Dumb question about varying speed/pressure with same weight projectiles

[HairyScandinavian]

So I still haven't started reloading yet, trying to educate myself and just soak in data. I"m looking at the Hodgdon data right now for 6mm ARC and see two 95gr loads that have quite a different starting weight grains to pressure ratio. Looking at the two loads it would appear the Barnes is probably longer projectile and more surface area/barrel contact resulting in more friction which would lead to higher pressures for similar powder charge. Am I on the right track? What else might be considerations? A friend suggested that the Barnes being longer may be compressing the powder which I could see at max load but doesn't seem likely at minimum. Same COAL on both of these. I'm not interested in loading Barnes and that TMK load data looks great so probably use that, but I just want to know for my education why the difference between these two with the same weight.

 

[ID_Matt]

You're on the right track... some bullets have more bearing surface contacting the barrel so will reach pressure sooner than others. I think in your case of looking at a copper bullet vs standard lead core that theory is even more exaggerated.

 

[TaperPin]

So I still haven't started reloading yet, trying to educate myself and just soak in data. I"m looking at the Hodgdon data right now for 6mm ARC and see two 95gr loads that have quite a different starting weight grains to pressure ratio. Looking at the two loads it would appear the Barnes is probably longer projectile and more surface area/barrel contact resulting in more friction which would lead to higher pressures for similar powder charge. Am I on the right track? What else might be considerations? A friend suggested that the Barnes being longer may be compressing the powder which I could see at max load but doesn't seem likely at minimum. Same COAL on both of these. I'm not interested in loading Barnes and that TMK load data looks great so probably use that, but I just want to know for my education why the difference between these two with the same weight.

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Loads from different sources will vary enough it’s hard to think of any of them as written in stone, especially in your rifle. For instance if you were to duplicate these loads, variances in lot to lot performance of each component as well as variances in your rifle chamber and bore won’t line up exactly with those velocities, and that’s ok.

Also, not all published data is for the same pressures. Your data above is for the SAAMI maximum pressure, however the cartridge was designed around a semi auto gas gun so those pressures are considered low for a bolt gun. For instance Sierra shows different data for bolt guns 10,000 psi higher. Unfortunately they don’t list your powder, but this shows the velocity difference based on max pressure for the application. CFE223 can safely gain 150 fps with 1.6 gr more powder in a bolt gun.

 

[TaperPin]

It’s not uncommon to use published data that isn’t quite right for your gun as a starting point. Many people will use data like you posted and work up from there for bolt guns. You might not get pressure signs until 1 gr to 2 gr more powder. Pressure signs seem cryptic at first, but they get easier to interpret with a little experience.