RCBS scale consistancy

[Sgt 45]

I have the RCBS Chargemaster 1500 scale and the first time I got to chronograph my .338 Lapua I found the first batch of 20 rounds had a SD = 25, the second batch had a SD = 12. Both lots were 90grs Retumbo and a 285gr Hornady match bullet, CCI 250 primers were used. I weighed every round to exactly 90 grs. The chronograph was the magneto speed and it stayed constant throughout the firing. Has anyone had problems with their scale not being accurate, or should I be looking in another direction?

 

[Steve Blair]

At that charge weight, the Chargemaster's probable error should not cause that much variance. Have you tried adjusting the charge up and down a little to find a better combustion point?

 

[Preacher]

Were the 1st and 2nd batch all with new cases ???
New cases will use up energy forming to the chamber, unlike once fired...

 

[people]

One thing you can try doing is remeasureing the powder. If it is 90gr it should be 90 on the second go.

 

[Max]

Has all of your brass been reloaded the same number of times? As you work the brass it progressively hardens and changes the grip on the bullet which can change the velocity.

 

[amlevin]

Sounds like you're questioning the scale when the issue might lie with the brass. Unequal neck tension could do just exactly what you're observing.

 

[Sgt 45]

Learn something new every time. Not sure on how many times each was fired or if one batch was new or not. All the brass was Lapua but some may have been 1st load while second batch may have been once fired. I will check those 40 cases next outing to see if velocities settle down. I did try weighing them again and the weights were dead on. The brass in now being neck sized only whereas the first batch was full length sized so I think I just answered my question: possible new full sized brass vs once fired neck sized. Thanks guys, that never even dawned on me.

 

[BoydAllen]

Start with new cases, fire them in rotation, so that each has the same number of firings and sizings, even so, you will see differences in shoulder bump with the same die setting. If that is an issue, anneal, using the most modern approach. Read Ken Light. When you seat bullets, look for differences in the amount of force that it takes to seat the bullets. Don't clean all the powder fouling out of the inside of the case necks.

 

[Turbulent Turtle]

You start off with virgin Lapua which you F/L resize before first firing for some unexplained reason and then you neck size from that point on and you are surprised to have case size differences?

I would be surprised if you did not have differences.