Question about big jump in velocity

[texwelder]

Loaded some 178gr Amax for my newly built .300WM. All powder charges were weighed twice, once on 1200 DPS, then again on RCBS beam scale. Using RL-22, loaded three rounds at 74.0 gr, three rounds at 74.5gr, and three rounds at 75.0gr. COAL is 3.340. Went to the range today and here is what I found. 74.0 gr averaged 3100fps with ES of 4fps, no signs of pressure. 74.5gr averaged 3150fps with ES of 8fps, no signs of pressure. 75.0gr averaged 3177 for first two rounds, with last round at 3352fps, and still no signs of pressure. All shot fairly well with 74.0 gr at around .5 (c-c), 74.5gr at around .3 (c-c) and 75.0gr at around .5 (c-c). What would cause this big change in velocity with everything being the same?
All brass is once fired Federal, trimmed to 2.61, deburred with VLD reamer, primed with Rem. 9 1/2. Rifle is Savage Stainless 112, Northland recoil lug, Shilen Stainless Match at 26" with a 10 twist.
I am not new to reloading, but I have never experienced such a large increase in velocity with the same charge of powder. The only difference I could find with the "mystery" case was that the neck thickness measured .017 and the rest of the cases measured .0155. Could this have caused it?

Thanks in advance,

texwelder

 

[cfvickers]

Battery is getting low on your chronograph or just a bad reading. if you were shooting a 178 out of a 300 win mag at over 3300 actual fps you WOULD have pressure signs. I see this pretty often with my chrony beta when the battery starts to get low. how much time between shots? if less than about 15 seconds mine will do it as well. My 6.5x55 was slinging 140 interlocks at 3950 the other day! most of the time the differences with a low battery are more subtle than 1300 fps fast though on mine at least.

 

[texwelder]

I didnt think about the battery.....it isnt that old. Time between shots was about 2-3 minutes.

Thanks,

texwelder