The light armed cohorts discussion and evidence

[Gaius Julius Caesar]

I would be inclined to agree with Crispus here. There are in fact more depictions of the 'ball' below the tang on the 'Pila' - for example, the Legionaries on the Adamklissi monument from Trajan's Dacian Wars are thus equipped. I believe the assumption that these were 'lead' comes by analogy with 'plumbata'. In fact they could be any material, from clay ( perhaps a little unlikely as being rather impermanent) through wood to Brass/Bronze, and just possibly Lead. Has anyone had experience of trying to throw a 'pilum' with a cricketball/baseball sized lump of Lead attached ? ( I don't think that stelae such as the 3 C Aurelius Mucianus one from Apamea show two such 'balls', but rather the tang's wooden surround with a single 'ball' below.) On balance, I tend to think the most likely candidate as wood, or just possibly, brass/bronze..... and whether the 'ball' was to add weight, or was just decorative, is an unknown......

I think it would be even less likely to be brass or bronze, with all due respect. we would more likely have remanents of them than a soft mallable lead ball.
And Yes, I do own a pilum wit ha lead weaight, and I can promise you, it is well weighted to throw , either directly in close proximity of the enemy lines, where it would most certainly punch through a shield into the opponens as well, or lobbedup over the sheild, or as John so apt points out, from a fortification down into the unlucky besieger assaulting your fortification.

Experimental archaology? Is it not a term oftem bandied about? I have no degree, but I have held, and marched 12 miles with one.
And I would say it would be guaranteed to punch through a shield, whereas I am a little skeptical about the unweighted ones to be honest.