I just started reloading about 4 months ago for my 300 win mag. I would like to get into long range shooting and I’m slowly building up my equipment list to get me there. I received a chronograph for Christmas and today was the first chance I had to try it out. In the past I purchased some cheap Winchester and Federal ammunition. I’ve kept the brass and now I’m reloading it. It was 25[FONT="]°[/FONT][FONT="][/FONT] Fahrenheit outside and my ammunition was in the garage so it was at outside temperature. I thought it would be a good opportunity to test how sensitive my powder is to outside temperature. I setup my chrony on a tripod and began to shoot through it. On the second shot the wires frame flew apart. My heart dropped as I figure I had just shot my brand new chrony. After finding the part and no nicks on it I put it back together and with each shot I could see it move and three shots later it came apart again. I then realized that since my Chrony was only 5 feet from the end of my barrel the Chrony was getting blown apart from the exhaust gases. I decided to put on my muzzle brake which ended any movement on the Chrony. I heated several cartridges by using my vehicle heater to approximate room temperature. The Federal brass is stamped with FC and the Winchester brass with W-W, so I will refer to them as such. I reloaded 155 grain Berger VLD bullets with 76 grains of Hodgdon 4831SC. I was expecting a velocity around 3050 fps from Hodgdon reloading data. I have a sensitive electronic scale and measured each load of powder very carefully and I’m shooting at a elevation of 7000 ft. Here are my results. FC at 25° (3152, 3180, 3214) FC at room temperature (3151, 3151, 3145, 3185) W-W at 25° (3155, 3150, 3122) W-W at room temperature (3148, 3146, 3124) I was surprised at the higher than expected velocity and the extreme spread (ES). Due to the large variance there is no way to say if temperature affected the velocity. I decided to measure the weight of each case to see if it indicates a large variance in the volume of the cases. Here is what I found in grains. FC (257, 256, 255, 258, 245, 256, 255, 255, 257, 253, 254, 257) W-W (244, 240, 242, 245, 243, 244) After that long diatribe I’m interested if anyone has suggestions for why the velocity varied so much. Is the case volume or could it be something else? This is the 3rd time this brass has been shot and I haven’t annealed the brass.