Hmm. If you are really interested in an "investment", then the way to approach it is the same as for antique gun collecting. Buy a classic piece, in the highest possible condition you can afford, and hold it for 10 years. Yea, I know that you don't want to spend that kind of money, but that is the way its done.
For example, buy as close to a mint condition 1903 Springfield as you can find, all original, matching parts, and then put it in the safe and leave it there. Don't shoot it. If you can't afford that, then buy the highest quality1903 A3 that you can afford, and hold it.
Otherwise, you will just be buying low to mid-quality surplus, and trading it back and forth every few years, for very small profit, if any. There is nothing wrong with this, and I know that some people derive great joy from it. But it is not the way to create a high yielding investment.
If you are looking for some type of quick turn-around profit on pieces, in 3 to 5 years, it isn't gonna happen. For every one piece you get lucky with in this regard, I believe you will lose money or only break even on two other pieces. When averaged out you probably earn little or nothing.
Regards,
Mannyrock