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:

Attachment 261262

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 !

Attachment 261258

I worked up a load in my new Mauser but with 165g NBT and found 44.5g of 2208 to be the most accurate. This load hasn't been chronographed but is accurate with no pressure signs so I stopped right there.

Greetings @Bagheera,
I chronographed 42.5 grains of AR2208 with the 140 grain Corelokt in a 7mm-08 recently on my 35P. Velocity was 2,710 fps at 3 metres from a 19 inch barrel. Your result of about 2,750 fps for the same load is in line with this assuming a standard 22 inch barrel. 43 grains, or 0.5 grains increase, should add around 25 fps giving about 2,780.
My first suspicion is barrel fouling in what is a pretty well used barrel with the fouling raising pressures as more shots are fired. Do the various loads, or more importantly the two lots of the 43 grain load make a single group on the target? If they doo then what is the problem? If not it is the first few shots out of the barrel that matter and if these are low due to the velocity you need to zero for them. You will need to test again with your new brass especially if it is a different make but please remember it is the first mag full or part thereof that matters for hunting not what happens after that.
Regards Grandpamac.

"I'm using an F1 Chrony and was blessed with an overcast afternoon and it captured every single shot. Absolute accuracy - who knows ?"

Before you read too much into the change in velocity of your loads you might want to consider finding a Labradar or Garmin to measure your loads with. The optical chronographs can be a bit interesting with changes in light conditions - I had one tell me my load was over 100fps different from an hour earlier also on an overcast day.

Quote:

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Originally Posted by Steelisreal

"I'm using an F1 Chrony and was blessed with an overcast afternoon and it captured every single shot. Absolute accuracy - who knows ?"

Before you read too much into the change in velocity of your loads you might want to consider finding a Labradar or Garmin to measure your loads with. The optical chronographs can be a bit interesting with changes in light conditions - I had one tell me my load was over 100fps different from an hour earlier also on an overcast day.

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Yep 80FPS variation is less than the reliable accuracy I would expect out of an optical chronograph.

It was raining a bit and some water had accumulated in the tray of the chrony by the end of the session.

Do you clean your carbon ring?

80fps between fastest and slowest...I see no problem here. Do maths at your desired range and may find it's not a deal breaker issue. That thread on nodes n random thirty shot groups etc make a fella think.

Wet brass will put your speed up.

Old brass with uneven neck tension will put your speed up or down (more tension, more pressure?)

But also yeah fouling from a clean (bare metal clean) barrel will speed up shots but from past experience usually this will happen after one or two shots.

The fastest of your slower strings at 43.0 appear to be at the bottom of the range of your faster shots at 43.0.

I don't think you have enough data to determine a trend here.
For the question regarding fouling with a half dozen shots before sighting, I don't think it's worth the investment unless you are shooting long range. 80fps won't make enough difference until you get way out there.