So far this is all I have found which is not very definitive IMO.
Bomber Command Ammuition 0.303-in.
Between January 1942 and May 1944 the operational sequence for belting
0.303-in. ammunition was as follows:
7 A.P., 2 incend., 1 tracer (day or night)
On the 26th May 1944, provision was made to alter the current sequence, should the necessity arise, to enable a quick turn-round of aircraft for day/night operations, to be effectec:
Rear Top left: all 70% A.P., 30% Incend., no Tracer
Top right: 1st 500 rounds like above
Reminder 50% A.P., 20% Incend., 30% Tracer (Night)
Bottom left: All 60% A.P., 20% Incend., 20% Tracer (Night)
Bottom right: All like above
Mid-Upper and front: Both 1st 300 rounds, 60% A.P., 20% Incend.,
20% Tracer (Night)
Reminder 60% A.P., 20% Incend. 20% Tracer (Day).
===============================
Part two:
Night Operations:
In the event of more than 300 rounds being fired from each gun of the rear turret, the top right-hand gun was made "safe" leaving three guns available for combat. It was considered that under reasonably light conditions, the small percentage of day tracer would not interfere with sightning, in which case the top right-hand gun could have been used. It was considered unlikley tha more than 300 rounds per gun would be fired from the mid-upper or front turrets.
Day Operations:
The 300 rounds of night ammunition in the top right-hand gun ammunitions tracks were removed befor the guns were used operationally. The first 300 rounds of each belt in the mid-upper and front turrets were removed and the tanks replenished with daylight sequence before the turrets were used.
Tracer Ammunition:
The points infavour of the discontinuance of the use of tracer were summarised as follwos:
a). The very reald danger of the gunner "hosepiping" tracer in the direction (approximate) of the fighter, disregarding this sight altogether.
b). In view of the adoption on the zone method of sighting, tracer ammuntion only served to indicate line accuracy. In the corkscrew this value was lost.
c.) It was considered that the use of tracer ammunition was so distracting, that with qyro gunsight Mark IIC, the gunner could not properly track a target with the moving graticule.
=====================================
Part three:
Bomber Command planes with 0.50-in Ammunition:
By No. 2 Group from 18th September 1942 onwards:
40% A.P., 40% Incend., 20 % Tracer
From:
17th January 1944: 3 A.P.I., 1 incend., 1 night-tracer
10th February 1944: 1 A.P.I., 2 A.P., 1 incend., 1 night-tracer
29th January 1945: 4 A.P.I., 1 night-tracer
30th April 1945: 100% A.P.I.
===========================
After a combat of 7th August 1941, Kingaby (92 Sq, Spitfire Mk.Vb) commented that his ammunition had included no tracer, and if they had the results would have been much more positive.
====================
Tony Williams
also see also see
http://www.quarryhs.co.uk/BoB.htm
From my book ‘Flying Guns: World War 2’:
The RAF fighters armed with eight .303" guns differed from the usual practice by loading each gun with only one type of ammunition. During the Battle of Britain in 1940, the reported use was three guns loaded with ball, two with AP, two with Mk IV incendiary tracer and one with Mk VI incendiary (the “De Wilde”). It is not clear why the Mk VI was used so little or why ball was used at all; possibly there was a shortage of the more effective loadings.
RAF Bomber Command principally used .303" guns for defensive fire. In 1942 the approved ratios for loading the belts in four-gun turrets were 65% AP, 25% incendiary and 10% tracer (two-gun turrets had 20% tracer with the others reduced accordingly). Between 1942 and 1944 the approved sequence was 70% AP, 20% incendiary and 10% tracer. This was modified in May 1944 to increase the proportion of incendiary to 20-30%, and changed again just before the end of the war to 100% incendiary, on the grounds that the German fighters were too heavily armoured for rifle-calibre AP to have any effect, whereas incendiaries at least stood a chance of setting light to something. Fixed .303" guns for air fighting were loaded 50% AP and 50% incendiary; for attacking ground targets the figures changes to 75% AP, 25% incendiary. The .303" Mosquito FB nose guns were loaded with unmixed belts of AP and incendiary rounds; i.e. two guns fired only AP, two only incendiaries (this also appeared to be standard practice with Spitfire .303s).
and
An extract from 'Flying Guns: World War 2':
"There was a considerable debate (which continues to this day) about the desirability of tracer ammunition. It was generally felt to be useful in bomber defensive weapons as the sight of tracers curving towards them was observed to distract attacking fighter pilots. In fact, the USAAF even used a tracer round for bomber defence, the M21 "Headlight," which was specially developed to be as visible as possible from the front for exactly this reason. On the other hand, Air Marshal Harris disapproved of RAF gunners using tracers as he felt that this encouraged them to "hose" the target instead of using their sights. In night bombing, some crews were cautious about firing as tracers merely gave away their position; so were their opponents. The Luftwaffe used Leuchtspur during the day (L'spur) but Glimmspur (dim trace) at night. Upward-firing guns in Nachtjäger (night-fighters) usually used no tracer at all, although some pilots liked to include some Glimmspur.
There was also a debate about the use of tracers in fighter aircraft. Some argued that tracers were useful in correcting their aim, or even in firing to one side of enemy aircraft to persuade them to change course (which sounds suspiciously like a theoretical concept; if enemy aircraft were in range, most fighter pilots would sensibly be shooting straight at them). The counter-argument is that the sight of tracers flashing past them gave enemy aircraft instant warning of attack and enabled them to take prompt evasive action. It has been reported that USAAF fighter units in Europe which did not use tracers scored considerably more successes, and suffered fewer losses, than those which did, which would seem to settle the argument. On the other hand, tracers could help the pilot in ground attack or in judging any sideways drift.
One general problem with tracers was the impossibility of matching the trajectory of other ammunition types, because of two conflicting factors. First, projectile weight was usually lighter than standard, and reduced as the tracer element was burnt up anyway; also the gas generated by the tracer burn reduced the pressure differential between the front and back of the projectile and therefore reduced drag. Tracers were generally specified to match the aiming point of other ammunition at some specified distance and remain within set tolerances at other distances. Another problem in explosive cannon ammunition was that the tracer used up some of the shell volume and thus reduced the space available for HE."
==========================
I'll give those a go later.