This is a tough question to answer, but I'll try my best. About the only ones who could answer this are those who were/are military armorers or military shooters and you have to realize that NM military shooters were not allowed to change stocks without their Armorer at least knowing about it.
Something else that has to be added to the mix even when we are talking about a real, G.I. M14 receiver is that G.I. fiberglass stocks will also wear over time and use in the bedding area. G.I. fiberglass stocks were not fitted with the steel stock liners that wood stocks have. M14 receivers stayed tight in the liner longer in wood stocks than say an M1 Garand wood stock that did not have a steel liner or an M14 fiberglass stock that did not have a liner. Now, even a steel liner would eventually wear down and loosen up from wear as well.
Most G.I. NM M14's had the stock liners cut so a layer of fiberglass would go between the liner and receiver legs in the front and back of the receiver legs. Now, I think Ted Brown has talked about some G.I. rifles with wood stocks that the stock liners were forced inward for a tight fit on the receiver legs without bedding compound between the liner and receiver legs. We didn't do that, so I don't have experience with that.
I have to go back to the 1970's to the time when we buillt NM M14 rifles without rear lugs and rack my brain to try to remember if we ever put a NM M14 in a standard G.I. fiberglass stock and shot it for testing against a glass bedded wood stock. I don't remember us doing it in my time, BUT they had done it before because they already knew the standard G.I. fiberglass stock had too much flex in the forearm vs a wood stock. Our shooters put one heck of a lot of tension on the forearm from sling tension and that screwed up the accuracy with a standard G.I. fiberglass stock a bit.
When that testing was accomplished, we did not have a good and inexpensive way to stiffen the forearm of the G.I. fiberglass stock. The Fenwall compound we used to glass bed the rifles in those days would not do it and even the then "new" bisonite would not firm up that forearm very well and quite frankly we hadn't figured out how best to do it. Some folks used to take aluminum flat stock and mill them so you could screw a long aluminum support to the inside of both sides of the stock. That was a lot of work and it didn't stiffen the forearm enough.
It wasn't until after Gail McMillan started supplying us with his excellent fiberglass stocks that we had a fiberglass stock stiff enough so the forearm would not twist under NM sling tension. So we never got around to "officially" doing much with the G.I. fiberglass stocks after that. But that doesn't mean we didn't do some unofficial work and testing.
Our Women Marines who shot on the team had to learn to lift and lug the heavy wood stocks or fiberglass stocks. Always had to give them credit for that especially in the Off Hand shooting because women naturally do not have the upper body strength that men have. However, for some of our Women Marine and some smaller Male Marine shooters, we had to use the wood stocks so we could trim down the grip so they could use them better. That led to some unofficial experimentation again with the G.I. fiberglass stocks that would better fit their hands. But it wasn't successful with the Bisonite glass bedding we had at the time. Also and what some folks may find strange, we were not allowed to use laminated stocks for Service Rifle competition until AFTER we had been using the McMillan stocks for quite a while in the early to mid 1980's. It required a specific rule change for Service Rifle Competition.
I don't remember exactly when, but sometime in the early 1980's, my brother was working at Mullins Machine Company in Richmond and he gave me a few kits of Marine Tex to try. They used it in their high speed cigarette machines and had done a huge test of all the major fiberglass bedding compounds available at the time. Marine Tex had handily beat Bisonite as to less shrinkage and superior strength against the pounding of recoil. It was developed for the boat repair industry, so it had to be virtually water proof and shock proof. It also had to have superior structural strength for that use. The only "problem" with Marine Tex is that you can NOT eyeball the quantities of the mixture or the glass will be soft. You have to measure it by volume and I learned to use metal kitchen spoons to measure it out. I could not use it on Marine Corps guns because it wasn't approved, but I used it on some stocks I worked on for some shooter's private rifles and some civilians. After a while and from their reports, I believed we had a significant improvement in bedding material over Bisonite. So I gave a couple of kits to the Marine Corps Rifle Team Armorers to try and told them what I had learned about it. At that time I was the Instructor of OJT's (Apprenticeship Instructor), so I could not do official testing of the stuff. Tim Fischer on the team tried it and began to really like the stuff. Even when we began using Titanium Devcon, Marine Tex was still our "bedding material of choice" for the base coat because the Devcon was expensive and harder to work with at the time. We would skim glass with the Devcon over a Marine Tex base coat.
When I got transferred from the RTE Shop in 1988, they were still using Bisonite on the Post and Station and Marine Corps Match rifles as well as the base coat in many of The Big Team's and even The Big Reserve Team guns. However, they were beginning to use more and more Marine Tex on The Big Rifle Team. When I got back to the Shop in 1994, the Bisonite had been completely replaced by Marine Tex for Post and Station and Marine Corps Match rifles as well as at least the base coat for The Big Rifle Team AND Model 40A1 Sniper Rifles and our special Match Bolt guns.
Now, I don't want to sound like I'm trying to take huge credit for introducing Marine Tex to the Marine Corps. Other people outside the Corps had tried it or were trying it when I brought those first kits in. If Tim Fischer on The Big Team had not tried it enough to test it, we would not have gone over to it so totally. And of course had Marine Tex not been so good, all of that would have been moot.
OK, I'm running out of room and still have more to tell, so it will be in Part II.