hello,,wards part numbers can sometime be a tough nut to crack ,,your rifle may be a mossberg md 46B ,,thats the only western feild model thats listed in the cross refrence chart with a 491a,,i looked at gunparts parts listing for a 46 b figureing they just may have a pic but this model isn't listed with pics ,,you have to know who made it for wards before ya can try to figure out whats wrong,,it is a bolt action right?if so remove the bolt and check the fireing pin ,,when you try to fire it does the unfired case have a dent in the rim? this may be a round dent or a rectangular dent ,,if not your fireing pin isn't hitting the rim to fire the cartridge ,,try some light oil on the fireing pin ,,if it hasn't been shot in some time it may have dirt or light rust in or on the pin which is keeping it from striking the rim hard enough if the dent is very light ,,if theres no sign of a mark or dent then theres a good chance your fireing pin is broken or damaged ,,this is a common problem with 22 rifles if they have been dry fired alot,,,,but first off try to figure out who made the rifle for wards ,,you could look at different mossberg rifles online and see which one resembles yours by searching "mossberg 22 rifle"then look at 46B rifles and see if it looks like what ya have ,,heres a site for cross refrence charts and also if ya need parts
http://www.e-gunparts.com/dept.asp
bigcurt