7.9 grain will do no damage whatsoever. You will have to experiment a bit to find which pellet your specific gun prefers.Both my break barrel Dianas (34 & 35) like the RWS Super Domes (8.3 grain) better than anything else I've tried. My fixed barrel Diana has more expensive taste. It thinks there's nothing to compare with a JSB Exact in 8.44 grains.It's all a matter of trial and error.
That is all I ever put in mine.