If you have a proper machine shop in your area, they can determine for you the groove diameter.

Are you running the under sized bullets because that's all you have?
With your fired / not resized cases, that bullet will pass through the neck with no problems, right?

Patching could get them to fit.

Next time at the range, hold the rifle like you're shooting it in a hunting situation.
Good isometric hold like it counts.
You can rest the back of your support hand on a rest or tripod.