PA-28
Interested to know where you get a cyclic rate of 1200rpm, I suspect that to be a bit generous. Current day GPMG (piston operated) is 650-900 dependendant on gas regulator setting with an optimum of 750 rpm. The main limitation is the spring being able to fully compress under the the effect of the gasses far enough rearwards to eject the spent round and exert itself again with enough force to collect the next round to feed into the breech and strike the firing pin hard enough to cause the round to expend. This is especially prevelant using a delayed action blowback theory as I understand the .303 Browning to be. Curent Rifle and LSW (locking spline piston delayed blowback) also is around the 750rpm figure and even mechanical (chain) gun is still around the 625 rpm (Apache, although I fully realise that 30mm ammunition is far harder to shift around). To increase the cyclic rate, you would need to introduce more barrels (Dillon mini-gun M134) or start getting really clever with caseless rounds and computer generated firing sequences.
What I do not know is this.....When a BoB Pilot operated the trigger, did all 6/8 guns fire at the same time or was there a delay in the electrical circuit introduced to each gun. If this was the case, a collective cyclic rate of all 6/8 guns of 1200rpm would be believable. As has been said before, the collective effect of lots of .303 is greater than the single effect of a higher calibre weapon. I am sure a very clever man in the Wartime RAF mathematically proved this.