Upvote:12
WW2 Pacific Theater means US, British, or Japanese; let's start with the US. Long, solid, and pointy means anti-tank round, and it's capped. US anti-tank guns of 3" diameter means 75mm, 76mm, or 3 inch shell. 76mm and 3 inch would be overkill against Japanese armor, so probably a shell for the 75mm M2 tank gun. That cuts it down to an M72 or M61 shell. M72 is not capped, the M61 is. The M61's projectile is 13.2" long.
It is the projectile from a M61 or M61A1 75mm APCBC-HE-T shell (Armor-Piercing Capped Ballistic Capped High Explosive with Tracer). It is lacking its casing, which is good because that's a lot of gunpowder you don't have to worry about.
Because the ballistic cap is intact it is unlikely to have been fired.
The hollow bottom is where the explosive filler would go. It's empty so it's probably safe, but I'd still have an expert look at it. Perhaps call a local military museum.
Punching a 75mm hole in armor usually produced plenty of havoc and shrapnel on its own, and the explosive filler was often omitted.