The M63 is stable shooting "up" and the buffered mount helps. but it is an area weapon then, not a sniper rifle and does expend a lot more ammo to get hits due to movement in the cradle and mount in recoil. the gun fired off the tripod with T&E is sniper rifle accurate and Trophy wife will shoot the wheels off cars and trucks with it. The Naval MK 16 cradle ( 3 legged) is great, but you need to bolt it to very heavy weighted plate or big load of sand bags to prevent the same "tipping moment" . the Moment arm of the gun on the tripod pedestal/cradle provides a LOT of torque to the base and will cause the plate, plywood or even HUMVEE to rise and move around when firing. The MK 26 naval cradle is neat, but requires the removal of the spade grip back plate and at least on my gun a re-timing of the gun. The MK 93 uses the guns spades and trigger.
We were testing a gun and flex mount for a foreign contract helo application and part of the testing required dispersion tests in 5 and 10 round bursts at 100 yds. You would lock the gun in adjusted for a 4x4 ft target at 100 yards, and fire a 5 rd burst. We tried a mount in a USGI HUMVEE first - The truck moved so much that by the third round the bullets were off the 4x4 target. Next tried a 1 ton dodge dually (photo below) with a similar result. finally had to bolt a steel frame to the concrete pad and mount the cradle and gun to get adequate results.
The gun truck below was set up for a little demo shoot for a group of "folks" . I was running the M2HB on top and letting shooters fire off about 50 rds while others guys were letting shooters fire the 249, 240 and PKM. Targets were swinging steel plates at about 150 yds. The M2HB moved the truck so much that the other gunners could not hit anything while the 50 was firing - again - moment arm of the recoil acting on the height of the tuck over the axles. We had to trade off shooting time.
I have found that the MK 93 cradle with the Short riser used in a 50 tripod is a good compromise between laying in the dirt and being able to hit stuff with out extended bursts of expensive 50 BMG. No pic of that yet.