accounting for different bullet legnths

[odocoileus]

Shooting a 7mm rem mag. Ran out of a box of bullets that my gun was shooting very well with 65 grains of 7828ssc. Started into a new box but the CVTO of the new box (different lot) is .010 longer than the last box and they aren't shooting as well. How should I change the powder load or seating depth to account for this?

 

[Wyo7200]

Drop it to 64.5 gr and test one group at -.01, one at the old length, and one at +.01. May give you some direction.

 

[Rick Richard]

I would just change seating depth first before I changed load. In other words, change only one variable at a time.

 

[barefooter56]

odocoilems,
Rick is right. Change one thing at a time. If the base to o-jive measurement of the bullet is .010 longer than the last batch of bullets you are seating the bullet .010 deeper in the case. Adjust the seating stem up until the cartridge base to o-jive measurement is the same as the group that shot well. You MAY have to adjust the load by a 1/10th grain or so to get back on your velocity accuracy node.
Hope this helps !

 

[Wyo7200]

In a magnum case like the 7mm, 1/10 of a grain wont have much telling effect, if any.

 

[barefooter56]

Wyo7200,
Yep. But we need to work up slow so we don't miss the velocity node or go to an over pressure situation if the load is on the edge for safety's sake is why the 1/10th grain " or so" suggestion.
Good point though.

 

[Wyo7200]

I was just going by what the industry reccomends... Whenever a component is changed, we are to lower the charge and work up again for safety. I can see the eyes rolling over from that one sentence Ive cut that corner too. It's not like he's going to miss his accuracy/velocity node in his work ups with this new bullet. Heck- we don't even know what bullet he was shooting and is shooting now...

Good luck with the adjustment!

 

[barefooter56]

Wyo7200,
Yep. I just fudged it a little more to the low side. No sweat. And your 2nd sentence is spot on. You never know when not doing that will end in tears. Been asked too many ties on the range " why did it do that?"

 

[Wyo7200]

What if he's .01 off the lands with the old bullet and adjusting his seating stem to accommodate for the .01 difference puts him on the lands? Without decreasing the powder charge, wouldn't that run the risk of having high(er) initial shot pressures? but, that's another variable we don't know about.

 

[barefooter56]

Wyo7200,
In that case because he is so close he should go down in powder charge and work back up ,yes. But since he is seating the bullet farther out the case capacity has increased and that MAY decrease the pressure enough that the .001 longer MAY not be an issue. This is all hypothetical though because of the small distance involved. Go to our website and open the "Berger Bullet Blog" and read Bryan Litz's
Articles " The effects of OAL,COAL and CBTO" parts 1 and 2 for a more complete explanation on this.