I would agree with most of the posts here. Don't buy for investment because firearms are too easy a target. I had a massive collection of Lugers and European .32's in the 1980's that became worth their weight in scrap metal with the OIC bans. Having said that, I have a crate of SKS's sitting in the expectation they will appreciate over time. If I could go back in time I would tell my 1986 self to buy a bunch of Swedish Mausers at $45 each and sit on them rather than trading them off all the time like I did.
The South American Mausers are often in lovely condition and are pre-WWI works of art. They don't seem to be appreciating much though because they don't have a "cachet"--definitely agree with the Hollywood connection. I think even Russian stuff went up somewhat after the highly fanciful "Enemy at the Gates" came out.
At the end of the day, if things are fairly cheap you could try to buy for investment, but in general, buy what appeals to you. ALWAYS try to buy the best example you can. Even if there were only 5 of some particular gun left in the world, a rusty, beat-up example with a pitted bore would still to be worth little. For milsurps, none of this stuff is being made anymore, and with tightening international arms sales agreements (unless you are the government and have some armoured cars to sell) it is going to be very hard to see any more coming in from anywhere. Russian items have great potential to increase in value, are not expensive now, ammunition is available, they are fun to shoot, have a ton of history associated with them, so they are a good bet. Swiss K31's are not horribly expensive, will likely go up, and are beautiful works of art to look at, play with, and shoot. German stuff in my opinion is reaching its peak--decent actual VBB Mausers used to be about $300 and now it is $600+ for a Russian shellacked mismatch. Not sure how much more they will appreciate. Franken-Garands are ridiculous in price. Long Branch Lee Enfields are excellent quality, are definitely more expensive now, but are worth it because they are often in excellent condition.
My main concern with the rising prices in some areas like Long Branch Lees is whether or not new shooters and young collectors will be able to afford to buy any of these things in the future?
Ed