whats beat up on it ?? I have a old moss 500AT ( thats whats stamped on it ) i've had it for over 20 yrs and who knows how long my dad had it .. maybe @meketrefe might have some info for me on its age / era ... i can't find anything on it .. 10 videos on teardown and still couldnt get the forearm slid off ..
this gun you have to unscrew the mag tube because of a spring welded to it ... i hated shooting it .. kicked like a mule with slugs .. plastic butt plate .. so after 20 + yrs .. limbsaver 1 inch pad , cut down the stock , sand and steamed out the dings in the wood and sanded some more , new stain and poly .. its a whole new shooting feel and love shooting it now .. I just can't see putting plastic on this old girl . ...
I think the mount on the side are so so no matter what firearm.
drilling and tapping the receiver is quite easy. It should take around 1hr. with a drill press and making sure you measure well.
on top you could tap with a slight larger thread for extra strength and even bedded including a hardened steel pin that will work
as recoil lug. But only if you plan to us a heavier scope otherwise not needed.
I have to do a rem 870 so I could put up some pics on how this is done but it is not rocket science. The main thing is to measure
well before you punch and start the holes the rest is just tapping and sing a song.
If one wants to bed jd weld works fine. total cost shoudl be below $10 that is better and cheaper. just the extra time for tapping.
oh... might have to add the cost of a tap if you don't have one but perhaps you can borrow one.
Can't help you with the grip question.
But I had a collapsible stock/pistol grip combo on my HD 500 for some time, and I hated it. The pistol grip makes operating the safety and the slide release very cumbersome. You have to severely alter your grip to run either. I ended up replacing it with the Magpul SGA and I couldn't be happier.
The Remington 870 controls with the cross bolt safety and slide release in front of the trigger guard are much better for the pistol grip.