Plain and simple, Ball Type Powder. Reliable consistent ignition across the entire spectrum.

Look at any older Speer manual and you'll see their test commonly use magnum primers with non magnum calibers using BTP's. Speer testing shows some combos even low pressure low velocity loads using BTP's are more consistent with magnum primers.

Do I use magnum primers with HP38/W231? Rarely, if ever but I know why some data recommends it.

Especially in rifle loads, I use magnum primers more often than not now. Even though I use a lot of extruded like IMR4064, 4350, 4831, I like W760/H414, W748, H335, Accurate 2520 etc, and it's easier to keep one type primer on hand. I still test accuracy using test both standard and magnum, if there's little to no difference I generally use the magnum large rifle to work up my loads.

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GOA
USA Shooting Supporter