Bingley... if you look at the pictures, you'll notice a shoulder at one end of the insert...in a single shot shotgun, there is a step machined into the barrel into which the rim of the shot shell fits... it keeps it from moving forward into the barrel, and holds it against the breech face so the firing pin can strike the primer to set it off...the insert is machined of rifle barrel steel (4140) and rifled, so it is the barrel for the sub caliber... the .22lr is probably offset slightly as it is a rim fire and not a center fired primer...the accuracy would be similar to any firearm with that length of barrel...modern rifle propellants typically need 65 calibers of barrel length to adequately burn the propellant... most pistol powders do a pretty good job in 4"...