oh. whatever. here, I found a picture of a cannon in the anime. The second I saw it, I knew i should show you. This cannon, which is used by the marines, is rifled. Of course, that doesn't mean their "rifles" are.
I also snapped a pic of the rifles.
The ones the marines use are flintlock muzzle loaders with no rifling. The grip isn't very big. It's a wonder these guys can hit anything at all. Not only does it take a while to load, the "bell" at the end, and the lack of rifling would decrease accuracy. The grip is pretty small, so it would be difficult to handle properly, which would also decrease accuracy.
Perhaps these are just ment for shortrange combat, not as long range as true rifles. If you may have noticed, whenever I see muskettes and guns like these used, it's not the sniper position. It's pretty much just a bigger, clumsier pistol.
As for what Van Auger uses, he seems to use a blunderbuss rifle (yes, rifle). A blunderbuss isn't what I'd call accurate, but it is one of the more deadlier of older guns. A blunderbuss pistol would clear the deck pretty quickly back in the day. However, the rifles had a pretty nasty kick to them, so you'd have to really steady yourself and hold the gun by your hip, or somewhere else that can take the kick.
http://netlinkenterprises.com/prodimages/22-1053G.jpg
Most blunderbuss's's are pretty big. Even the pistols. But, because of the bell-like barrel, it would be hard to use as a sniper weapon.
Of course, that bell on Auger's rifle might be there to only help with muzzle-loading. It would make loading easier.
Crickets gun is just a flint-lock, muzzle-loaded pistol. It didn't have rifling. That, combined with Sanji's speed would make him miss. From what I've noticed, most of the guns in One Piece aren't rifled, but some of the cannons are. There may be some rifled gun, but probably only on long-range guns.
Perhaps the main word used when talking about firearms in One Piece is "Juuhou" (which I have heard a few times, actually), which translates to "gun/firearm". Now, a long-barreled firearm, despite it being rifled or not, may be called a rifle by someone who doesn't know better. If someone translates something, and they come across "Juuhou!" and they see longbarreled firearms, they may just translate is as "rifle". The few things that have been called "Pistol" "Shotgun" and "Rifle" had the proper mechanics behind them (Luffy's attacks).
So, like in our world, the long-range shooters use rifles, the foot-soldiers use shotguns, some use pistols, and the cannons depend on what the cannon is meant for.