You can change the projectile of a weapon, but not a kit...since a kit doesn't have a projectile.
If you want 1 weapon only to have special properties you need to remove it from the rest of the level. Say you want the No4 (sniper) to be modded (shoot wespe shell). Remember, the No4 and No4sniper fire the same bullet.
In the maps init.con file, you need to change the allied kit for sniper from US_Scout to German_Scout, and US_Engineer to German_Engineer. This will give everyone the K98, and you can place the No4 rifle anywhere you'd like on the map.
From there you would:
ObjectTemplate.Active No4Projectile
ObjectTemplate.projectileTemplate <whatever>