Which bullseye event are you wanting to compete in? ISSF or NRA?
For ISSF Standard/Sport Pistol:
Pistols can be any 22LR repeater (revolvers are allowed but not altogether practical) with a maximum barrel length of 153mm, maximum sight radius of 220mm and it must fit into a box with internal dimensions of 300 x 150 x 50mm. There are specific grip and other dimensional restrictions, such as height of bore line, but all pistols built for ISSF competition would conform to these rules. There is also a minimum trigger weight of 1000 grams.
For the NRA rimfire portion of the 1800/2700:
NRA Rule 3.4 .22 Caliber Pistol or Revolver - Any pistol or revolver using a .22 caliber rimfire, cartridge having an over-all length of not more than 1.1 inches and with lead or alloy bullet not greater than .23 inches in diameter and weighing not more than 40 grains; barrel length, including cylinder, not more than 10 inches; sights may be adjustable but not over ten inches apart measured from the apex of the rear sight to the apex of the front sight. Trigger pull not less than 2 pounds. All standard safety features of the gun must operate properly. Any sights, including telescopic, are permitted with the exception of those sights which project an image on the target. Any sighting device programmed to activate the firing mechanism is prohibited.
NRA Rule 3.6 Grips - The grip, or any part of the firearm, may not be extended or constructed in any way which would give any support beyond the hand. The wrist must remain absolutely free and visible when the pistol is held in the firing position.
No part of the grip or accessory may encircle the hand. Upward curvature of the heel of the grip may not exceed one inch ... Adjustable Grips are permitted as long as they conform to the dimensions as [described.]
