617 makes a great .22 gun for any purpose. Good choice!
I like the new Ruger SP101 8-shot .22lr also. It is more accurate than I am, but that isn't saying alot.
Trying to understand gun design is a great thing to do. From the original post I wasn't clear about if that was your intention, or if you were looking for a 'woods carry' semi-auto recommendation, other than the MK series.
So: summarizing what others have so eloquently brought up:
Possible causes of semi-auto firing when unintended:
Trigger is pulled, thumb safety has slipped 'off', actuating sear, causing hammer to fall, striking firing pin.
Trigger is pulled, [no external safeties] actuating sear, causing release of striker/firing pin.
Solutions: a holster that covers trigger from brush, preferably with a thumb strap so brush won't 'lift' the handle/grip far enough while crawling for it to make the trigger snag on something.
Trigger is NOT pulled, thumb safety has slipped off, [see above]:
should NOT fire, except for rapid impact causing a 'possible' sear bounce. If that happens, it could fire: theoretically. in a worst case scenario.
Solution: keep gun in good working order, making sure there is a firm engagement and disengagement 'feel' to the thumb safety, AND get a holster that covers the thumb safety so that it can't get caught on clothing/brush to 'sweep it into the fire position'.
Thumb safety is on, but impact causes hammer to bounce off sear engagement hooks: Not likely. Hammer springs are rated usually between 14 and 32 [low end is 1911 with light reloads in a tuned 5" model, 32 is the weight in a new BHP] lbs, while sear springs are usually closer to 4-6lbs, I believe. If the sear doesn't move, it seems almost imposible for me to visualize how a hammer can bounce off the sear engagement lugs to the rear, and then move forward to fire the gun.
For the hammer to move forward, the sear HAS to move out of position. Thus, if the sear is 'frozen' in place by the thumb safety ON, the hammer should [in theory] be able to 'bounce' of the hooks and back on. Unless it BREAKS the hooks, and then the half-notch should catch it from firing.
Keep your gun in good working order and inspect it regularly.
The above does not apply to Star Model B handguns, where the hammer is blocked by the thumb safety. Nor does it apply to the Baby Eagle, Walther P38, Ruger P-89-97 series [and Beretta M9 92?], where the safety is part of the slide. See inertia firing regarding these designs.
Inertia firing: dropping the gun in such a way that the inertia of the motion causes the firing pin to move on its own, without the FCG being utilized: resulting in a gun firing while the thumb safety is on [or just no trigger pulled: I.E. A dropped gun].
Reality: not likely in a holster-unless you jump off a 5-10 foot drop onto a hard surface. And then, still not likely.
Theoretically impossible on some modern gun [with all design features operating properly] designs that incorporate a firing pin block into the slide design.
Some are trigger actuated [Modern Browning Hi Power, Colt series 80, Kimber's Swartz system, Sig Pistols I believe, Ruger P97, M&P, Glock and others].
Others are part of a slide-mounted safety system. I believe the Walther P38 design was mentioned as one that the firing pin block worked, as long as the safety was on. I don't remember if the Beretta was in that category or not. I think the Baby Eagle I owned [pre-2008 model all steel] was like that [baby eagle = IMI version of Witness, Witness = EAA version of CZ].
Basic rule: if the gun was
originally designed in
1980 or newer, it
probably has a firing pin block designed into it. Heck, the Browning Hi Power was converted to a firing pin block sometime before 1989. You can research when, but the original design was 1935.
Most Colt 1911s were designed in, 1911
No firing pin block in this original design: just stiff firing pin springs. Same with Star Model B, and I believe the Balistar Molina .45s.
The Ruger MK series goes back to the 1950s. The Buckmark is a modernized Challenger, which can trace its' history back to designs of the 1960s. The point? No firing pin blocks in these designs either.
Instead, they have a firing pin spring designed to hold the firing pin away from the primer until the hammer is fired. That way the gun is safe to carry.
I do not know as much about the actions on striker/firing-pin guns [Glock, M&P/XD/ etc]. I believe they all have a firing pin block built in, but am not sure. I believe it is
possible that a hard knock 'might' cause a M&P [or any of the others] to have the pin 'bounce' off the sear, but then the firing pin block stops the pin from moving far enough to fire the gun. So no accidental discharge.
I do NOT know if all conversion kits ALSO have a firing pin block built in, if the original design had one. I believe the AA Glock LE models do, but I could be wrong.
Sorry: this is all I've got.
One last thing: read gun manufacturers manuals carefully. I can't remember any manual saying it was ok to jump around with a loaded gun in a holster. I may have missed something. Of course, I remember more comments in rifle manuals about unloading from the chamber before climbing fences or trees.
I think the experience of people like Monty Stratton [back in 1938: check out the movie some time] encouraged gun makers to start listing this in their manuals early. His was a holstered handgun while crossing a fence. The movie made it into a rifle. Handgun references bounce between .22 revolver, .22 auto and .32 auto. However, the accident seems to be what you indicated you were concerned about.
This is in the semi-auto category, so I won't try to include any revolver references.
And everyone, PLEASE, if I have made an error, IDENTIFY IT. I am trying to explain the systems, the possible faults, and using what I've read here, in other posts, and my experience/knowledge to flesh this out. I don't claim this is all-inclusive, or exactly accurate. Point out inaccuracies for the sake of clarity to the OP. Thanks!