I'd think mounting it sideways would be best - since you're attaching the mount directly to the body of the rifle anyway, you shouldn't have the usual issue of torque pulling the aim off to the side, and it would let you mount the rifle closer to the body of the robot, reducing the torque currently tipping the robot backwards.
I'd think mounting it sideways would be best - since you're attaching the mount directly to the body of the rifle anyway, you shouldn't have the usual issue of torque pulling the aim off to the side, and it would let you mount the rifle closer to the body of the robot, reducing the torque currently tipping the robot backwards.