For my .243 Winchester I've been running 115gr Berger VLD hunting bullets with 46.0 grains of H1000 in Hornady cases and a CCI BR-2 primer. I have been getting very consistent chronograph readings of 2890 fps all summer. We've had a couple cool days, so I decided to get some 60 degree F data to go along with my 70-80 degree data. I brought with me ten rounds of the batch I took to Bang Steel, the only difference being these were in brand new cases, five of a more recent batch in once-fired cases, and five in once-fired cases with bullets I'd pulled from other loads. For brevity's sake, I'll call them Loads 1, 2, and 3 respectively.

Straight out of the case the rifle put a round of Load 1 right in the center of the bullseye at 100 yards, which was not bad for having not cleaned it since my last range trip. I attached my MagnetoSpeed to the barrel and took another shot: 2916fps. "That's a little hot for cool ammo," I thought. The shot landed just beside the first shot, despite my awkward, semi-prone shooting position. Four subsequent shots showed pretty good ES of 9, but the average was 2911 fps. Load 2 was slightly hotter at 2919 with a 14 fps ES. When I finally shot Load 3, I found much more familiar numbers: 2894 fps and ES of 36 (2885 and 23 fps excluding one 2900+ outlier).

My load is 1% over the published max Lee has for 117gr bulets, but the velocities have seemed in line with a lighter projectile and longer barrel. Still, I'm wondering why a cooler round on a cooler day could be having such high velocity readings. I tried (somewhat unsuccessfully, I might add, chilling a baggie of rounds in a bucket of ice water for varying lengths of time before shooting to get velocity data only to find there was no appreciable difference between 60 and 80 degree F ammo and only a 20 fps difference between that and 40 degree F ammo, though that was not too far from my ES from either temperature.

I should also say I didn't fire them all in a string, one load after the other. I tried going round-robin with a few generous cool downs for the barrel. I also noticed the MagnetoSpeed had slid forward slightly, though it seems like that would make the velocities measure as slower rather than faster.