Is there a formula to calculate how much powder to add or back off for different bullet weight? Example: I have published load data for powder X with bullet weight X. That’s not for the bullet weight im loading for. My bullet is 50 grains more. So I guess my other more simple question is how much do I add to the published charged weight to get safe starting weights for my bullet?
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Good question. I’m sure there is a way to do it very scientifically per cartridge per gun per projectile per seating depth etc, but it’s not easy to account for all of that and say “here’s the formula.” As a general rule of thumb, if you use a published safe charge for a HEAVIER bullet when actually seating a LIGHTER bullet and you MAINTAIN THE SAME SEATING DEPTH (read: same case volume available for expansion) you will be ok. It may be on the slower/minimum end of the spectrum but so long as the weight difference between the two projos isn’t huge (ie you can’t load a 150 gr with data for a 300 grainer), you shouldn’t get a projo stuck in the barrel and with a chrono and watching for pressure you can safely work up. Some cartridges have VERY little margin for error in this, and I can’t stress enough you need to maintain the same seating depth and the projos need to have similar base profiles (seating depth for a flat base bullet will NOT be equal to a boat tail of the same, much less different, projectile weight). For instance, in a teeny tiny case like the 300 blackout, you can’t sub the charge for a 220 grain boat tail using a 150 gr flat base. But if you use the charge for a 220 gr boat tail and sub in a 208 gr boattail, seating them to the same seating depth and thusly there’s the same amount of expansion space within the cartridge/space between projo and powder, you should be relatively safe. It may be too much or too little powder for the velocity, but you shouldn’t blow up. Pm me if this doesn’t make sense.