Both of my 300's shoot better with JSB Match pellets in either heavy or middle weight.
Don't discount aperture sights, once you find the front aperture size you like for the particular range and get your technique down these old rifles can shoot some tiny groups. I have two 300S's and a 601 and all have aperture sights. Just purchased another 300S that should be here Monday, it is wearing a scope but will probably end up with apertures.