Screwdrivers for gunsmithing is not rocket science. I've found as soon as the label "for gunsmithing" is attached, the price skyrockets but you get exactly the same products sold in other sets.
There are two basic types of flat tip screwdrivers ... one has a "tapered" tip, the other has a "hollow ground" tip. Taper tips are typically used for wood screws and should not be used for gun screws. Hollow ground tips are used for machine screws, like those found in guns. The only exception for guns that I can think of is screws used to hold a butt plate, which are wood screws that use a tapered tip.
With the popularity of hex bits, you can find high quality hollow ground tip bits and handles at very modest prices. Most are sold in a set but you can also buy the bits separately. I have a Craftsman's set that cost about $15 and included 8 flat tip hollow ground bits in varying sizes, #0, #1, #2, and #3 Phillips bits, plus T-10, T-15, and T-20 Torks bits. It also included a nice handle that has a "lock in" shaft that secures the bits without using a magnet. I've found the bit sockets that use magnets are a real pain because the screw will get magnetized, which can make inserting a screw into a hole a major challenge.
I would avoid the cheap Chinese tools (ie Harbor Freight) and stay with quality US brand name tools because the bits are hardened and don't end up twisting when you remove a tight screw.
The concept for the "right tool for the right job" is simple. Find a bit that is almost as wide as the screw head. If it is too wide, you will mar the area surrounding the screw and if the bit is too narrow, you will likely damage the screw slot. In addition to width, you want a bit with the proper thickness that best fits the screw slot. Again. if you use a bit that is too thin, you may damage the screw slot. You may find gun screws with very narrow slots. This is very common with guns such as a Browning A5 shotgun and many other European made guns. The solution is quite simple ... either buy a special bit or use a bench grinder and shape a thicker bit down to size. #8-10 flat tip hex bits are very inexpensive and serve well for custom bits. Rugers all use conventional hollow ground standard sizes, with the exception of SAs split tip screws in the base pin latch. If you are handy, you can make a split tip bit out of a donor bit. If not, Brownell's sells a special split tip bit for S&W rear sights that works perfect.