Gary Jones, Bob Forringer, Bruce Karn
see also Bob Forringer's video ……………………………………………………………………………………………………………………………………… 
The M129 Grenade Launcher was a redesign of the M75 grenade launcher that featured a concentric cam that reduced recoil and improved mounting. It had an improved rate of fire of up to 400 rpm.
Gary Jones: The forward mounted 40MM was a test system, which I think was developed by Hughes Aircraft and tested in our unit. I remember flying with the system installed on one occasion when I flew COL McGowan (Squadron Commander) after his C&C LOH was down for maintenance. We (I) fired the system from altitude at a hooch near the Bo Loi Woods, where one of the Mech Troops was in contact. I was able to walk the rounds in just like mortar fire. It was a belt fed system and worked pretty well except for the time I blew out the chin bubble on another mission and scared the crap out of my observer, Rocky Rhodes. We were flying through the trees and one of the rounds hit a tree branch just as we were flying by. The 40MM didn't travel that fast and we out flew the rounds. I had forgotten about the system untilI saw Jim Kreil's photo (below).
Bob Forringer: I found the 40mm launcher a very accurate, very reliable, and a very enjoyable system to operate. The round itself had a ten meter burst radius which would severely wound or kill anyone or anything with this range. I also found that the rounds would devastate any target hiding behind bushes or shrubs.
The major down fall of the system was related to our mission flight envelope, low and slow. The 40mm round would detonate on small limbs and branches between the aircraft and the target thus putting us within the five meter burst radius. I returned to base many times with my windshield and chin bubble peppered with 40mm shrapnel. Our maintenance personnel were not happy campers on these missions.
It is an excellent system when utilized at higher elevation reducing the premature detonation of the rounds. Shallow or steep gun runs would produce first round on targets with deadly accuracy. I believe the current Task Force 160th SOAR (Night Stalkers) have some AH-6 aircraft (Boeing Little Birds) with the system installed. Bob's Video
Bruce Karn: Then there was the short period of time that we had two experimental 40mm systems. They worked super but if the pilot failed to elevate it before firing it the rounds impacted right in front of the aircraft and normally take out the chin bubble on the left side. (The XM-27 mini-gun on the left side would do it too if not elevated prior to firing.)