With thanks to Guy & Lesley Field of The Yeaveley Estate https://yeaveley-estate.co.uk/ BotR and The Wirksworth Gunroom bring you the definitive, and quite obvious, answer to this strangely contentious conundrum. Simply by shooting the same course of fire in three different manners, with .303 Lee-Enfield No.4 rifles:

1. 10 round mag loaded to capacity at the start.
2. 5 round mag loaded at the start.
3. 10 round mag loaded to 5 rounds at the start.

How did it turn out? And did anyone get a rimjam (new PPU Prvi Partizan ammo, loaded down-up-down-up-down in chargers) that couldn't be solved by just slamming the bolt with a bit of élan? (Spoiler: no)