There are, as it seems, as many opinions as there are people to hold them about this topic, and any similar topic. I have found myself asking the same question among others; especially recently. I am a regular person with a regular job and plenty of dept. i know, typical "loser" american right? Anyway, I have only ever owned one or two "cheap" guns for plinking and basic home defense. Within the past six months, I have spent over three thousand dollars on tactical weapons and ammunition. I know that is not a lot, but for me it is a huge investment, or at least a great start. After carefully researching and asking those with knowledge and experience, I concluded that, as far as investment is concerned, popularity and availability are two of the most important factors for this kind of investment.
An "investment" can be looked at in more than one way. The obvious is the hope for an increase in value for the purpose of re-sale, or trade. I was also, however, enlightened to consider the infamous "SHTF" scenario. So, depending on how you look at it, this sort of investment may have a two-fold purpose.
Okay so more to the point. The most common small arms calibers used in our police and military is 12g, .40S&W, 9mm, .308 or 7.62x51, .223 or 5.56, and maybe even 7.62x39. As it was suggested to me by many, "think about it". In a "SHTF" scenario, these are the ammunitions that will be most readily available for obvious reasons. Everyone will be using them almost predominantly. So the next thing is, which guns to buy for these calibers? Among the top choices are any make/model of AR-15 or derivative including DPMS, Colt, Bushmaster, RockRiver, Smith & Wesson. Glock is a very reliable, cost effective manufacturer that is extremely popular in our law enforcement. Savage makes great high power rifles, and Mossberg is one of the top sellers for a 12g shotgun, particularly the 500 series. In fact the military prefers Mossberg over Remington in this area; I was amazed. Romanian AK's are a great choice for cost effective, accurate/reliable assault weapons.
The most expensive of all these weapons is obviously the AR's, with which you cannot go wrong. all of the other weapons can be purchased for under $500, most of the time well under that figure if you know where to get em. I have even found that older military surplus rifles, such as enfields in .308 or the SKS in 7.62x39 are a great pick for those calibers, and can be purchased for well under 200.00. Surplus ammunition for these guns can be purchased at a huge fraction from that of "recent make" factory ammunitions, and the ammo functions flawlessly.
Quantity is still a question to me. I really didnt buy guns/ammo for the intent to re-sell or trade, but for defense and "just in case", if you know what I mean. I don't have a rack full of any particular model, for example 15 DPMS AR's or something silly like that. But I do have a variety in different calibers. If I had 25-30k extra to spend on an arsenal, I would spend about half of it for such a purpose. Maybe 4 each: AR-15's, Glocks, Mossberg 12g, Savage 308's, and a stockpile of about 2-3k rounds for each caliber. And dont forget the kevlar, gas masks, food, water, and survival gear
*guns don't kill people; people kill people.