The barrel channel is 1 inch / 25mm diameter so anything under an inch would fit.
I guess ideal would be couple of mm each side for floating / cooling.
I had a 10/22 Target model in it which fitted snugly.
I hope that helps?
Lovely Boyds Thumbhole stock for Ruger 10/22 in excellent condition.
Lovely stock £185 new, so your's for £110 BACS or Paypal (plus fees).
Collect from Bisley or from SL6 or will send via tracked dleivery for £10