lol.
I remember the instructors saying about 100 times before we went in:
About 10 seconds after the door closed, dude next to me is grabbing his throat, saying it. Sigh. Gas mask soccer game commences.
lol.
this is very accurate, though. the way we did it, everyone (a 30ish man platoon) files in with their masks on and stands in a circle around the pit that the gas comes from. the DI's can make it more or less intense, i'm not sure exactly how it works, but it's not like they just chuck grenades at you...although we got inert ones thrown at us all the time, and you either had to mask up super quick (for CX) or somebody had to dive on it (for frags).
anyway, everyone stands there, every exposed part of your skin burns, and it slowly creeps up loose clothing (like shirtsleeves). they'll talk for a bit just make sure you get a good coating of itchy burning on you. then you lift your masks temporarily. the smart people have a good clean breath in their lungs and do not open their eyes (i used the lower lip of my mask to conceal that i didn't open them). then you put it back on and clear it. i was 100% fine. others either breathed gas with the mask off, or didn't clear it right...rough fucking time for them.
one guy, a slightly older (than 18, at least) big beefy dude that was one of our squad leaders and generally VERY competent, panicked, threw a drill sergeant to the floor and ran out. mild hilariousness.
the rest of us stood in a line waiting to get out. one at a time, the drill sergeants stop you at the door, and you pull your mask completely off and MUST show them open eyes and breathing. again, the dumb ones did really bad here, and the DS's would keep them there as long as they could just to mess with them.
me, i took the mask off with open eyes and immediately sucked in a big obvious breath- and immediately began choking as if someone was standing on my throat. i got let out immediately with no questioning/fucking with, heh.
afterwards everything, especially your mucus membranes, of course, will burn badly for a bit and then just be an extended mild sting.
best advice you can possibly get- if you have to use the bathroom afterwords, do not, i repeat, DO NOT, TOUCH YOUR DICK.
oh, and yeah, the OP is some weak shit. they made up walk in a big circle for a while yelling 'MY ARMS ARE FLAPPING AND MY EYES ARE OPEN,' all the while trailing tears and snot to the ground. not a lot of vomiting, though.
edit- also we had a brick or concrete building that was EXTREMELY foggy. not trying to be e-tough, i'm just confused by that clear tent in the OP. i guess that's how relaxin' jackson does it.