My primary weapon is the AR15 platform. I've buried 2 full length AR's. My secondary weapon was an HK-91. I've now swithced to the FN, so the HK is buried with 17 magazines.

If you can find SKS's cheap enough, they are good for caching. It's even cheaper cache bolt actions. If you use the FN, I would look at caching a couple of the Lee-Enfields in .308.

My opinion is to cache a weapon that uses the same caliber as your primary weapon. In my case I can't find any inexpensive 5.56 weapons so I use the AR.
The HK was a weapon that I already had so it was cached. Since the magazines are not interchangeable with the FN, I cached them also.