"You can buy a 6" one like this at Harbor Freight for around $20."
Actually, we can buy those for $10-12 when on sale as they often are in Rifleman magazine and others. The H.F. calipers are exactly the same as those sold by reloading companies like RCBS for as much as $50. I have two 6" and one 12" Harbor Freight dials plus one 6" digital. All are within a half thou of my 6" B&S caliper (perhaps slightly better than Starrett/Mitotoyo) according to my Jo blocks and that's as good as calipers need to be. Anyone wanting better accuracy than a thou better be reaching for a micrometer, not a caliper of any brand.
Given that I can buy 12-20 H.F. calipers for the cost of one 'pro-grade' caliper I really don't care if they don't last a lifetime in a full time machine shop or not, that's not were I use 'em! And I'll cry far fewer tears if I drop a cheap-o and have to replace it myself. (Never again!)