Adrian,
 
Thanks for the great information and images.  I have often wondered how the feed mechanism worked for the Hispano 20mm cannon.  If you have any additional information or images/drawings on the feed system that you could post, I would be interested in seeing them.
 
Thanks.
 
Ron

Old thread, new info.......  Just got an orig. copy of TM 9-227 20mm Automatic Gun M1 and AN-M2
 
It states that the 60 round magazine " should be used with the M7 adapter and muzzle brake only. "
 
Also lists four different 20mm designations used by the USAAF:
 
Type A -- Consists of the basic M1 or AN-M2 Gun with AN-M1 Adapter, AN-M1 electric trigger, and M2 manual charger
 
Type B -- Consists of the basic M1 or AN-M2 Gun with M6 Adapter, AN-M1 electric trigger, and M2 manual charger
 
Type C -- Consists of the basic M1 or AN-M2 Gun with M7 Adapter (with M1 thread protector), AN-M1 electric trigger, and M2 manual charger
 
Type D -- Consists of the basic M1 or AN-M2 Gun with M7 Adapter (with M1 muzzle brake), AN-M1 electric trigger, and M2 manual charger
 
Still a mystery ... the 60 round can would be used ONLY with the Type D Gun, and to modify this type would require the changing of the adapter, as the M7 adapter with muzzle brake " is used ONLY with the 60-round magazine which does not utilize the recoil energy of the gun for its operation. "
 
The muzzle brake had exhaust ports that deflected a portion of the blast gases to the rear, absorbing about 35% of the recoil action. 
 
In other words, with the muzzle brake, there wouldn't be enough recoil to operate the AN-M1 feed mechanism of the 150-round magazine, so it would not just be a question of using the larger magazine once a plane got to its destination, the gun would have to be modified before the larger capacity magazine could have been used, in addition to requiring the installation of an AN-M1 Feed Mechanism (spring and recoil operated), which the 60-round magazine did not use.  
 
Maybe the factory only had Type D guns, with the understanding that the guns would be modified after delivery when parts (muzzle brake and/or AN-M1 Feed Mechanism) became available ?

SHAEF1944 American Veterans Museum

The Hispano-Suiza was originally designed to fire through a propeller hub. The unusually high profile of the drum feed limited the original gun’s service application. Recognizing increasing pressure from the U.S. Navy for large ammunition capacity wing mounting and early British success with the same, U.S. testing of disintegrating link belt feed systems began in May 1941. By December ’41 contracts were let to three firms for the production of Mechanism, Feed, AN-M1 and AN-M1A1 with an additional firm added in the summer of 1943 for a total wartime production of 119,216 mechanisms. Some early installations were obviously Type D s, but the widespread preference for feed mechanism led to the M7 Adapter, Brake, Muzzle, M1 and 60 round drum magazine becoming available by special order only. -Adrian

The Hispano-Suiza I believe was used in the earlier models, based on the MACR's I have gone thru on the P-38 so far about 500+ the main manufacture of the 20mm cannon used in the P-38 was International-Harvester, which I sure had made a majority of improvements over the orginal Hispano-Suiza design.
 
Terry T.