Thge ugly truth is that to be exactly right you need fitted screwdrivers. The easy approach is to measure the length and width of the various screw slots with a dial caliper and a set of marked thickness/feeler gauges. Go to Brownell's and look at their screwdriver bits and handles. Buy ones that are the correct length or a skosh under and a skosh wider than the slot. Also buy a grindstone sold for these bits - it is chucked up in a drill press of drill held in a vise. Gently thin the bits for a snug fit in the slots.
No offense intended, but if you can't do the above, you maybe should consider not taking it apart. It is oh so easy to burr a screw slot and oh so unsightly. Do you have a vise to hold the gun while working on it?
