PDA

View Full Version : Velocity Question



craigrowe58
01-07-2008, 03:52 PM
I recently purchased a cronograph from Sinclair a CDM2,took it to the range today 47degrees,shot some loads out of my 300 win mag 81grs of h-1000,78.0grs of imr7828ssc and 73grs of I-4831 all behind a 180gr bullet the velocities were as follows 2731-2739 for h-1000,2863-2866 for i-7828,&2766 for I-4831,now these loads are 1gr to 1/2gr under maximum,also shot some loads that I tested over my friends ohler last fall that produced 2918fps in 60degree weather these loads now registered 2734,where would you guys think the problem lies,cronograph,powder scale,rifle,Iam confused would appreciate your input,I was thinking that these velocities would all be over 3000fps.Also the load tested over my friends ohler was 71grs of I-4831.

Squeal
01-07-2008, 05:20 PM
Two different chronos are going to give different readings. Also how far from the muzzle were you?? EXACTLY the same as before?? Is this the same powder or maybe a different lot? Also you will lose some because of temp. Not a lot, but add all the factors together and you might be suprised at the end result.... Dont panic. Remember those velocities in the book are just guidelines and will vary from rifle to rifle. I have rarely reached book velocities with my rifles except for one. Doesnt mean they arent accurate.

Larry Elliott
01-07-2008, 07:00 PM
Temperatures can make a difference in velocity, more with some powders than others, and the distance of the chronograph from the muzzle will make some difference too. If the chronograph is too close to the muzzle, especially with something like a .300 Mag is muzzle blast, the gasses coming out of the muzzle are faster than the bullet and can trip the screens. I set the center of the screens on my chronograph 10 ft (4x30" paces) from the muzzle, but don't shoot a magnum either.

Different lots of powders can also vary which can produce more or less velocity and pressure than other lots of the "same" powder. Bullets of the same weight but from different maker may not deliver the same velocities either. Things like bearing surface length, jacket material, and hardness (or softness) can cause a difference in velocity as well as weight.

If you can shoot the same loads over your chronograph and your friend's Oehler with one set up right behind the other. The farthest back chronograph will read a little slower velocity, but not much. It'll be a pain in the tail to get them both lined up, but you'll know if one of the other is reading fast or slow.