Below is a copy of the rule section applying just to rests / bags in the 3-Gun competition (Sporter, 10.5, and 13.5).
For a full read on all rule sections go to IR505.com, and then the Rules Section.
As noted by Indiana James, the UNLIMITED event is just that, "UNLIMITED." Basically the only limitation in the UNLIMITED event is the gun must shoot standard .22 RimFire ammunition.
Hope this helps, Doug Miller
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"SAND BAGS
Front rest must be topped with some form of sand bag. Rear rest must be a sand bag with no adjustable or mechanical means. Sand bags must be made of leather and/or cloth. The front and rear rests will not be connected to each other, the bench or the gun. Only sand bag can touch sides and bottom of stock. When rifle is raised straight up, the front and/or rear rest must not rise up with rifle. If the front and/or rear rest rise up with rifle, the front and/or rear rest must be weighed with rifle to determine eligibility. Front and/or rear rifle rest can not restrain the recoil of rifle. Front and rear rifle rest must be placed directly on top of the bench with the following exceptions. Spacers may be placed under the rear rest to attain elevation. A coin, disk, bearing, or similar device may be placed under the legs of the front rest.
Nothing can be attached to the bench that would impede the movement of the front or rear rifle rests."