Changing to Revolver for IDPA & Insteresting Ammo Info

[ORIGINAL TEX]

This a bit long, but you may find it interesting and there is a question a the end.

I recently pulled my old Colt Diamondback, four inch, out of the safe, dusted it off and took it to the range to play with in addition to practicing with my Glock and regulating new sights on an AR. Shooting it was like putting on an old well worn, well-formed pair of boots that had been custom fitted just by wearing them for a long time. I forgot how much I enjoyed that revolver – even if the cylinder latch and cylinder spin are backwards for some of you S&W fans.

Since I don’t reload, I gathered up a number of different 38 Special loads from different suppliers and then took them to the range to chronograph and make sure they made the 105,000 required power factor for revolvers. I figured that was the first step and then picking the one that shot the best groups would be the second step. I had; Aquila 130 grain, Winchester White Box 130 grain, Blazer Brass 125 grain, Blazer Aluminum 158 grain, Magtech 125 grain, S&B 158 grain, Remington Green & White Box 130 grain, and some 130 grain reloads from Ventura Tactical.

To make the power factor of 105,000 a 158 grain load would have to move out at 665-fps, the 130 grain loads at 808-fps, and finally the 125s at 840-fps.

I was in for a heck of a surprise and thought the chrony was malfunctioning until I tested it with a couple of 9mm rounds from my Glock - it was correct. The temperature was about 60 degrees with a light overcast.

All of the 158 grains loads made the power factor, but only by about 20 fps. Absolutely none of the lighter weight rounds made the power factor, except for the Magtech 125-grain which exceeded the power factor by almost 60-fps. None of the others even made the manufacture’s advertised velocity. I shot six rounds and averaged the speeds. Most of the lighter rounds missed the mark by as much as 80-fps and even the Winchester White Box, which I consider to be a slightly hot factory round, missed it by 15-fps. In addition the shot speed deviation on the lighter loads ran anywhere from 50-fps for factory to 145-fps with the Ventura Tactical reloads. None of this was what I was expecting. I should think any 130-grain factory load should at least move out at 850-pfs – which should clear the power factor floor by about five percent.

I could tell the Magtech was a slightly hotter round when I shot it so I was not surprised it made the power factor and it shot a pretty good group – averaging 1&1/8th inch away from group center at 15 yards.

As experiment I place six of the Winchester Whit Box 130s in a towel with two of the open and shake type throw away hand warmers – and left them for about 20 minutes. When I took them out and tested them, they had picked up speed by about of 40-pfs – making the power factor. The odd thing was that each successive round seemed to chronograph at a slightly slower speed until the last few rounds – I suspect because the now cold cylinder was cooling the cartridges and then the cylinder heating up near the last two shots. Shots were about 20 seconds apart and were as follows; 890, 852, 831, 814, 816, 823. An average of 838-fps compared to an average of 793-fps when cold (but not really that cold). As you can see, dropping speed for the first four rounds, leveling off by the 5th round and then picking up a bit of speed with the last round. I well know that there will be deviations, but this seemed to be a pattern. I didn’t have time to heat up any additional loads to see if I saw the same pattern. I can understand a temperature of sub 40 degrees having an effect on temperature sensitive powders, but was only about 60 degrees that day.

Have any of you experienced something similar with factory 38 Special loads and such drops in velocity on a cool, not cold day?

[LTUME1978]

I don't have any experience with 38 special ammo but I did shoot rimfire metallic silhouette years ago. On cold days, we would put our ammo out in the sun on in a pocket that held it close to the body to warm the ammo. It seemed to increase the velocity and help with drop. When shooting a 22 standard velocity round a 100 meters, we needed all the help we could get to get the round to shoot at the sight setting for warmer weather.

[Mike S]

I've never chrono'd pistol ammo, but temperature change has an effect on muzzle velocity. A drop in temp will cause a drop in MV, more for some powders than with others. (And conversely, a rise in temp = a rise in MV).

It doesn't require -40° F weather, but I'd imagine at that temp I wouldn't be outside in the weather anyway