RE: Swiss K31 7.5x55
The best ammo by far accuracy wise is the Swiss GP-11 ammo, its cheap when compared to commercial ammo, but is expensive for surplus ammo. Its worth the price if you just want match grade ammo. My handloads are about as good as GP-11 ammo, its just an unescapable fact that GP-11 ammo and the K-31 rifle were made for eachother.
You can easily reload for the K-31 by necking up 284 win brass to 30 caliber by simply lubing the cases up really well and running them through a 7.5x55 full length die. I do this very easily using RCBS dies, though RCBS dies need a bit of adjusting as they are more suited for reloading for a 1896 or 1911 swiss rifle. Pay careful attention to seating of the bullets as the throats on these rifles are very short, and the land to land dimension is closer to 0.299" than 0.300" as normally seen on 30 caliber rifles. Use .308 winchester reloading data even though the case capacity is very close to that of a 30-06.