Why does some 45 acp brass need large pistol primers & some 45 acp brass need small pistol brass? Thanks

To give you two chances to have the correct ones?

Because someone decided that a small pistol primer is all that is needed for a low pressure .45acp case and someone else decided that it might actually be more accurate because a small pistol primer was not as powerful as a large pistol primer and the variability would be more controllable by variations in powder charges.

In reality it was done to frustrate all reloaders, especially ones using progressive presses, because they now have to carefully sort their brass and stock both primers for just one caliber

I sort the two sizes and load the small primer brass with cheaper components and shoot it where i don't intend to ever pick up my brass.

I was thinking the same then realized someone might trade something for it and if I run out of one size primer I could still load and shoot the other.

But to your point, I have a collection of the small primer stuff I have never reloaded.

To dog knot hand loaders.

There were a couple companies that made "eco friendly" ammo used with the small pistol primers. I know Speer did these, as well as a few others. The 45 ACP brass I reload is the Speer small primer pocket. I find the small primers actually flow and load better through my Dillon's, also. But loading 45 ACP and having 2 size primer pockets is a PITA to sort out.

old vs new