Buy a stainless steel 6" dial caliper. RCBS are about $40.00 U.S., Princess Auto are $29.00 CDN., those are the regular, non-sale, prices.
They are both the same, Chinese made, except the RCBS costs more for the name. They are reasonably accurate if not abused, and re-loading isn't precision machine work anyway, so if they are off by a thou. or two, it's not the end of the world.
Canadian Tire also sell a 6" electronic caliper, convertible from imperial to metric measurements, for $39.99 regular price, on sale fairly regularly.
Downside is your battery will power out when you need to use it, upside is batteries are cheap at any Dollar Store, so keep a couple of spares.
Check them fairly frequently using a known size something, say a 0.309" or 0.458" bullet for example, or whatever else you have handy, and this will also let you get a feel for how they work, avoiding any flex from pushing too hard.
I prefer the older dial type personally, but either kind will more than do for reloading, and it doesn't matter if it's for pistol or rifle use. Whichever you get, when not in use store it in the fitted case it came in to avoid dirt and/or accidental damage.