I am loading 180 gr cast FN bullets with from 8 - 9.5 grs of Longshot powder, OAL of 1.26 and all are showing signs of excess pressure. I am getting flat primers and deformed primer into the firing pin hole. Also, accuracy is terrible and even look to be tumbling. Gun is a Glock 20.
What am I doing wrong?

Cast lead bullets in an original Glock barrel is strictly a big NO-NO! A replacement barrel that has the regular cut or button lands and grooves is OK for cast lead. The polygonal (?) rifling in the factory barrel will clog with the lead and cause excessive pressure to build with each bullet fired and stripped down bore. The Glock 23's are famous for blowing apart with cast bullets and factory barrels.