As I mentioned in a previous post, I have been working up a load for a 7mm STW using a 140gr. Nosler Partition. I started with 74 grains of IMR 7828 and worked up to 78 grains with no pressure signs and now plan to take it up in smaller increments to 80 grains.
I have now bought a box of Hornady SST's in 139 grain size. Since the bullet weight is almost the same and I am using the same cases and powder, can I start the new loads out at 76 or 77 grains, instead of going back to 74 grains (this is the starting load in the Hodgdon manual, but a high load in the Nosler manual)? Can I expect a pressure spike by going to a different shape of bullet, or have you found pressure to remain relatively consistent by bullet weight?
Always start out at the lower powder amount when changing anything. Bullets of different makes but the same weight can have different shapes that cause the bearing surface to be different. This can cause pressures to be much different between the two types with the same load.
As Reloader 2 pointed out bullets have different shapes and many Hornady bullets have that shoulder on the ogive. This changes the distance to the lands a lot. Bullets of that shape also tend to have longer bearing surfaces as pointed out.
Rick, just check a few of the past/present posts on the sciroccos to verify what these guys have said. A safe load with another bullet may not be safe with them, as an example.
Would I be able to tell by the velocity that I get with the SST's whether I am getting a pressure rise? In other words if I load it up with 77 grains of the same powder and I get the same velocity I got from the Partitions with no other pressure signs, could I assume that pressures are running pretty equal for both bullets? Is higher pressure normally accompanied by higher velocity?
I will have to look back for the posts on the Sciroco because it has a pretty similar profile to the SST.
IF velocities are the same with the same powder charges, the pressures may be SIMILAR, but probably aren't exactly the same, because one type of bullet might be easier to accelerate than another brand. However, if you are safely shooting 78 grains with a Nosler Partiton, you would not have to go all the way down to 74 grains when changing to a Hornady. You could start at 75 or 76, and go up from there. In my experience, the Noslers are tougher than a standard bullet because of the partition, and the Barnes X is even harder. If you were going from a Nosler Partition to a Barnes X, then the idea of dropping down and starting all over to work up a load would be more imperative. Note that HIGHER PRESSURE IS NOT ALWAYS ACCOMPANIED BY HIGHER VELOCITY!! In some instances, increased powder charges give higher pressures without an increase in speed. SOMETIMES, VELOCITY ACTUALLY DROPS WITH INCREASED POWDER WEIGHT!!
