Being a 7.7 one assumes it's a Type 99?
That "gap" is called headspace. Isn't actually measured because it's a rifle manufacturing tolerance, only. Headspace just gets checked with proper gauges.
Pacific Tool and Gauge custom makes 7.7 Jap headspace gauges upon order for $37US each.
You can rent a 'Go' and 'No-Go' for 3 days for $7 from Reamer Rentals too. Probably better for a one time thing.
7.7 mm x 58 Arisaka Headspace Gauges - Pacific Tool and Gauge
7.7mm Japanese Arisaka Headspace Gauges
Put the rifle in a padded vise then slip the gauge under the extractor(if you know how to remove it, that's a bit better.) and gently close the bolt. The bolt should close completely on the 'Go', but not close completely on the 'No-Go'. 'Completely' being the operative word.
Plan 'B' is to take the rifle and gauges to your local smithy. Do not let him tell you he must buy the gauges. You will pay for 'em and not get to keep 'em.