First, rifles or pistol?
Then, if pistols, how old. If they are pre-WW2, they most likely have .454 barrels, and will like these better. Then, even if they are post-WW2, they still will likely shoot .454 bullets just fine. Many of the Cowboy Action Shooters use .454 bullets, and most of them shoot modern guns.
If it's a rifle, it would depend (and in some of the pistols, too) on how tight the chamber is. I had a 38/40 rifle. Barrel is supposed to be .401. Mine slugged .413. I loaded some using 41 magnum bullets, .411. Probably would have shot fine,but they were too large to chamber.
So, if I was you, I'd load up a couple and make sure they chamber in all the guns I plan to use them in. If it's revolvers, remember to try them in all six chambers of the cylinder, not just one.
If you do decide you need to resize them, the easiest and cheapest way is the Lee resizer. It screws into your loading press. They cost less than 20 bucks.
Meddle not in the affairs of dragons, for thou art crunchy, and taste good with catsup - George of Lod, Year of Our Lord 297
I always take precautions.
Beware the Evil Bullet Fairies.