Velocity question...

[Bill2662]

Shootong 175 smk’s average velocity was 2666. But weren’t grouping. Reworked the load and backed the coal down to 2.8. Now up to same charge weight and velocity is average 2700fps. Grouping good and no pressure signs. Surprised in a velocity increased though. Any thoughts? Same can of powder.
Initial load the bullet was .020 off the lands. New load is it at .060 from lands. Curious because I figured moving away from the lands would also slow it down a bit.

Rifle:
Vanguard 308
24” barrel
1:12

New Load:
175smk
44.5g Varget
Fire formed Hornady Match brass
Fed 210 Primers.
2.8 COAL
.060jump
2700fps avg.

Old load
175smk
44.5g Varget
Fire formed Hornady Match brass
Fed 210 Primers.
2.840 COAL
.020 jump)
2666fps avg.

Alt. 1880
Temp 66
Basically the same both days out.

 

[918v]

How much do your Hornady cases weigh?

 

[Bill2662]

Good question, I have to weight them. Nowhere near compressed with that load though.
Just popped out a primer and weighed one. 166.2

 

[918v]

I think your chrony is lying

 

[Bill2662]

I’ve thought of that too. It’s a magneto speed. I’m going to have to shoot and check what the drops are.

 

[918v]

I’m getting those velocities with 43.5grs in 164gr Hornady match brass out of my 26”

 

[1moaoff]

New barrel?

 

[Bill2662]

No not all. With 150’s and 45.5 of imr4895 I’m getting 2850 avg velocities. Starting to wonder if it’s just a fast barrel?

 

[ShtrRdy]

If you seat the bullet more into the case it will decrease the volume inside the case. This will raise the initial pressure and increase the velocity.

 

[abn31c]

By seating deeper you are giving the bullet a running start, think freebore. Roy Weatherby was a fan of this theory. Also as ShtrRdy say more initial pressure.

 

[Bill2662]

Makes sense. Not going to worry about it, was just curious how I was at that velocity 1/2g under the max load. If I remember right win cases hold a bit more than hornady. Either way thanks all. No pressure signs and groups are good so no worries. Might go back down to 44g play with seating depth.

 

[Doom]

By seating deeper you are giving the bullet a running start, think freebore. Roy Weatherby was a fan of this theory. Also as ShtrRdy say more initial pressure.

With the same charge and deeper seating you also have less air in the case. The air is a dilute that reduces the peak temperature and pressure of the fired charge. Small effect but still there.