Digital calipers allow for great precision when measuring groups and don’t cost much. Good units can be had for about $30.
To measure groups the right way, do this: Place a single shot on the side of the target and measure that with a caliper. Subtract that value from the edge-to-edge distance between the two farthest shots of your groups. Depending on the quality of the paper and the type of target backing, that calibration shot might be larger or smaller than the bullet caliber you’re shooting.
Another option is to use caliber-specific gauges that attach to calipers. Sinclair International sells these. They offer two models, both of which measure three calibers. One has cutouts for .22-, 6mm-, and .30-caliber bullets, the other handles .25, 6.5mm, and 7mm bullets. ($26 each; sinclairintl.com)