Just look at the indentation in the seal in the lids. Some have more of a mark than any real indentation in the black seal. That means the lid is not pulling in far enough all the way around the lid to provide an airtight seal.

The other issue is the indentation in the seal may be on the very outer edge of the seal in some area of the seal rather than centered in the seal. This is because these cheap ammo cans made in China and Vietnam have sides that bow out a lot.

I put 1/2" of water in some ammo cans in which I discovered the color indicating rechargeable gel beads turned from orange to black while the beads in other ammo cans was still orange. I put the can upside down and notice water leaking out from a corner of both ammo cans.

In the past I put some rocks in ammo cans to weight them down and put some rock on top of them while they were placed in a 5 gallon bucket of water. I check for water inside the cans after say 15 minutes. This was a hassle compared to just putting water inside and turning the cans upside down to confirm leak.

Last year I examined about 50 of the 50 cal ammo cans at HF and only found 4 or 5 to meet my standards. At Rural King I when through more than 50 ammo can and only found about 2 to be acceptable. They sell them in tan, black and green colors. The green can were so consistently bad regarding the seals that I stopped checking them after see that 20 or more all had the same poor seal. The seals in the black and tan can varied much more. I suspect the green can were processed on a different manufacturing line or plant where the equipment was more out of spec.

I think the quality of the sealing of these cans has gotten much worse in the last few years.