If you want to be able to stab with it, then the core can't be too close to the end. But if the core is a long way from the end, then the foam points will break off because they're not supported.
Fabric glued in the place of core might help prevent tip breakage, but I dunno if it will be enough.
For stab-safe spears, I believe there's a construction method in the UK that involves using softer foam for the tips. The softer foam takes the initial impact of the stab, then is backed up by the firmer foam underneath. Coreless soft foam is less likely to snap than coreless hard foam, but perhaps more likely to tear.
I'm looking for instructions online, but here's all I've found:
I'm not entirely sure what that means. I think the thick leather is the puncture protector, to stop the fibreglass core pushing through (like arrow construction, except I'd use a coin for that purpose not leather). On top of it is some closed-cell foam, so that the protector has something firm to press against. The rest is open-cell foam. But how much of the length of the head is closed-cell and how much is open-cell? And what's this talk about thin layers of closed cell foam "top and bottom" to give it support? Is the spear head he's suggesting made of vertical layers of closed and open cell foam? Seems a bit weird.