Reference your request to remove the buttstock of your Mossberg 500 shotgun.
I have been gunsmithing since about 1967 and I was our local Mounted Patrol police armorer for many years and I have worked on all models of the Mossberg shotguns thru the years and the buttstock is easy to remove if you follow this proceedure:
Get a long screwdriver (which you indicate you have) that fits the screw slot in the buttstock (I use a Craftsman screwdriver that I ground the blade to be slightly thicker on the end by grinding away part of the blade tip. The Craftsman screwdriver shank is square allowing me to fit a wrench on it for turning.) Then use an adjustable wrench, such as a cresent wrench, fit on the shank of the screwdriver to give you good leverage to turn the screwdriver as you press down on the screwdriver firmly while turning in a COUNTER-CLOCKWISE direction. If you place the shotgun receiver in a padded vise it is easy to get a firm grip on the screw in the buttstock.
The key to avoid stripping out the head of the buttstock screw is to grind the screwdriver tip to fit the screw slot frimely.
Some models of the Mossberg have a buttstock screw that has a hexhead and a slot to turn the screw. If yours has a hexhead and you have a long extension for a socket wrench, use that. You can get better leverage with the socket wrench than with a screwdriver.

After you have removed the scew, the buttstock can be removed easy by pulling on it. If it is rusty, spray it with WD-40 and clean off as much of the rust as you can, then spray it again with WD-40 before replacing the screw.

When you reinstall the buttstock, make sure you tighten the screw down firmly, BUT be careful so as to not strip out the screw slot. When you tighten the screw down, remember to push frimly on the screwdriver to hold the blade tip in the screw slot. If you are re-installing a wood buttstock, the screw needs to be tight to prevent the stock from splitting out under recoil of shooting the gun. I have seen many older guns that had split, or cracked stocks due to the owner not keeping the buttstock screw tight.
Remember wood shrinks with age and that allows the buttstock screw to loosen. So every year before hunting season, make sure to check the tightness of the buttstock screw.
That should give you years of good service. The Mossberg 500 series are good solid shotguns.