Brownells

[hornetguy]

If you are making a single shot rifle or working on something with damaged fasteners and want it to retain some semblance of originality or period correctness then it's a problem.

Stick a Phillips or torx screw in the side of a lever action and see what that looks like.

Oh, I understand the need/desire to match what's on the gun, but in reality, a hardware store very likely is not going to have the right thread pitch, so having or not having a slot is sort of moot. I have to "match" machine screws at work on a pretty regular basis, but I'm fortunate in that most of my stuff uses more standard threads, either SAE or metric, coarse or fine. Finding a 6-48 screw at the average hardware store just isn't going to happen.
Many times I have to go to an online specialty place, like MSC or someplace like that. Occasionally McMaster Carr will have oddball sizes, but B&M hardware stores generally don't.
I just purchased a base for a red dot sight, and the manufacturer sent 3 different sizes/thread screws to try to fit nearly any drilled and tapped firearm. The firearm industry is not known for consistency between brands, at least in my experience.