Greetings. Oznog of Amtgard here, I've been an LARP archery engineer for over 15 years and have done a lot of work with crossbows. Jikanta has been so kind as to make an exception to the forum permissions to allow me to provide a consulting opinion on this subject.
I believe the calculations listed on the BelegarthWiki
http://www.geddon.org/index.php/Crossbow_Rules_Development are incorrect due to a misunderstanding of the definition of draw length. I have put my comments on this in the Discussion area of the wiki article.
The math discussion aside, the important point is that a rule must multiply power stroke (string's travel distance) by poundage to be useful. Power stroke is just as important as the poundage is in determining the strength of the crossbow. Double the stroke distance, double the power. If you don't specify the stroke distance, you don't know what the power is.
We do not allow headshots in Amtgard, but we do use the exact same poundage handbows. 450 inch-pounds is about 2/3rds draw (or a 24 lb bow at full draw), has been shown to be safe and complaint-free at close range with well constructed arrows. Getting much below 400ip starts to slow them down enough that many fail to even notice them. 350ip borders on impractical for this reason. As I mention in the Discussion article, it is important to note that crossbow bolts have less shaft weight which lessens the job of what the impact padding needs to do.
Crossbow design is often done with fairly crude fiberglass or wooden prods. These can be less efficient and shoot at lower energies than the inch-pounds predict. I can say that the 450 ip we decided on was determined using what is probably the most efficient crossbow prod possible.
In some ways I can say that crossbows actually have a safety benefit. Handbows have the capacity for archers to reduce the draw at close range while crossbows cannot. This sounds like a benefit at first, but then you realize that you're relying on the shooter to actually do this and many don't do it with 100% reliability. Limiting crossbows to a lower power IS 100% reliable and this hard limit is generally desirable.
Incidentally, as far as speed is concerned- a projectile's energy is the mass times velocity
squared. Shooting an arrow out of the same mass from a bow of half the poundage will launch at 70.7% of the initial speed, not 50%. The impact energy will be half as hard like you'd expect.
I use 2.5" diameter arrows that are
very light on a very good 35# bow and have shot up into the 115+ fps range as measured on a chrono. Crossbow bolts are more in the range of 97 fps or so, but they are somewhat lighter due to the shorter shaft. With a heavy design, you might be down to 90 fps on a good bow, could be down to under 80 fps on a poor 35# bow.