I would like to make two suggestions to increase the versatility of your allowed two guns:

1. A single shot shotgun/rifle like made by New England Firearms/H&R or Rossi can be had as combination guns or additional barrels of different calibers may be able to be purchased. Since the frame part is usually considered "the gun", you might be able to have a single, very versatile gun with multiple barrels this way.

2. You might also look at the various adapter things made by a company called MCA Sports. http://www.mcace.com/ They would also allow you to get multiple use from a single gun.