I've not had any problems with loose in cans without absorber packs and have had ammo stored over 10 years. Still have a tight vacuum seal when I open the lid. Ammo looks clean as when it went into storage.
I used to keep my handgun ammo in the boxes but after awhile it became a pain to keep track of so I put it loose in ammo cans. I can fit 1,100-1,200 .45acp in a 50cal can but it's heavy as hell so I keep them to 1,000. No clue on 9mm. My rifle ammo I keep in boxes unless it's shipped loose then it's put on stripper clips & put in ammo cans. I can easily fit 1,000rds .223/556 on stripper clips in a 50cal can w/ space left over for another 100-200rds if I want.
What I posted on arfcomm a couple weeks ago when the question was asked there:
About six months ago I posted about doing this. I figured out about how much I shoot each of my calibers on average and then
I figured out what kind of mix to put into a 50 caliber ammo can so that if I open the can I would run out at about the same time.
The primary reason for doing this was so that when I am out buying ammo I know what type I need most so I don't end
up with a ten year supply of 10mm ammo while the .308 is almost gone (which is what prompted all this).
I'm up to can number 7 and I'm still using some of my previous loosely stored stock (still have a bunch of 10mm ammo
without a home yet) and each can holds about 1400 rounds of mixed calibers in original boxes (except a few loose shotgun shells
I threw in so there would be some ammo for each of my guns if SHTF). Once a can is full, I put the date on it so I can rotate
stock.
I have the cans stashed all over the place between houses and shop and other outbuildings, so a fire or theft won't
wipe out my entire stock again.
It's been working great, and I even have an idea of how much each can is worth.
A few exceptions are my carry ammo which I keep seperate since I like to cycle it through
every couple months, and all reloading stuff which is being tracked seperately.
I dump the ammo straight into the can. Not like the thin cardboard box is going to help much IMO. I do have some shot shells separated in one of the cans but still no boxes.
I don't think keeping them in the can would present any type of problem. If there is a good seal on the can, not excessive moisture or exposure to other chemicals (solvents/oil) you should be good to go.