If you have a milspec stock and try to put it on a commercial receiver extension (aka: 'buffer tube'), it won't fit. Conversely, if you try and put a commercial stock on a milspec receiver extension it will be a sloppy fit. Most, (but not all) commercial receiver extensions have a slant back to readily identify them as such. Since you tried 2 different stocks that you believe to be of the milspec configuration(s), my guess is that you have a commercial receiver extension on your AR.
