I see nothing wrong with the name as it is, but you could simply add RIFLE to it, ie: Big Bore Rifles.

The "DOUBLE RIFLE" forum, is all inclusive, from .22 rimfire, to 4 bore, as long as it is a double rifle, and the "SINGLE SHOT RIFLE forum are both for a particular type of rifle, regardless of caliber.

The "Big BORE rifles"forum would include any rifle from 400 cal up.

A "Medium Bore Rifles"forum would include any rifle from 9.3,(.366) to .399 cal.

A "Small Bore Rifle" forum, would include any rifle smaller that 9.3 (.366)

I, personally, think all are fine the way they are, but that isn't my call! But, I guess that would be parr for the course, when you're an old BOAR!