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frame, barrel, sights
Receiver, stock, barrel, sights
A barrel does not have parts per se. You can have sights (front and rear) on a barrel, but the barrel is usually one piece, sometimes two.
action, barrel, trigger,sights, hammer
Typically shorter barrel, different sights, larger magazine capacity. And black or dark gray in color.
frame, barrel, sights, grips
Best left to a gunsmith
Fixed sights, adjustable sights, target sight, night sights, 3 dot sights, telescopic sights, red dot sighs, lasers, etc.
Shotgun barrel that has a raised rail (rib) that runs the length of it. Here is an auction site with pictures of a shotgun with vent rib barrel. http://www.gunbroker.com/Auction/ViewItem.asp?Item=80157268 on a firearm it is a raised bar above the barrell
A Smithh and Wesson model 19-2 revolver with a 6 barrel with Target sights and hammer can range in price from $50.00 to over $200.00.
Your Hawthorne Warrior M820B is in fact a Mossberg model 340, of which there were various designs. A lot of the parts are available through e-gunparts.com. But, be careful, and ask a lot of question when ordering parts. A lot of the parts are not a direct fit. I have a Mossberg 340KA, and had to replace the barrel. I ended up with a barrel from a Hawthorne M820B. The sights on the Mossberg barrel were dovetail mounted, and the sights on the M820B barrel are mounted with screws. That's just an example. This will get you started in the right direction.
Will depend on the exact model, barrel, sights, and condition. A well used Standard Automatic with basic barrel and sights might sell for $100, a higher end Mk II with extended heavy fluted bbl and target sights $350.