Reloading cartridges in the field is something that has been done by hunters and partisans for as long as we have had firearms. Modern soldiers and sportsmen have access to seemingly unlimited supplies of factory loaded ammunition, but handloading ammo while in the field is still a great skill to learn. You never know when the zombies may show up, or the Cubans paratroop into your town (see: Red Dawn).
For handloading while on the move, there are two kits made by Lee Precision that fit the bill. They both work, and both are inexpensive.
The first option is the Lee Loader.  The Lee Loader is a simple kit that contains everything you need to reload a single caliber (minus the components: powder, primer, bullet, brass). This kit allows you to use a hammer, rock or another heavy object to decap, resize, seat a new primer, add the powder and seat a bullet in about 30 seconds.