Slug Diameter

Those one or two thousandths of an inch in diameter can make a huge difference in how they shoot from specific guns. One gun may like a .249 in .25 caliber another the .250 and it's the same with .22. That little bit can mean inches of difference in groups when shooting at 100 yards. It's the same with powder burners.

The variable has always been there you just did not notice it till now. Constantly learning new stuff we are.

Edit; oops fergot to answer your question. 

Other than trying them out yourself ,or finding someone else that has already done that for you, there is no way of knowing what will shoot well in your gun.

You are lucky though cuz in this day and age there are allot of you Tubers out there doing testing on just about everything under the sun.