Velocity drop off.

[DaveMarine1]

While doing some final zero and chrony testing before a shoot this weekend, I noticed something that has happened maybe half the time. While shooting I shot in the order I load, that's just how I put them in the shell box and I grab them in that order...up to down. Anyway I sometimes see the velocity start at say 2760 fps and then it slowly tapers off to 2725 fps for an average of ES of say 35 and a SD of 14. I use a RCBS Chargemaster. Could my scale be drifting as I am loading giving me the velocity "slope"? Not a huge deal but if it's something you have seen before I would like to hear about it. Thank you.

 

[amlevin]

Is this a function of your ammunition or of the rifle as it heats up?

If you want the same results then the conditions of all components have to bee the same. I'd shoot the first few rounds and discard the chrono readings. Call them "fouling and warming shots". Then start recording chrono readings.

I've found on some of my rifles the first few rounds are fast and then drop to a speed that is stable as the dickens.

If you're concerned about your Chargemaster drifting as you load, two things help. I leave mine plugged in and turned on 24/7/365. I use it for short periods just about daily but I don't have to worry about warm-up or drift.

As a check, record the weight of your pan. Mine weighs exactly 157.5 grains. When I remove the pan with a dispensed charge I just look at the readout and if it doesn't read exactly -157.5 gr I merely pour the charge back, place the pan on the support, and "zero" again. This happens about once out of every 30-50 charges dispensed on some days, not at all on other days. It's a quick check and takes the scale out of the "worry equation".

 

[DaveMarine1]

Good thoughts. The barrel had been cleaned but had 5 previous shots down the tube. Perhaps though it is a heat issue.
I keep my scale on 24/7/365 as well. I see the pa. Weight drift from Time to time also. Maybe I need to zero more often. Thanks for the information.