Just curious what you guys aim for in general for ferrous/non-ferrous. I understand it'll depend on the particular part, material size, etc. In my case, I shoot for .001 with lathe or mill. Sometimes I get lucky too and somehow get .0005 once in a great while for a micro-second.
I can achieve a 0.0005" (half ten-thousandth) tolerance for a single operation using a little creativity with my benchtop machines if I need to.
For many hobby projects +/- 0.050" is usually OK. Very few hobby tasks will require better than +/- 0.010" accuracy.
For Gunsmith work, 0.001" tolerance is usually fine for the most critical operations.
Machinists are expected to make parts to a 0.001" accuracy standard and working to this standard usually requires 4 or more years of experience/training (8000 hours). Before you say this is easy consider a part with several bored holes, a turned radius and several machined pockets with a trapezoidal profile. Most students can't make a 1" square block to a 0.001" tolerance standard in one hour until they receive 3 or more full semesters of training. Fortunately, most parts fall within this 0.001" accuracy standard.
Tool and die makers are frequently expected to work within a 0.0001" accuracy standard and very few tool and die makers have less than 15-20 years experience as a machinist.
Tolerance is machine related as well... grinding machines easily work within a 0.0001" tolerance and better. To achieve high standards of accuracy requires a sound equipment foundation and machines with considerable mass to dampen and/or eliminate harmonic vibration. Very few benchtop machines can compete accuracy wise with machines that easily have 10 times the mass of the benchtop machines usually found in a home workshop.
Airguns are a gas