Britthunter, the Wal-mart Federal loads would be considered heavy, I think. They are 3 dram equiv, 1 1/8oz loads. Worc may have other suggestions.
That said, I've found that my I-12 doesn't always work reliably with the Federal loads available in the 100 round packs @ Wal-mart, but it appears to be due to inconsistent crimping of the Federal shells and not the ammo itself. It puzzled me at first because my I-12 blew through several hundred rounds of 1oz loads, both game and target, but when I tried these Federal 1 1/8oz loads, the shell feeding from the magazine would not always feed all the way onto the carrier.
Someone else on the board posted a picture of the crimps on these shells in response to a question I posted. One crimp was nice and rounded and the other bulging at the end. However, with these Federal shells, if I hit the carrier release button and manually feed the round onto the carrier, it works flawlessly, so I know the ammo is stout enough to function the action, it's just that a bad crimp can cause feeding problems. Mind you that I do this while shooting clays, so I only have to worry about 2 rounds and manually feeding the next round isn't a big deal.
Just thought I'd post this in case you run into it.