You may have seen them used with the stock fitted instead but that was essentially for Northern Ireland only. Should we have come down as a result of IRA action we could easily remove it from the aircraft and use it in the ground role.
That logic seems sound and would apply to most every location Helicopters are being flown in close proximity to enemy ground forces.
Yet, rarely is that done....the using of the standard Infantry style butt stock.
During my time on Chinooks we only had the aircraft style double grip type trigger assembly and the Huey Lift Ships had the same.
Huey Gun Ship Door gunners used the Infantry style as did the LOH (Loach Scout) aircraft as the Gunners aboard those two type of aircraft used the "Free Gun" method rather than the pintle mount.
In my Unit we were more prone to resort to Rifles, Pistols, and M-79 Grenade Launchers for self protection on the Ground but were prepared and trained to dismount the two sometimes three M-60's (all with double grip trigger assembly) for added firepower. Our real hope was a rapid arrival of other helicopters bearing QRF Infantry or arrival of Armored Units that we were supporting when we were shot down or forced down due to a mechanical failure. We carried about two thousand rounds of 7.62mm Ammo for Door Guns so we would have had plenty to shoot. In our case...if we could get off a good Mayday Call....put out our location accurately....there was usually plenty of help nearby that could respond. The common practice was upon hearing a Downed Aircraft Call....everyone nearby turned and headed to the location until someone arrived on scene and took command of the recovery/rescue/reinforce effort and called off the need for further assistance. It was most comforting to know help was on the way....as it got rather lonely standing next to the aircraft listening to the tinkling of the cooling exhaust stacks and the pounding of your heart in your Ears....and no sounds of Rotor Blades banging towards you. It never happened when you had onboard a full platoon of Infantry that could act as your own personal Security Element.