Haven't used Clamtainers, but they seem like they would be okay. I'd probably tape them closed to make sure nothing comes loose.
FWIW I've never had anyone say anything about ammo weight. I travel with my daughter and enough ammo for 2 shooters for a 2 day match. Most of the ammo goes in one checked bag, plus a bit in our pelican rifle cases.
I put ammo in blue 50 round MTM boxes and put a piece of foam inside the lids so the rounds don't rattle. I tape each MTM box closed, then stack them all together in one long row (6-7 boxes) then tape that so they stay together. Then I wrap the whole thing in a jacket or sweatshirt and put it in the checked bag. Never had an issue with TSA. A couple times they've opened the bag to look at it, but never unwrapped the tape or anything.