Lots of privately owned guns, such as General Pattons gun have found their way onto the front lines however.
I recently read a memoir by a Vietnam War OH-6A scout helicopter pilot; he managed to bring a personally-owned 6" Colt Python to the front lines and sometimes used it to take one-handed potshots at the VC when he didn't need both hands on the controls.
The doors of these helicopters were usually removed in combat, so he could shoot out either side, but he usually fired right-handed because they circled targets clockwise. This was primarily done so hot shell casings from the rear seat gunner / crew chief's M60 machine gun wouldn't blow back into the cabin, but it also helped minimize the muzzle blast from the Python (the pilot in Western helicopters usually sits on the RH side, so a right-handed shot would involve holding the gun out the door).
Of course, the helicopter was also armed with the rear-seat M60, and most of them were also subsequently equipped with forward-firing Minigun packs; the writer readily confessed that he mainly used the Python for shock value, and he doubts he ever hit any VC with it.
FWIW the US Army needed helicopter pilots so badly during the Vietnam War that they bypassed normal officer training and created a warrant officer program; these folks went through the same flight school as commissioned officers but didn't get the same administrative and leadership training and were given the option for shorter deployments. Since the typical warrant officer was a "short timer" who just wanted to fly helicopters for 2-4 years and then leave the military- rather than use the deployment as a stepping stone to a promotion to Colonel, command of a battalion, 20-year career, etc.- the warrant officers didn't have the same incentive to do things "by the book" and often got away with things that no 2LT would attempt.