The screw still takes shear forces, but it is actually strong enough to take them. A SHCS from a quality supplier like YFS is likely to be grade 12.9, which is ~190,000 psi material (off the top of my head). It's heat treated. The threads are rolled, so the grain flows correctly and continuously, rather than being cut. The head is formed with a radius at the transition to the shank. All the surfaces are burnished smooth, free of defects.
The Howa bolt stop screw is, to a fastener geek, a bit of an abomination. It incorporates multiple not-the-best practices, or "things to never do," in one part. It is designed with a sharp/square transition from the shoulder part to the shank of the screw, which is bad enough, concentrating stress. It is turned, not rolled and formed, so the grain structure of the metal doesn't flow around the form, it just gets cut discontinuously. The surface quality of the machining is variable. Some are smoother, some have worse grooving where the tool didn't leave it smooth. That's why some of the screws suck worse than others. I don't know what material it's made from or if it's heat treated.
If I was building a Howa/Vanguard to go after animals that would be very expensive to lose, or might take great exception to my attempts to kill them, I would probably upgrade to the M4 screw just to be safe. If I ever ran out of other stuff to do for engineering-like fun, I'd set up a test fixture and measure the force required to break a selection of Howa screws, as well as the M3 and M4 bushing concepts.
