Hard to say if it's a good investment in my opinion.
If god forbid restricted owners get grandfathered, our restricted milsurps will probably drop overtime just like 12.6 firearms - Or if say handguns are banned within cities like Toronto, many will hit the market at once and there are other possible political situations that *may* effect the values eventually. I often wonder who's buying Lugers these days, hasn't every 12.6 collector already got a few nice examples by this point? They will continue to drop in price, I can't see any 12.6 pistol as a wise investment. A few months ago I saw a RZM PPK on the EE for only a few hundred dollars.
For those who collected German WWII militaria, that market seems to be tanking. Prices simply got too high, dealers drove up prices out of greed and people just can't sell that stuff for what they've invested even after sitting on items for years. This is scaring other collectors from buying big items, and so many fakes has turned off new collectors. We could see a similar trend. I've noticed some members selling "all original" M1911A1's on the EE with very obvious problems, probably in hopes of fooling a new collector. I've also noticed at gun shows most East German re-build K98's are commonly being sold as and confused for all matching original examples.
-Steve