit out the bullet at a different place in the barrel whip.

I'm intrigued about the velocity change though. I think I need some clarification;

-Cartridge?
-same charge weight?
-same brass/bullet/primer/COL?

Could the new battery be the reason? Bumps speeds up when new, and depresses them when it's getting tired?

 

[Dan Newberry]

Clean your barrel really well, concentrating on a possible carbon accumulation just in front of the chamber. A tight spot there (you can feel it with a patch over a jag, or tight brush) can cause the pressure jump you're seeing.

I haven't heard of 4064 being that far off in a lot to lot variance... but stranger things happen.

 

[Grump]

.308 with CCI 200s and Hornady 178-gr A-Max.

The chrono battery factor is pretty consistent--a wildly different number when voltage is down, sometimes an even multiple of like 1/3 the expected speed, maybe a few 1/2s and I think more often something randomly close to 1,000 low with the rifle cartridges. I have to make sure there is no sunlight shining into the skyscreen port, or often nothing will register. The ports were in the shade. Using the diffusers, too, always (except on the rare high overcast day--thing works GREAT then).

I also keep the light sideways to bullet travel to avoid the gas cloud tripping the start sensor. Or I will shoot into the light sometimes.

Wish I could find some Varget to re-check the *other* standard load for that rifle. Actually a third one with RL-15 was good too, but I'm trying to reduce the number of different powders on the shelf.