elgrin said:
Here we get into Physics, which is not only not a complete answer in Runequest, but which I have not touched since nineteen hundred and ££££££££. So if anyone can actually do this correctly, I wuld apprectiate it.
Kinetic energy, which is the damage-dealing part, is 1/2*m*v^2. So the velocity is more important that the mass. This then goes back to the force*time(Pull length) exerted on the arrow while it is accelerated. My gut feeling (as I cannot remember how to work it out) is that despite the greater mass, there would be greater increase in energy of the missile by speeding it up, i.e. increasing the pulling power of the bow, then by increasing the mass of the missile.
You have to be careful about equating kinetic energy, and specifically velocity, to damage. As you increase velocity you also increase the likelihood that the projectile will go right through the target. A smaller high velocity round may go right through a person, leaving a neat little hole. The important thing is that the body has not absorbed all the kinetic energy in the round. A larger slower projectile may have less energy, but if it does not completely go through the target ALL of its energy is absorbed by the target, and therefore it is likely to do more damage. Another factor is the the composition of the projectile. A teflon coated bullet will likely cut right through a person while a lead one will mushroom, again much more of it's energy will be absorbed by the target and far greater tissue damage will be done. Tumbling rounds, hollow point and hydro-shock rounds are all designed to maximize tissue damage. The velocity and total kinetic energy has much less to do with damage than how that energy is applied to the target. As I mentioned before, increasing velocity can actually be counter productive.
The same holds true for arrows. A broad head and armor piercing head will hit with the same energy, but the damage will be very different. Again, it has to do with how the energy is applied to the target.
Of course the old real estate adage stands true as well: Location, Location, Location. A half inch whole in the heart is bad ju-ju no matter at what velocity it was created. But that is really a side note.
But back to bows.
In my example with the Giant I was referring to a Giant using a normal 'Human' shortbow. In the RAW he would do 1d8+2d12 with a normal shortbow, and I still hold it would break before he could put that much energy into it (also, arrows are not THAT much longer than the maximum pull the bow is designed for, and he would have a hard time manipulating the tiny bow and arrow in his huge hands, but that is all beside the point). The tree trunk bow would surely have different stats than a human shortbow - especially minimum STR required.
Which brings me to my point that bow stats already take STR into account. Bigger more powerful bows require more STR and do more damage.
Short Bow, STR 9, 1d8
Nomad Bow, STR 11, 1d10
Long Bow, STR 13, 2d8
Dragonewt Bow, STR 17, 2d10+2 (from Monsters)
So it seems the bow stats include the STR required to use them. In the case of the Dragonewt bow an average Warrior Dragonewt has a 1d6 damage bonus and only needs to be slightly above average to have a 1d8 damage bonus on top of that. Why would they ever close to melee range?
So following the established pattern for bows a Giant Bow would look something like:
Really Big Short Bow: STR 30, 2d12+2 (and that seems low compared to the bows listed above) and the Giant still gets 2d12 in a damage bonus. Ouch.
The RAW actually work well as is for humans, but once you start getting into slightly bigger than human creatures (Dragonewts for example) they become unbalanced, and the bigger/stronger the shooter the more unbalanced they become.
I like capping the damage bonus at 1d2 or 1d4, saying that you can only push a Bow so far past it's 'normal' potential. Otherwise a Giant could still apply a 1d12 bonus to a bow that in reality would break.