Thomas,
I have been looking at making my own AR butt tube to M-14 stock adapter. I will be using simple aluminum angle, with the AR pistol grip adapter welded on to the bottom. Not much machining, no fuss, no muss.
If you have the correct tap [ or can borrow it from someone we know who does have it ] then tapping the adapter for the AR-15 Buffer tube thread will always use the same size / thread. How high you pick to locate the buffer tube, and which buffer tube you choose, will be the major variables after that.
IMHO, the AR-15 buffer tubes ALONE are a bit too thin for a structural device supporting the weight, recoil, and potential abuse of an M-14. When they designed the first GI fiberglass stocks, they actually went and did bayonet practice with them on practice dunmmies, and had a set number of times the stock had to survive vertical and horizontal butt strokes. Not surprisingly, the horizontal butt strokes were harder on the stock.
So, if by MILSPEC, you mean we get to buttstroke somebody till the stock breaks, and record this number, I am all up for that. Wonder who we can get to volunteer as the dummy?
Personally, although thousands of ACE fixed skeleton stocks are successfully out there, and have stood up well, for my AR fixed stocks, I prefer the extra strength of the YHM design, which adds another tube around the existing buffer tube. I will be using this for my fixed stock M-14 / AR crossbreeding experiments.
As for FOLDING stocks, I have built about half a dozen of these, mostly on GI WOOD front ends, with Bulter Creek folders. I also have done one each Chinese wood stock, and one US GI Fiberglass stocked folders.
The only problem I ever had with a folder, was when one of my clients, who weighs about 280, dived into rollover prone with the folder extended, and the 3" thin /hard drywall screws came out of the VERY soft Chinese mystery wood. Fatter screws, and more epoxy fixed that one.
If you are planning on making the stock to use a folder, then check into some of the other designs [ the FN folder and copies are probably one of the best ].
However, if you want to use the vast array of SLIDING stocks that fir the AR-15 shorty buffer tube, then do whatever works fastest / cheapest /best [ choose any two ] for you.
I personally have never met a sliding stock that I liked, but maybe you can convince me different?
good luck with your R&D
LAZ 1