Double for deer?

[Doc]

You can use any shot size you want in any shotgun safe to fire, it makes no difference.

This used to be absolutely true. But with the advent of steel shot, we should be a bit careful because newbies may not know the difference.
Doc,

 

[JohnL]

Toot told me to try this with my 150 year old double; man that thing rang like church bells!!

 

[nhmoose]

Here in NH a double is illegal in the special ML season Firearm must only be capable of 1 shot

Regular firearms season it is legal go for it.

 

[megasupermagnum]

This used to be absolutely true. But with the advent of steel shot, we should be a bit careful because newbies may not know the difference.
Doc,

Well there's always a caveat, but it's mostly true with steel shot as well. I would say just flat out do no shoot steel of any kind out of an old muzzleloader, espeically an original with dmascus or twist steel barrels. In a modern steel muzzleloader like a Pedersoli, then the size of steel you use makes no difference. Caveat of course, steel buckshot wouldn't be good. Anything up to T shot would make no difference in a muzzleloader.