[paul.fr]

A bullet or a harpoon?

[LeeE]

If neither of them float they will both end up on the bottom:)

More seriously, if both are fired at the same speed, the harpoon, having a much greater mass than the bullet, is likely to travel further.  Bullets aren't such a good shape for water either - the flattened rear end of the bullet results in a relatively large amount of drag for it's mass.

[lyner]

A harpoon goes much better; it is more massive and goes slower so the drag force is less. But the comparison is not really a fair test - too many factors involved.

[lightarrow]

I only add to the previous posts: an harpoon (in addition to having less speed than a bullet) has a greater mass *at equal friction forces*, because of its shape. A bullet could travel as much as an harpoon if it had a much thinner profile. The problem is how to fire such a kind of bullet.

[lyner]

It would probably be fairer to compare a harpoon with a small artillery shell. Then it would be more a matter of the initial speed.

[LeeE]

I guess we should really be comparing objects with the same cross sectional area but unequal lengths here:)

We'd need to also consider whether the objects are to have the same mass or the same density.

It that case, if they have the same nose profile and density but differing lengths, the longer object would have greater surface friction but also greater mass to play off against the friction.  With the same nose profile and mass I'd guess that there'd be little difference (the energy would be the same).  Length plays a large part in the speed of surface vessels (when they're not 'planing') but I believe it's much less important underwater where it's similar to aerodynamics and where the max speed of an aircraft is not limited by it's length.

[Schiz]

The real answer to this question is surely another question. How long is a piece of string(well rope). A harpoon is attached to a rope so it can be retrieved so even if they have the potential to reach the same distance, the bullet would go further as the harpoon would be forced to stop.
On a more serious note, a bullet could not possibly travel further, upon impact with the water a small calibre bullet would break up. However if you are attempting to fire the gun underwater, many guns won't fire at all.

[Madidus_Scientia]

I remember mythbusters did a thing on firing bullets into water, if memory serves me even high calibre shots only go into the water no more than a foot with any kind of significant velocity, then just sink. Their mass is just too tiny and they have too much drag. A harpoon would travel further through water by a long way.

[JimBob]

Actually, it is a matter of the speed of impact. The faster the speed of the projectile, the harder it hits. That is why a bullet will shatter, especially a high velocity rifle bullet, when it hits water while a harpoon with it slower speed will penetrate further. The Arrow fishing technique is the perfect example of this principal. You don't go around fishing with a gun.

(actually the best and most efficient way to fish is with a stick of dynamite. But since it bursts swim bladders, it is outlawed most places.)