as I posted on the other gun thread, I open carry a Shield .40 in my center jersey pocket. Never needed to draw it (have only drawn once since i moved to the SW part of the country- inside my vehicle with a person who exited theirs and advanced on mine...they turned around and went back to their vehicle and drove off). It sits very snug and secure, I've emptied it (the ammo) and the jumped up and down in different manners to try to eject it, never budged from either the holster nor did the holster try to leave the pocket. Just a simple leather IWB holster, but works like a champ.
I too have issues with riding on the road, but I do my best to limit my riding to less traveled roads (sometimes unavoidable, I know) and weekdays when the schedule permits. Won't help you when the texter plows through you at 65mph from behind with no warning - that has happened to me once- should have been dead, no idea how I walked away- they were literally going 65mph and plowed right through me, slid the hood, launched off the windshield, then landed and stood up as soon as I determined I was (hopefully) ok. just a few stitches and good to go (sore as hell of course).
The MtB can be dangerous on technical trails but I'm smart enough to stay within my limits, ride mellow on rocky trails, and always wear my skid lid. Also a good idea on MtB rides to make sure you are on well traveled trails unless you are with a buddy...you get hurt bad, better not be stuck 6mi off the road with a broken body and broken bike, and no cell service. Have yet to really feel the need to carry on the MtB, as the only reason I carry is the same as the above poster who was run off the road, hopefully the only reason that you'll (also hopefully) *never* need it.