It's unlikely that those living in areas, such as Bangladesh, where rats are really a problem could afford a firearm, let alone sufficient ammunition, to deal with the problem. Even using cheap .22 bulk ammunition at around 3 cents a round, the cost in ammo to kill 80 thousand odd rats would add up rather quickly.
Traps are probably the best option, that way you can use the meat. It tastes better on a stick for some reason, but then it is said that most meat does.
There are anecdotal reports of 40 pound rats the size of medium dogs in some areas. That'd be a big rat.
Since rats like to frolic at night, one might want to equip themselves for after dark shooting if they were going on rat safari. A suppressed 10/22 and a PVS-14 would probably be just the ticket, perhaps in combination with an IR illuminator. If you use a PEQ2, it'lll actually cook the rats insides if you leave it focused long enough. (Fun with science, you can cook an egg in its shell with one.) Though since rats may be able to see IR light, that might be counter productive.