All directions are given with the picture above in mind.
There is a flat tab on the bottom right that is soldered in place. Grasp it with some vice grips or pliers and pull it away from the can. You can roll it up around the vice grips as you pull it out. When done, you should have a vertical strip removed on the right side (going top to bottom). Once the strip is removed, you can use the vice grips/pliers to start pulling the top off the can. Basically, you want to pry it off from right to left alternating between the top and bottom...kind of like you are folding down the covers off of a bed.
Just remember the weak point of the can is the solder. Pull on the aluminum to break the solder and separate the top of the can from the rest. Wear gloves...the aluminum is sharp.
BTW, where did you get the ammo?