I suppose you could charge a large(ish) capacitor up then dump it across the luxeon? Do you know the firing rate of the tank? this could be used to give some realism to the flash rate. I'd use a 'standard' LED, look on eBay and you can find some pretty impressive output capacity from small form factors with out having to resort to using luxeon, which also has quite a large form factor, to big to fit into the barrel.

Yours Simon M.