thanks for that added info, i realize they're not British/RAF figures but it's all I've got so bear with me........
the original question posed i believe was did the
ammunition and guns carried by the .50cal rear turrets incour greater weight upon the lanc than the FN.20/120 series ammunition guns? I'll probably pass comment on the actual turrets as we go
so, standard ammo count for lanc's rear turret= 2,500 rpg, 10,000x .303 rounds in total for the rear turret, now including links and fixed ammo bins that's 661.5lbs
now the guns, 4x browning .303s weighed 92lbs
ammo and guns for rear turret= 753.5lbs (but bear in mind some of the official modifications to the lanc would reduce this very slightly)
and that's for 130 seconds of firing time, which even by day you'd be hard pressed to use up
OK the .50cals, the Rose Rice turret carried 335rpg and 2x .50cals, so using the rough figure 100 linked rounds= 35lbs, 35x 3.35= 117.25lbs, times that by two 'cos of the two guns= 234.5lbs of ammo carried, each .50cal weighed 64lbs, giving a ammo and gun weight of 362.5lbs, and that will give you 24 seconds of firing time, which by night is fine but by day you might want a little more, so what happens if you double the ammo count...
48 seconds of .50cal firing time plus guns= 597lbs, so even if you double the ammo count of the .50cals the all up weight is less than that of the 4x .303s, and it can be assumed the actual turret weights will be roughly similar with the Rose Rice and FN.82 and FN.20/120, the .50cal turrets will weigh more but certainly not enough to make up the 156.5lb difference, so the .50cals do offer a weight advantage........
but i believe this's in the context of daytime missions, for which you will be wanting a ventral turret no doubt? well this poses a problem... can you do without H2S in daytime is the question? because if you still want H2S the best we can do is a single .50cal or .303 sticking through the rear of the fuselage somewhere, which would at least be enough to discourage attacks from the underside, the other question is would you have an extra gunner to man the position? with the FN.64 it was the responsibility of the dorsal gunner to man the ventral gun, so let's try out a few possibilities assuming we're going on a daylight mission with a .50cal rear turret with double the ammo count and, to make things easy, no H2S
option one- a single ventral gun and no extra gunner
when carrying a single ventral gun typical load was 1,000 rounds with a .303, it would only be a few hundred with a .50cal, so we shall assume the RAF's being it's typical self and gives this lanc a .303 and 1,000 rounds- all up weight roughly 90lbs including mount and ammo bin- for such a tiny amount the bomb load would not be adjusted
option two- a single gun plus extra gunner
carrying an extra gunner was very rare but a gunner plus 'chute weighs exactly 200lbs according to the RAF figures of the day, so this option has a hefty 290lbs to it's name
option 3- FN.64 turret and no extra gunner
this option would completely dissallow the H2S system so take that into consideration. weight of turret= 122lbs, two .303s= 44lbs and the standard 750rpg= 99.5lbs giving a total weight of 265.5 lbs- that's less than the option with a single gun and gunner! but does mean no H2S and one gunner manning two turrets (which is what the RAF did when the FN.64 was carried
option 4- can you guess what it is yet? yes, FN.64 turret plus extra gunner
all up weight- 465.5lbs, and for a weight like this the payload would be reduced by 500lbs, but it's not like the lanc would miss it, but this option would again dissallow H2S, but is it needed by day?
also bear in mind as this lanc has a twin .50cal rear turret you've saved roughly 150lbs over the quad .303 turrets, so if you take away 150lbs from each of the above options
that will be the actual weight penalty............
and i can now see Adler and FB wondering how this will effect the centre of gravity, well i can't exactly work it out myself however i am looking at loading charts giving the moments and the lanc has travel limits of 41-60.6 inches aft of the datum point, which looking at some of the moments any of the options above will suffice, and if not there are, as in any aircraft, hundreds of other things you can adjust to get the correct loadings............
ummm, I've spent so long doing this i can't actually remember if there's anything else that was being questioned? if there is ask and you may receive........
so given those figures it's up to make up your own mind as to which you think was the best option, remembering any weight increase will cost you fuel economy........
lastly my sources- with the exception of the weight of the .50cal rounds all my figures are from actual RAF sources and would have been exactly the same figures the crews would have used to work out loadings in their aircraft so they are the gospel, however bear in mind i'm only 16 and am simply messing round with figures so it's up to you how you see the data, any input on what I've done here will be welcomed, as long as it's reasonable