If you've ever heard someone describe a car as sexy, you'll know what I'm talking about. The gearhead community has this phenomenon which runs the gamut from using sexually charged language about their cars to being photographed in lingerie with bits of their anatomy inserted into the tailpipe. So it is with firearms enthusiasts. At one end of the spectrum there is simply the locker room language common in male-dominated hobbies. At the other end it develops into an outright paraphilia.
Starting at the relatively tame end, owners, operators and enthusiasts of firearms will describe certain weapons as beautiful. A rifle that has been customized, brought up from the ranks of the merely stock into the fully optimized artform that it can be is often aesthetically pleasing. The user and the aficionado can see the effort reflected in the careful selection of parts, the time and money spent finding, buying and installing them. A rifle you use often becomes a very familiar, almost intimate thing. The tastes of the user are reflected in many ways, down to the wear patterns on the gun itself. With such attention invested in them, is there any wonder that affection can develop?
Like other complex machinery, firearms can seem to have a personality. Slightly unpredictable behaviors, conditions they seem to like or dislike, favorite ammunition, and so on. With extensive use, you get to know what your gun likes and dislikes, how to treat it. And in the same way as ships and cars are sometimes given feminine pronouns, so too are guns by some owners. "She likes brass-cased ammo, not steel-cased." Over time, knowing the personality of your gun can develop into affection, especially if the gun in question has proven reliable or has a special characteristic. Something that makes it yours.
All of the above is reasonably normal. People tend to assign personalities to inanimate objects, and to feel affection for them over long-term use. It's more difficult to figure out where this turns into a fetish or paraphilia. On certain corners of the internet where such things are prevalent, I've been conducting my own research. The weapons board of 4chan is one such hangout. While the answers to my questions are usually unhelpful to say the least, some stand out. Common themes are sexual frustration on the part of the owner, and other prior deviances. Some media which anthropomorphise guns in various guises are appreciated here, such as "Upotte!!". These probably contribute to the attraction.
Looking for reason in anything related to human sexuality is foolishness of the highest order. Perhaps there is also the strange attraction to destructive potential, or the nihilistic fetishization of death à la Zardoz. The term ammosexual itself was originally a tongue-in-cheek attempt to make gun owners into a protected class. It can describe zealous advocates of gun ownership and rights or individuals who have sunk to using gun parts as sex toys. Much of the time the term is used in a joking manner. That being said, it's a real fetish.