Hmm, well your choices are 1) sealed with O2 supply (then you have to lug around extra weight, and you've got the equivalent of a small bomb on your back, try youtube: oxygen tank concrete wall, first link, mythbusters) or 2) not sealed, and hope that you can filter everything out. Of course, the other side's job is to try to make it get through your filter anyways. ;) Any damage to the suit would bypass the filter, so let's hope the suit can re-seal itself pretty fast.
EDIT: actually, O2 tank and bullet play chicken, O2 tank says 'you wouldn't like me when I'm angry..' (http://www.youtube.com/watch?v=zUP_7SX9pgM&feature=topics) | Mythbusters O2 tank vs wall (http://www.youtube.com/watch?v=ejEJGNLTo84) | EDIT2: O2 tank for teh lulz (http://www.youtube.com/watch?v=UlbXF-lmDio&feature=topics)
EDIT3: Holy friggin crap (http://www.youtube.com/watch?NR=1&feature=endscreen&v=9lw_fhNAIQc) (patience, watch it all, it's worth it).
From the comments of EDIT3: (read after watching, I should probably make a thread for this) Hows this for a theory
Fresh aluminum when exposed to air or O2 almost instantly grows an oxide film. This is an oxidation reaction that produces heat.
The bottle had about 3000 psi trying to push out the valve. Attempting to remove the valve with 3000 or so lbs friction force on the threads exposes fresh aluminum which will then go through an exothermic reaction with the pure oxygen. Reaction rate doubles with every 10 c increase in temp which may have caused thermal runaway .
Just a theory.