Greetings from Georgia,
I think Quick summed it all up concerning a 7mm Mag in a Handi. There are some limitations when considering introducing some cartridges into a handi rifle. I would love to have a 300 Mag in a single shot Handi, but it is the magnum performance really needed, not for me. The .280 shoots the same bullet as a 7 Mag and is only slightly behind in velocity. I must ask, is the "Magnum" really needed to harvest game ? Assuredly not. A lot of people are lured into the magnum realm simply because it is faster and flatter than what they have now, not because their non magnum guns are inadequate. Yes, I have several bolt mags, but when I unlock my safe and remove my weapon of choice for that particular hunt, I always ask myself, " What do I need today ?" My choice is ususlly modest, either a .308 Handi or .280 Handi for the long shots, or if hunting close the Old 45/70.
Sure, the 7 Mag is awesome, fast, flat and accurate, but it just does not fit into the persona of a simple push button rifle like the Handi. If you research and study information on the .280 rem. I bet you will agree that it is quite capable of doing everything that you would ask of a 7mm Mag. Good luck and God Bless.
Dave