When I was going through boot camp, we were put in formation and the drill instructors threw gas canisters at us and we had to put on the gas masks as fast as possible. That's about when the wind shifted and we didn't need to anymore, but we did have to keep from laughing at scrambling brown rounds.
See my first experience was shafted. I know the purpose of the gas chamber is to teach you how to put on your mask and seal it properly. My first experience was us in standing in front of the container, after jumping up and down, with them waving the gas in our face until we broke. Since I shouted my information, they let me go when I started twitching.
The funniest thing is when I got out, I wasn't affected like everyone else. I could actually see and didn't have a ton of snot coming out of my nose. They guy kept saying "follow my voice" and I kept replying "I can see you"....
Fortunately, when I got to my first BN, we did it properly and I put on my mask and didn't feel anything.