... as Renrich pointed out, the projectile, moving with a speed close to the point when the filler explodes (except a solid shot) is not a good solution at all.
You could calculate barrel length, riffling, and best weight for a specific muzzle velocity but there is little sense for such a tool.
It could be useful as an easily portable, long range sniper weapon but here again, the low weight of the projectile would cause larger errors in the mean point of impact, all other factors beeing identic.