As to the "autotrigger" issue, as a general rule the early guns and the M&P guns don't have disconnectors. As long as you are aware of it and act accordingly, practicing the standard rules of safe gun handling there is no problem.
Older guns are 2 3/4, but new production is 3".
"The rifle itself has no moral stature, since it has no will of its own. Naturally, it may be used by evil men for evil purposes, but there are more good men than evil, and while the latter cannot be persuaded to the path of righteousness by propaganda, they can certainly be corrected by good men with rifles."J. Cooper