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Cowl guns are not mounted to the engine, so only one gun out of three is mounted on the engine. The engine is mounted on elastic mounts (rubber bushings) to cut down on vibration transmitted to the airframe.
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The "light-as-possible aluminum mounting" may not be aluminium. It has to not only withstand the the recoil but hold the gun in place while the plane does a 6-7 "G" pull out or turn ( 22lb gun is now putting 132-154lbs of load on the mount) wing gun mount is usually attached to the spar/s. Some later mounts incorporated spring buffers to reduce the recoil load.
OK, I can tell you from eprsonal experience that good groups are possible out to 1,000 yards.
Actually the .303x4 or .50x2 was not there as backup in case of a jam, it was there to give the pilot options with what he engaged with, if you read through encounter reports considerable numbers of Spit pilots seem to fire the 20mm's alone then use the MG when ammunition ran out, the 4x.303 gave a far greater volume of fire and beaten area against soft targets when straffing, and I believe this is why they were often retained instead of the potentially more destructive 2x.50 setup!
put it in context and no wonder the aces said get in close, no matter either few aircraft that knew they were under attack were hit!
I often hear it said the vast majority of victims had no idea they were under attack!
I did see a report on the increase in the number of successfull attacks once the gyro sight became available, but can't find it at the mo, I understand it made a considerable difference?