In WW2 the 100Mil or 100mph gunsight ring was a standard across all of the countries in the war. A con at 1000 feet(333yds) seen through your 100Mil ring, traveling at 100mph, would travel from the edge of the ring to the center dot in 1\3 of a second. That 1\3 of a second was also how long the .50 or Hisso 20mm took to reach 1000ft. Other guns took different times resulting in different amounts of lead. Part of the basis for many being confused by estimating lead for snapshots in Aces High.
Some players flying only one ride for a very long time simply get it visa repetition. Then promptly fall on their face the first time in a different ride with a gun that shoots it's round a different distance in 1\3 of a second. Some are naturals adjusting between rides. Some vets tell you to shoot leading with the edges of the windscreen. Some never seem to get it.
Below are modified screen captures show the MK108 30mm 300mph lead. The MG151/20 20mm 300mph lead. And the AN\M2 .50 and Hisso 20mm 300mph lead. I did the first three in a 109 as an illustration of how 1\3 sec is different for different guns. Then one in a spit for a different cockpit. The gunsight ring in both is 100Mil or a 100mph ring. Across both rides you will see how the differences in the velocity of each round type effects the lead allowance. And how players can become confused. I added a first blue ring around the 100Mil Revi ring to show a 100mph ring for 500m\sec and 720m\sec. This is to illustrate with slower rounds, your con will travel that much farther in 1\3 of a second.
At least you can see where the old AH snapshot gunnery rule of thumb "lead with the windscreen bars" came from.
Bf109 MK108 30mm, 300mph lead for a 500m\sec 30mm round.
Bf109 MG151/20 20mm, 300mph lead for a 720m\sec 20mm round.
Bf109 300mph lead for a 880m\sec .50cal and 20mm round. Red lines are the 720m\sec and 500m\sec. 
Sptifire 300mph lead for a 880m\sec .50cal and 20mm round. Red lines are the 720m\sec and 500m\sec. 
