This bit of your text caught my eye : "There are many articles regarding the fact that when in the dark and shooting an airgun, without a bullet, sparks or a blue light could be seen. It was postulated that this was what made some tanks explode. If I read it right, that's what this article is mostly about."

I can't comment on pcp's as regards what I refer to by my made up name of "Muzzle Aura", but certainly many years ago when I regularly used my FWB Sport in the dark and when firing it there was a visible red glow at the Muzzle. I put this down to the rapid compression of the air on the compression stroke raising the temperature of the air being compressed, possibly along with the "Cardew effect" (lubricant vapour igniting). However, more importantly, when firing at Spinners in the dark, every time one was hit there was a streak of red or white light, due I believe to the pellet ricochet vapourising in part (or at least getting very hot) ?

Anyway, to cut a long story short, I could well believe that a fast moving tight fitting ball in a vintage PCP could well also generate hot particles of lead which would glow in their own right but also ignite lubricant vapour which I'm guessing could well be present in small quantities.

Just a daft suggestion that the C19 observations could well be observable now along with one possible cause (along I'm sure with many others with more science behind them to back them up).

Vic Thompson.