I think one significant concern is that flashlights are still not considered a hobby by most people. Most hobbies that have magazines devoted to them are already widely considered and accepted by others as hobbies (e.g. RCs, knives, knitting, cars, guns, shortwave radio, guitars) even if they themselves do not enjoy that specific hobby. If you told someone that your hobby was collecting knives or playing the guitar they might ask why you selected that particular hobby but they never question that it is a "real" hobby. If you tell someone your hobby is flashlights the usual reply is, "collecting flashlights is a hobby?
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Now, having said that, while one could argue that a flashlight magazine is not viable because the internet (e.g. CPF) will always have more up-to-date information than a printed magazine, one could pose the counter-argument that the same is true for most other hobbies anyway (e.g. knife forums are probably more up-to-date than knife magazines) yet hobby magazines still continue to be printed. So I disagree that the existence of online forums are a deterrent to the successful publishing of a magazine.
In my personal opinion, content is key. If a magazine contained some premier content (as a flashlight hobby example, perhaps an interview with Paul Kim of Surefire during SHOT Show, some exclusive photos of Fenix's coming lights or a behind-the-scenes look at 4sevens.com) that is not readily gained from an internet forum then it would have some possibility of success. Of course, to get good content you would need a team of reporters, editors, editorial assistants, photographers, etc. which would cost money. Then you would need layout artists, etc. to actually create the magazine layout. And of course, you need industry contacts so you are actually in a position to get the premier content that you need. And then of course, there will be the costs of printing (unless you do some online PDF only magazine and only use open source tools such as GIMP and Scribus to avoid license costs). And you need money for salaries for people because if you want the magazine to be successful then you will need at least a couple of full time people working on it and if you want good people who will help the magazine succeed, then you need to be able to afford good people.
Please do not get me wrong, I would also love to see some sort of flashlight hobby magazine but I think there is a lot that needs to happen before we will ever see one. I do not wish to sound so negative but I do know people who are in the publishing industry (my sister and brother-in-law work in advertising, my brother-in-law's sister has worked as an assistant editor in a widely published magazine and her husband has had his photographs published in magazines, and my wife worked for several years as an editorial assistant in a magazine) and my impression of the magazine publishing industry is that it is a very complex industry where everything has to come together just right (the planets all need to align on the 10th leap year of the century on February 29th at exactly 12 noon kind of stuff) or else the magazine will never even get a premiere issue published.