Cold drawn 4140, 4130 are both reasonable to machine and quite strong as-is. They can be oil quenched and tempered if you need really hard. If you need really strong and still machinable, find some ETD150 or "stressproof" (which I think is the same stuff). 150ksi yield, but still tough, and machinable. Normal cold rolled/drawn stuff is pretty good too, just machine it symmetrically, or round bar to round part wise.
If you need corrosion resistant then that's another post ;-)
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