Would it be safe to use .451 ball in 45 cal (.440) Kentucky pistol?

[dr.s.goerisch]

It looks like ball would go down nice and tight without a patch. Can I do it?

Shooting "naked balls" (that's what a distinguished British attendant at my club's blackpowder tournament chose to call it!) may be fun, but it will not procure decent groups.
Besides, forced loading might damage your gun. A former member of the German national team broke my musket, while attempting to force a ball (of correct size, .690) down barrel with an oversized patch (.020) down the barrel, using a hammer. The L'an neuf musket (Italian replica) broke at the neck, leaving me heart-broken.

I just suggest you get the right size balls, rather than apply force, which will become necessary after the 2nd shot fired "naked", due to fouling buildup.

 

[Rifleman1776]

Will probably be hard to load. But if it will go in it will come out. Not likely to give best results.

 

[The Crisco Kid]

Does the Kentucky pistol we're discussing have a .440 bore or a .450 bore? It seems like there's a lot of confusion in this thread. If it's a .450 bore like most 45's a .451 bare ball without a patch will load easily. But if it's a .440 bore that they call a 45 it may be a tight fit.