I thought of annealing the cap and then reforming them over a longer form. That, or simply rounding out the closed end to look more bullet-like.
In doing so, we could fill the cap roughly half way - or whatever volume of lead gets to the right weight - leaving the skirt open.
My thinking is that the projectile would act as a minie ball does and expand out to fit tight in the bore.
Also, theoretically, the projectile will fly straighter such as a dart does since it is nose heavy.
Could end up about 450gr and be pretty fast to boot.