FX Panthera .25 with 700 mm barrel

Hi,

Does anybody have found good accurate settings to shoot 34 or 36 grain H & N slugs with a FX Panthera in .25 cal with the 700 barrel.
I have the reg set at 125 bar,the macro wheel at 19 and the micro at 3 ,speed is around 297 m/s.
Three shots group well and then the 4 and 5 th shot wander of the reservation.

Can this be temperature related,or hammerspring tension I shoot the gun with a external bottle,the reg on the bottle is set at 180 bar.

Any thougts are welcome

Rgs

Beemer 1300