Are set screw front sight bases just as stable as pinned on? Anybody use the Yankee Hill Machine kinds?
Hell no - not even close.
Generally the order for stability is (most to least)
1) Pinned with taper pins.
2) Clamp on
3) Set Screws
One company epoxies on their FSBs for their DCM Match rifles. It's stabil but it's not a easy on/easy off setup.
I would never use a set screw FSB on any of my rifles that I needed to run 100%. If I used a set screw FSB on a 'plinking rifle' then I'd drill dimples for the set screws to fit into (making them more like pins).
What Forest said
Whenever I get the money I will have many uppers. Lightweights, 24 inch varminters, Hbars, most freefloated, most flattops, but ALL will have standard A2 pinned front sight base. Even 24 inch varmint rifle.
Theres no better way to have a guaranteed front sight and the FSB hurts nothign but like 1 oz of weight. You will never see it through an optic other than red dots, and even then it wont hurt anything.