Check this: see if the recoil tube is bottoming out in the stock. A couple of the early M81s I've looked at have had this problem. Use a long 9/16 or 5/8 wood bit to deepen the hole about a 1/4". (Be Sure it fits the original hole diameter) It's possible this can be a issue for the early semi-pistol grip Model 8s. Not sure, I've only had one.
The drill bit needs to be about 8-10" long, Don't use a regular twist bit, it can grab and cut too deep.
Have you tried to find 3/16-40 Tpi screws lately? How bout a tap? Don't damage your tang screws for a 1/4" of walnut.
