Some .223 brass is kinda soft, meaning it is more likely to spread out at the base. Federal used to have this kind of problem, with loosening primer pockets, primer pockets getting more shallow, and I believe I heard this complaint too.
Some mil-surp brass for sale has been fired in light machineguns, which usually have loose chambers and headspace. If you are not using commercial brass, the small base dies may well be what you need.
Some chambers are cut really tight. In the past, Mini-14 rifles have not been, but Ruger has been trying to improve the rifle's accuracy.
If you are only reloading brass you have previously fired in your rifle, the chances are very small that you will need a small base die.