Consider what is available to you in terms of wad, round ball, cards, etc. in purchasing another gun. If you can make your own, perfect. If you have to order things in, are they readily available and inexpensive?
A young guy I know was interested in buying a rifle chambered for .257 Roberts as a deer gun. My advice to him was to research the availability and price of the ammo. If it is expensive he might consider a .270 or .308, which is cheaper and locally available. The less available the cartridge the more the local gun store is going to charge for them...
There is some truth to this, but it really matters very little to a muzzleloading smoothbore. In the case of a round ball, you can make oddball sizes work fine. This isn't a sport where you need bullets to be plus or minus .02 of a grain, and need to be sized down to the .001". There is a pretty wide array of ball mold sizes out there, and currently three good options for custom size balls, including buckshot. Patches are patches, no discrimination on bore size there. Wads are the one area it might matter, but it matters very little.
Really it comes down to as you say, can you make your own? We aren't talking about huge investments here. Let's pick an oddball size, say 17 gauge mentioned earlier, bore of .650". A quick search shows 17mm is .669", which would be perfect size for most wads. You can find a 17mm punch easy enough, and it doesn't cost that much. Combine that with a hammer and whatever you can find, card stock, felt, leather, etc. and you have a huge supply of wads that is no harder or easier to make than any other gauge. Even if you had to buy wads, you probably aren't going to have that much trouble using 16 gauge wads. As for a round ball, about the only sizes I can ever count on finding in a typical store are .440", .490", and .530". Again, this isn't high precision bullet casting here. All you need is a steel or cast iron bowl to melt some lead in, you can do it on a camp fire, or turkey fryer, grill, possibly a hot plate if it gets hot enough. New Lee casting pots could be had for around $50 not that long ago. Lee makes a .600" round ball mold for around $20 which you should be able to get to shoot reasonably well from a 17 gauge. If that's not good enough, you can order any size your heart desires from JT ball molds for around $50-60 shipped. And if you absolutely want to buy them, Track of the Wolf sells both .626" and .648" balls.
You can apply this logic to just about any muzzleloader within reason. I think it safe to say a 4 bore is not going to top the list of those looking for shooting round balls with an emphasis on accuracy and efficiency.
Since this poster specifically asked about round ball being a top priority, and small game like rabbit and squirrel being the usage for presumably shot, then the choice in this case seems rather clear. The smaller the gauge, the more efficient it will be with round ball. A 28 gauge would work just fine for rabbits and squirrel, and it should be top tier for round ball shooting. There's no reason you can't load a 10 gauge with only 1/2 oz of shot, and 35 gr of powder, but there's no getting around the fact that a 10 gauge ball of even remotely close size weights around 700-750 grains, and it needs a decent charge of powder to be accurate. They don't shoot that great with 40 grain poof loads.
