No ammo can lock is particularly secure. Ammo cans are made of thin sheet metal and the lids are held in place by thin metal rod, easily cut through with a hack saw, or even more quickly with a set of bolt cutters. My interest in locking an ammo can is to prevent curious kids from getting into something potentially dangerous. I even store household chemicals in some to keep them out of reach of the kids. Some locks degrade the air/water- tightness of the ammo can by drilling holes. My method is one step and avoids degrading the can itself. I use the 1/2" hole in the can's latch and the carrying handle beneath it to create a reasonably secure means of locking the can.

Materials needed:

50 cal ammo can (the dimensions of parts for this lock may also work on 30 cal or other cans)
Padlock with 2.5" clearance in shackle and 1/4" shackle diameter (this is a pretty common size)
7.62x54r steel shell case, fired/empty - Other shells may work but the case diameter must be slightly less than 1/2" and it must have a rim that is larger than 1/2". You could use a brass case but it would be much weaker.
Drill with 1/4" bit (or slightly larger than the diameter of your shackle) and a smaller bit (to start the hole for the larger bit)
A file to de-burr the edges of the drilled hole
Optional: A bicycle (or similar) cable lock to anchor several cans to a fixed object

Step 1: Drill a Hole in the Shell

Drill a hole in through the shell case. Your hole should be as close to the base as possible while still leaving space for the thickness of the can's latch. It should be centered across the diameter of the shell. You may want to use a small bit to start the hole and then move up to the larger 1/4" bit. Once the hole is drilled all of the way through, you may need to use a file to de-burr the edges.

Your lock is done!

Push the shell case into the can latch's .5" hole from behind and then run the padlock's shackle through the hole you drilled. Lift up the can's front carrying handle into the lock and close the lock on it.

Optional: Thread a long bicycle cable lock through the shackles of padlock(s) and around a fixed object to keep the can(s) from 'walking away'.