I have never had my ammo weighed in the dozen or so times i've flown. All of my clothes and gear goes in a huge, locked pelican case. Within that I lock my pistols into a hard locking case. And all my ammo is in a factory ammo box (whether reloads or not) and inside of a plastic ammo can. And I always fly with about 400 rounds of 9mm, which by my scale comes to 12.5lbs with out the packaging and ammo can.
If I am traveling for just a weekend, I can get my bag to be less than 50lbs with careful planning and use of my carry on. If I'm gone more than one day then I say 'screw it' and pack that bag full to like 65lbs and just pay the overage fee.
I've found that by kinda over-doing it with the multiple locks and hard cases I get much less scrutiny. And the $50 for overweight isn't that big of deal. I've flown my guns and ammo out of San Diego, Little Rock, Vegas, Birmingham, Houston, Phoenix. Some were better than others but all seemed concerned with just two things, that the pistol was locked up and the overall weight of my bag.
Again, by being overprepared it puts them at ease, that I'm practiced at this and have my act together. Then they normally do to.