For reloading, any decent stainless steel 6" caliper is fine. Absolute accuracy isn't really as important as repeatability. The main dimension you would be measuring with this instrument is going to be overall cartridge length. Unless you are using a tipped bullet, the variation in the bullet's deformed lead nose is going to overshadow the variability in the tool. To really be set up for measuring case head diameter you'll also need a 0-1" micrometer. Again, absolute accuracy is not as important as repeatability. As a reloader we aren't measuring a case to fit in any gun, but for our gun only so if your tools aren't calibrated properly, it's not that big a deal as long as you only have the one tool. The plastic 6" calipers sold for reloaders may work fine if you are really careful with them, but I've always been hesitant to get one because it would be really easy to make a nick or gouge on the face of the jaws and then they are messed up for good. I got some mid priced stainless steel calipers years ago and also a Craftsmen caliper, both of which have been dependable. I don't recall the brand of the 0-1" micrometer that I use for reloading but it was given to me by a machinist. I also have a 0-1" Craftsman and a cheapo Chinese model. They all work for reloading purposes. Sure, I'd love to have a Starret set but don't really see the need for an individual to spend that much to just to reload unless you have the money simply want to.
In the above I was assuming the caliper would be a dial type. I've been to too many companies where the Quality department would not allow the use of digital calipers to accept product. Also, the only advantage I can see with digital compared to a dial type caliper is the ease of reading the measurement. Once you get used to a dial type, they really aren't hard to read at all.
