? on BC

[mike33]

I have a 25-284 i been using for whitetail close range and had good luck with sieera 100 spitzer flat base. My current load was tad being hot but worked was 58 grains ImR 4831. I just got a box of swift sccorr.11 looking for a better bc and decided to try some. I loaded op some backing off to strigs of 56 g 57 and 58 just playing it safe. The first string of 56 grains of powder was to hot only fired 2 shots stiff bolt lift and plunger mark. Back to the bench pulling bullets and dumping powder i started over again. This time strings of 53 grains 54, and 55 . 53 wasent so good no pressure, 54 3 shot group .540 good bolt lift case was fine this load will work for me. 55 grains bolt was just hair sticky not bad but group opened up. I did not have my chrono. with me due to getting late. My big? is why did i have to back off 4 grains of powder with the same 100 grain bullet weight was it because of the higher BC i got pressure?
mike

 

[Kevin Thomas]

Mike,

BC and pressures have, essentially, nothing to do with one another. Totally different matters. However, some feature that may contribute to the changes in BC, could also affect pressures; bearing surface, ogive radius, rifling engagement at a given OAl, that sort of thing. Bottom line here is, you changed a component, which means starting from scratch where the load is concerned. Different ogive shape, jacket thickness or core hardness, all this goes into the mix. But those are the issues that have created the pressure problem, not the BC per se.

 

[rscott5028]

Expounding on what Kevin already explained...

Scirrocco having a much thicker copper jacket vs the Sierra FB Spitzer will have a greater copper to lead ratio will likely have a much longer bearing surface.

Similarly Barnes all copper bullets exhibited high pressures which lead to the releif rings that you see on them now.

-- richard

 

[Kevin Thomas]

Preeee-cisely!

 

[mike33]

Makes good sence to me, thanks for your help.
mike