Thread: Velocity Increase During A Shooting Session

I was trying out a couple of reduced powder loads in my 7mm08.
Brass is quite old and I am looking for something to avoid case base separation while I work up new brass.
Bullets are 140 gr Nosler Ballistic Tips and powder is AR2208.
I'm using an F1 Chrony and was blessed with an overcast afternoon and it captured every single shot. Absolute accuracy - who knows ?
I've previously estimated the velocity of the 43.0gr load at 2820 f/s by truing from drop at 500m and come up at 800m using Shooter then Strelok.
Now, don't laugh at my ES which are around 30 f/s for each string of 5...

Here are the velocities of each shot, in the order I fired them:



I think this shows a progressive increase in velocity over the session. No, it was not accompanied by an increase in ambient temperature !
There isn't much obvious increase within each string of 5 so probably it isn't due to heating of the barrel.
More likely, I expect it's due to progressive fouling. It's quite an old barrel, with about 3000 rounds down it. I clean for carbon each session and use a brush about every 100 rounds and copper maybe every 500 (randomly).

So, I'm thinking that, for this gun I need to fire half a dozen foulers before serious sighting in and trajectory testing ?

Has anyone else found increases in velocity as they fire more shots over the day ?

One effect of this would be that I'd over-estimate the effect of increasing powder weight if I worked up from starting lighter to heavier as most people would do for safety.
By chance, I started and finished with the highest load (43.0gr) which I considered safe and with the middle (42.5gr), and the lightest load (42.0gr) out of order.

Leaving out the first 6 shots, I had overlapping velocities across the 3 powder loads but more of less a progression as expected, being about 40 f/s for one grain of powder. Incidentally, this is equivalent of an inch or two of barrel, I'm told !