Pom-pom belt mixes and tracer ballistics

Did Royal Navy pom-pom belts use purpose-designed ammunition mixes like American bomber machine guns?

Usually not at first; belts tended to carry one shell type, while later mixes remained uncommon and tracer introduced its own ballistic complications.

Early pom-pom belts did not normally use the carefully patterned mixtures familiar from American .50-caliber bomber ammunition. A belt was more likely to contain one shell type, with mixed arrangements appearing only later and remaining uncommon. Tracer helped a gunner see the stream, but its burning composition changed the projectile's mass and flight characteristics as it travelled. That meant the visible tracer did not always follow exactly the same trajectory as the service rounds it was supposed to represent. The Royal Navy's concern was technically grounded, even if another service might accept the error because engagement ranges were short enough to make it operationally negligible. A 40 mm shell also has enough room to carry tracer material alongside its other filling, so tracer and explosive are not necessarily separate types of round.