I have one of Gary's bore gauges and really like it. Can't go wrong. Buy a good one and keep it for years. Great guy to deal with too.
I use a Mitutoyo 209-111 and a .750 setting ring. It's probably not quite as accurate as a purpose built bore gauge (mainly b/c it requires a little technique/finesse to use), but it's good enough for what I've needed it for. You can usually get them on ebay for $75-100, and they work from .4 to 1.4 so it covers all gauges. Since constriction is a comparative measurement, it will give you a very accurate amount of constriction (bore - choke), but it's probably +/- 1 or 2 thou of you want to know the actual measurement of the bore or choke. There's a couple SPI ones on ebay right now for around 50 bucks.