I should have all three. I know I have the first two. I think the third one is the same as the first, but just a different stock number (you really miss playmodb when you want to use it ...).
I would use the word rifle. Rifling is the spiral grooves (lines) cut into the inside of the barrel to make the bullet "spin" (spiral) as it travels. This rifling makes the shot (bullet) travel straighter.
Rifles started to gain popularity during the beginning of the 19th Century. Muskets would have long barrels to help the bullet travel straighter but when rifling was introduced you could have a short barrel and it would be more accurate.
If you combine the two - long barrel and rifling - then you really have a tool that will shoot very accurately over a very long distance.
As for gun #2 the receiver (the tube on the bottom below the barrel) holds extra rounds. It's like a magazine on an assault rifle but does not stick down - it points ahead. As the gun fires, the empty cartridge is cycled out and a new one from below is chambered in its place. This is what happens with a pump action shot gun, but with a pump action gun it is manual. You can get shotguns that are semi automatic and even fully automatic (so they chamber the rounds by use of gas exchange - the round you previously fired has a lot of blow back gas (the equal and opposite reaction effect - as the bullet fires forward the action cycles the old round out and chambers the new one).