7mm08 139g s 140g 2 g difference in max charge WHY?

[Bull Nutria]

3 minutes ago
i have a pet load for 7mmO8 42.5g imr 4064 with 139g HornadyBTSP. i am NEARLY out of the 139s. i have 200 speer 140 g sierra prohunter. reloading manuals say max load 40.5 max 4064-
does one grain bullet weight make max charge 2 whole grains less of powder.

what say you ? can you explain this? i am tempted to load my 42.5 g with the 140 g pro hunters.


Bull

 

[conan32120]

Me, i'd load a new ladder but I'd start very near sierras max if just looking for max speed but much lower if looking for a new Accuracy node which is rarely at max in MY experience, ymmv

 

[Blue68f100]

Different bullet different jacket and core causes different frictional properties. Do a load workup with the new bullets and powder.

 

[FROGO207]

Different bullet different jacket and core causes different frictional properties. Do a load workup with the new bullets and powder.

THIS
even though it is one grain the composition/shape of the bullet might make it easier to push down the barrel so the different charge. Also could be a different test setup. It'll probably be close to your old load but at least do a minimal workup to be safe. In the past I have found no two rifles act the exact same neither do two slightly different bullets.

 

[jmr40]

Look at different sources. The loads you see in loading manuals don't necessarily show the max safe load. That's just where they stopped testing. Another manufacturer will keep developing loads beyond where another stopped. They may have been using a different brand of brass or primers which changes things too.

I wouldn't use the 42.5 gr load until I'd reduced my powder charge and worked back up with the new bullets. Chances are good you can safely load more than 40.5 gr but the odds are that a different load works best with the 140 gr bullets.

It's not just the weight, it's a lot of other factors. This is one of those situations where a chronograph is useful. It's entirely possible that the 40.5 gr load will shoot the 140 gr bullets to the same speed, or even faster, than the 42.5 gr load will shoot 139's.

 

[TEXASJD]

It’s not just bullet weight difference it’s powder lot difference and where the gun they were testing hit pressure. When making a change in powder lot, bullet, brass, barrel I always back off and work back up

 

[jmorris]

What are the OAL's? Where is the base of the bullet?

This bullet,



Looks like it will have a more contact with the bore (friction) than this one.

 

[horsey300]

3 minutes ago
i have a pet load for 7mmO8 42.5g imr 4064 with 139g HornadyBTSP. i am NEARLY out of the 139s. i have 200 speer 140 g sierra prohunter. reloading manuals say max load 40.5 max 4064-
does one grain bullet weight make max charge 2 whole grains less of powder.

what say you ? can you explain this? i am tempted to load my 42.5 g with the 140 g pro hunters.


Bull

Prohunter is HARDER bullet= more friction =more pressure, that ph will out penetrate the Hornady too.