112 Savage J Series Help

I have this rifle in a 243 heavy barrel single shot action. Id like to convert it to a 223 hb rifle so I bought a 223 bolt head for a model 12 short action but upon receiving it I can see its not even close . Its shorter in length and the firing pin sizes are different. Can anyone tell me what size bolt head I need and is it even available? Thanks, Rick

112 Js are long action with some mods to feed short action rounds better. the 12s are a true short action. the 110/112 .223 bolt head has not been made for a few years. Hard to come by used also. the only place I know of is sharp shooter supply the make a kit to adapt a short bolt head to the long action.