I agree with Tim - an old fashioned tapered screw causes the plug to open up and expand into the hole. A modern screw tends to merely cut a thread into the plug without opening it up. We have gone backwards!
Personally I have found that QS are one of the better screws for resisting head wear. However, you *must* use a bit that fits them well. SF own brand bits (Erbauer) seem to work well, as do Wiha and Vira. Ordinary steel bits are not really up to it - especially with the more powerful drills and impact drivers.
The moral of the story is don't reuse screws - or at least not more than once. Its a false economy. On the rare ocations that a head does get damaged mid way through driving, I will take the screw out and use another rather than persist in trying to drive a damaged one.
The old tapered screws cause a tapered expansion of the plug - with the taper widest at the shallowest point. They also cut a thread in the plug, but by their nature tend to destroy it again in the process of driving the screw further into the hole. I would say it is more important to drill the right sized clearance hole with parallel thread screws, but personally I have never had any problem getting a decent fixing with them into plugs.
(modern brown plugs tend to open up at the deep end more than along the full length of the shank - anyway, and so are well suited to modern screws)
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