Rookie mistake

Working up some new Barnes TTSX loads for the 300 Weatherby 
Lubed,  sized, trimmed, chamfered, uniformed the flash hole sorted by headstamp loaded up a ladder and went to the range. 
1st group varied over 50fps, second group not good either, 3rd or 4th group max sammi had 1st 3 within 5fps spread and last 2 almost 100fps slower.
Same with the max + .5gr batch, WTFO?
Luckily last 2 batches were 3 good ones in a row and 2 slow.
Looking at the brass when I got home I had mixed Weatherby R-P 300 Mag and Weatherby 300 Mag brass. 
Case volume 6.41 cc h20 vs 6.67cc h20, dry weight almost 15 grains more with the R-P, checked some Norma pretty close to the RP
Looking over the data and assuming the slow rounds were the wheatherby brass the R-P with 79 gr H4350 was close to the same velocity as Weatherby with 80.5. 

That range session was about 25 bucks worth of schooling!
Got to pay a little more attention at the reloading bench.