My .577/450 had a .460" groove diameter near as I could measure it.
: The use of smaller diameter bullets, if paper patched and the cores of pure lead, will work just fine as it does with Sharps and other BP ctgs. The BP load expands the bullet to fill the bore. A good wad and perhaps a lube wad helps to clean the bore and protect the patch from blowby gasses.
: Enough powder should be used to reach the bottom of the neck, in my opinion. Loose BP charges shoot poorly, and can be dangerous if left to extremes. Loaded slightly up the neck, with a card wad pushing it to the bottom of theneck will suffice. The coiled ctg. of the day were well undersized and even they held the 85gr. charge. If the capacity is increased, so must be the load. this is comon BP loading practise.