Played around with these for a few hours in the DA a while back.
We found two methods that worked "ok".
1. Dead 6 approach, aim a good ways above the bombers, fire at d1.2-d1.4. Yes, in other words you'll have been under fire from .50s for a while already. Depending on how much speed advantage you have over the bombers, you want to be farther away. I think with the bombers and fighter both doing 250 we were firing at 1.4 and the rockets would blow up and damage the bombers. They only kill on a direct hit though, but if they do hit directly sometimes the explosion will kill 2 or all 3 of the bombers.
2. 12 approach, aim above the bombers and fire at d3.0 to d2.5, depending on how fast you are closing. We didn't have as much luck with this approach, but it is possible to score hits this way.
Approaching from other angles is very hard.