Looks like beanbags are out... Still think that would have been the most satisfying. Heavy enough and they give a nice THUD when they hit. Also no bouncing around like a tennis ball would. I think you probably could have made pretty durable ones (they use them in those riot guns)

I can see why people would want to standardise on something like a tennis ball - but man are they lousy to throw. If the max weight is currently 3oz, seems like you need to find something around that size, and spongy/foam. Dog balls? You could probably still _make_ custom balls, but I don't think many people would want to put in the effort. The rules for archery actually seem a bit lax to me... can you imagine being shot with a 50lb bow and a (even padded) wooden arrow?

From: http://www.sca.org/officers/marshal/combat/archery/missile_combat_plus_supplemen...

OTHER MISSILE WEAPONS
General Rules:
1. A missile shall not contain any material, such as beans, sand, etc., which could enter the
eyes if the missile came loose or broke.
2. All missile weapons shall be marked with the name of their user or maker. For
interkingdom wars they shall also be marked with the name of the kingdom to make it
easier to locate the maker/user of the weapons.
Hand Thrown Missiles:
1. Hand thrown missiles may include, but are not limited to: axes, throwing knives, etc.
2. Hand thrown missiles shall not exceed two pounds in weight. They should have
progressive give similar to a legal javelin head. Lighter weight missiles may have less
progressive give.
Hand-Held Mechanically Launched Missiles:
1. Hand-held mechanically launched missiles may include, but are not limited to: hand and
staff slings, spear/dart throwers, tennis ball shooting crossbows. Hand or shoulder held
guns are not allowed.
2. Tennis ball missiles shall not exceed 3 ounces in weight. The balls may be perforated
with a 1/4 inch maximum diameter hole by drilling. To prevent their collapsing when
stepped on, they may not be slotted.
3. Darts or javelins used with spear throwers shall be of golf tube construction. The spear
thrower shall not be capable of throwing a golf tube dart/javelin more than eighty
yards. The spear thrower must not be able to pass through the bars of a SCA legal
helm.
4. The maximum range for three-ounce tennis ball missiles launched from a hand held
weapon is 90 yards and for two-ounce tennis ball missiles, 100 yards.