a few questions/ observations

[aikijoe12]

I just started chronoing my reloads, and I could use some help with few questions/ observations. I use Zero 125 gr FMJFP bullets and use Universal Clays in all my reloads.

1. Is the velocity affected by the overall length of the ammo?

I have a load with 4.6 grains with OAL of 1.121", and my average of 10 shots is 1049 giving me a power factor of 131.1. The other load I had is with 4.5 grains with OAL of 1.090", and average is 1064 giving a pf of 133.0. So, lower powder weight with shorter OAL gives more velocity?

2. I also tested one with 4.4 grains, and OAL of 1.046", and velocity is around 1068, pf of 133.5.

3. The other ones are 4.4 grains, and OAL of 1.121" gave me a velocity of 1002, but there were some that fell below 1000 velocity (about 4 out of 10 shots). I need to have at least 1000 velocity to make power factor of 125.

I guess I'm thinking that the shorter the OAL, it will give off more pressure and hence the velocity will be faster. Is this correct or am I way off? thanks for your help.

Also, if I was to try different bullets, say from Precision Delta, or Montana Gold with the same 125 grns.and get some JHP instead of the FMJ Flat point, will the velocity also changed?

Thanks for the help.

[G-ManBart]

Shorter oal will increase the pressure and velocity compared with an otherwise identical load that is longer.

It is possible to have a higher velocity with less powder, but loaded shorter. Normally it takes a fairly big difference in oal to cause this. Also, if you're using mixed brass that will create some variation since they all have slightly different capacities (lower capacity, higher pressure). The other thing is that the same load chronographed twice in a row won't give you identical results...close, but not identical. The sample size we use is usually too small to get rid of that sort of variation. Similarly, testing the same load on different days will give slightly different results....lighting, temperature etc.

Bullets of the same style, weight and diameter will be pretty similar in velocity, but it really depends on how much case capacity they use. Normally a JHP loaded to the same oal as an FMJ will take up more capacity, raise pressures and increase velocity. The reverse is also true.

If you change from jacketed to plated, or lead, moly coated etc, that will cause a significant difference as well. R,

[boz1911]

Bart's pretty well nailed it. Even crimp can affect pressure thus affecting velocity.

[cnote]

When making changes, and when you want to compare one to another, you can only change one variable at a time, like OAL or charge weight or bullet type/manufacturer. Then you will still have slight differences. Try to use the same brass, shoot in a relative stable ambient temp ( don't compare shots fired at 8 AM 60 degrees to others shot at high noon 80 degrees. Some powders are temp sensitive.

[aikijoe12]

Thanks Guys.....I appreciate the help.