This is what I've learnt and take it all with a pinch of salt as I have no idea how to use this properly and have been experimenting.
You get a separate rubber piece (like a sucker) with a small central dish. You put your snuff (it will hold a pinch or two) in the dish part and use the cannon to snort the snuff. When using it in this manner I believe the flat end is used to pick up the snuff from the dish and the larger end is pressed to the nostril. I found this to place the snuff too far up my nose (into the back of my nasal cavity), perhaps I can get it to work better with practice. I believe this is the intended method of using this device.
Also you can unscrew one end, load it with snuff, screw it back on and sniff. Be careful as it will fire a large load of snuff into your throat this way. this I do not advise as I found it VERY uncomfortable.
My third and preferred method is to use it as a box, opening it and placing snuff inside and tapping onto my hand before sniffing.
The cannon itself is well made and sturdy but not suited to the purpose I was looking for (a snuff bullet) hence the 3 stars.