But I have to ask why do this to a 642? J frames are not range guns, and are not good for target shooting. It is an excellent, up close, point blank defensive revolver. Good at contact distances to 10 feet. Instinctivly points well, and shoots to point of aim. Yes, many can and do shoot them well at greater distances, but the snubbie's bread and butter is as a contact distance defensive weapon.
My recommendation is for the money of installing a new front sight, buy ammo and practice.