1. The model 7400 is semi-auto, not pump action. The model 7600 is a pump action.
2. I strongly discourage you from purchasing USA mags. I have not found them to be any more than marginal quality. Besides, ten rounds is illegal for most huting uses. For other "non-sporting" uses, the USA mags are not reliable enough.
3. Mine was in .270 Winchester and shot very well. It was, however, somewhat ammo sensitive and had to be kept very clean and lightly lubricated, especiallyinthe chamber. The model 7600 is likely to be less ammo picky due to being pum-action, not gas-operated. Accuracy hovered about 2" at 100 yards with Remington 130-grain Core-Lokt ammo.
My $0.02 - Get the model 7600, not the 7400. Use the factory mags only. Put on a decent scope. Have fun!