The reason why gunsmith screwdrivers are needed requires only to look at most used guns to see the chewed up screw slots.
Ordinary screwdrivers are wedge shaped and only grip a screw at the very top of the slot. The force against the top edges of the slot bend or chip the steel.
Another problem is, get a screw in a deep hole and a standard screwdriver will chew up the top edges of the hole.
A classic is the grip frame screw holes in a Colt Single Action that are in a deep hole. Try a standard bit and the finish is forever ruined.
A real gunsmith bit is straight on both sides and faces. This type of bit contacts the SIDES of the screw slot and directs the force to the strongest part of the slot.
Since the bit is parallel on all sides, it fits down into a deep hole without scarring the finish or dinking up the hole edges.
The Brownell's Magna-Tip bits are the industry standard.
Then make more sizes then anyone, and unlike most others, Brownell's are American made of the finest quality.
Handle a Brownell's bit and you'll note how glassy smooth they are.
Break or damage a Brownell's bit, and they will replace it FREE.
All you have to do is send an email about which size you need and they'll ship you a new one.
The best handles are the Brownell's "law enforcement" shorter handles. These get you close to the work for much better control and fewer slips then the bigger handle or the fairly useless stubby handle.
These handles come in magnetic and clip-retention. They're cheap, buy both types, you use both.
Finally, if you don't want a full set, buy a handle, then just buy the bits needed for a specific gun.
As example they sell a 4 bit set especially to fit S&W revolvers. Instead of buying the bits and handle, just buy the bit sets for the guns you own.
This is very much a case of "Buy once, cry once". Since replacement bits are free, you only have to buy one.
The first time you dink up a gun screw that may not even be available you'll wish you'd bought the Brownell's Magna-Tip bits.
Even if the screw is available, you'll have to pay more for the screw then the bit costs, plus shipping charges and waiting time.