Most commonly found on internet forums for first-person-shooter games set in modern times like Counter-Strike and Battlefield 2, the Armchair Infantry spends his day browsing gun sites (chiefly http://world.guns.ru) memorizing every factual statistic about every weapon known to man. In any instant, the Armchair Infantry can deliver to you every possible statistic about any of an obscure line of military-grade weapons that never even saw live combat.
Armchair Infantry are easy to identify. If they have an avatar, more often then not it is the logo of the United States Marine Corp. If they have a signature, it's almost always a Barrett sniper rifle, or a guy hiding in a bush with a rifle (that may or may not be a Barrett). Your typical Armchair Infantry will hide behind a veil of lies to 'reinforce' their point; the most common and effective being that they own several of the military weapons and are pure marksmen with them. Slightly rarer, but more effective, is the Armchair Veteran, who protects his ass with unearned respect by insisting that they've fought in ANY number of real-world conflicts: Iraq, Afghanistan, Kuwait, Bosnia, Grenada... If the debate is about the Vietnam war, you damn well know that these guys will crawl out of the woodwork, insisting that they served 4 tours in Vietnam. If it's about Kuwait, suddenly they tell you about Operation Desert Storm as if they were there firsthand.
When provoked, the Armchair Infantry will pull out a dazzling number of 'facts' that they've almost completely copy-pasted off another website. In a conversation about a military asset of some sort, they will be determined to convince you beyond a shadow of a doubt that they've fired/flown/driven whatever it is they're talking about. The goal being, of course, to convince you, someone who isn't hiding behind the Armchair Infantry persona, that they have much more experience then you (which they don't), and therefore are right, and you're an idiot.
In their FPS games of choice, these 'veterans' generally play as a sniper whenever they can, clinging to an emo-esque image of a lonewolf cold-blooded killer out for revenge, or some cliche bullshit like that.
The irony of the Armchair Infantry is that most have never even been around a real gun outside of Boyscout Camp. Even more aren't even old enough to enlist.
If you ever meet an Armchair Infantry, patronize and mock them. There's a slim chance that yes, they DID fly an A-10 in the Gulf War (because you know how many 40-year-old Veterans play FPS games to recreate a war they saw people die in), but there's a far greater chance that they're utterly full of shit.
Rather then the use of the term 'Armchair Infantry', here's an example of some in action:
"The accuracy and range of the rifle is way off. I have three and go to the range four times a week and I can put 20 bullets within a half inch of each other at 500 meters."
"I flew a Cobra in Kuwait and I can tell you that they do NOT fly like that."
"I risked my life for the United States and it's disrespectful that you guys got the turbine sound of the F/A 18 TOTALLY wrong."