Harry Sanders and Tubes have it pretty much correct. I have spent the last 30 yrs. working in the industrial water treatment business so hopefully I have picked up a thing or two. A residential RO that produces about 5-50 gallons of water per day will cost about $150. Depending on the quality of the incoming water, it will produce a water with a conductivity of anywhere from 10 to 75 micromhos (6 to 45 TDS). Generally this is good enough. Since I have ready access to ultrapure water I use RO water polished with a mixed bed deionizer that generally gives me less than 1 micromho. I would suggest that for gunsmiths that would use a fair amount of water to use a residential RO and polish it with a small throw away cartridge mixed bed DI. By using RO water as the makeup the mixed bed cartridge lasts a long time. Usually the resin in the mixed bed changes color as it become exhausted so you will know when to change it.
As far as using softening chemicals such as SHMP (sodium hexametaphosphate) I would strongly recommend against it. When heated to a high temp. or boiled, some of the SHMP will breakdown to orthophosphate and combine with the calcium hardness to form a sludge - hydroxyapatite. This certainly is not what you want when rust blueing a barrel as it will result is a white film on the barrel.
Also, the bottled water in stores that is labeled "Filtered" or "Drinking" water is pretty much what you have at your tap. If you are purchasing water look for the "Distilled" water.