If you spend a lot of time on a sporting clays range, at 3-gun matches, or anywhere where moving long guns is something you need to do, Rugged Gear produces really nice gun carts for this purpose.
Rugged Gear has multiple models which can hold 2, 3, or even 4 guns on a cart. The carts are solid, made of 1″ diameter steel frame tubing, flat free tires, and neoprene lined gun holders.
The carts folds down for storage/transportation, and simply breaks in half to reduce its profile. You can easily take the wheels off if you need to reduce its profile even further.
As you can see in the picture, there is plenty of storage space for a range bag, cooler, ammo, and other gear.
Rugged Gear gun carts range from $259-$299. They also make other gear such as gun racks, ATV mounts, and archery mounts.