450Bushmaster.net

Is it common, or unheard of, to lose between 100-150fps of velocity when your barrel is warm? I weighed every load and I lost that much velocity from my first group to my fourth group of three shots. It was very sunny and my first two groups were at 100 yards and my last two were at 300. I had to move the chrono a few inches to get to the 300 yard target. That is where I lost the speed. The two 100yard groups were within 30fps of each other and the two 300yard groups were also within that range

That's weird. my velocities usually inch up, especially if each round sits in the chamber long enough for some of heat to soak into it.
Though the 450b drops velocity over distance like a shuttle cock, that shouldn't have anything to do with velocity measured near the muzzle.
Are you running a gas auto or bolt action?

Hoot

It could be that since you moved the Chrono, the lighting changed how it was picking up on the bullets giving you the different read out. Or, Is this a new rifle? if so, it could be that you are "breaking in" the barrel and once that is done your numbers should settle. I try not to ever fine tune any loads on a new barrel as they will take some rounds through them before they settle.

It’s an AR carbine length Bushmaster that is about 7 years old. I was thinking that it was because I moved the chrono especially since the first two groups seemed abnormally fast

Cutright wrote:It’s an AR carbine length Bushmaster that is about 7 years old. I was thinking that it was because I moved the chrono especially since the first two groups seemed abnormally fast

I remember when I first started chronographing my 450b. I discovered that unlike most of my bottleneck calibers, I had to set the optical chronograph out around 15ft or it detected other ejecta, usually measuring some ridiculous velocity. Not a problem with my LabRadar however.

Hoot