To correct a forum post, DPS(Texas Dept of Public Safety) Texas Rangers are now issued SIG P226R .357sig pistols but by SOP can carry nearly any pistol or sidearm they choose on duty.
In the hit CBS TV series; Walker Texas Ranger, actor Chuck Norris(who is also a real sworn LE officer/reserve) packed a few sidearms. A stainless N frame .44magnum, a Taurus 9x19mm, a custom 3rd gen S&W pistol(which I think was either a .40 or 9x19mm).
See www.IMFdb.org .
Sorry about the misquote, I was watching some special on them a while back, about Texas Rangers using 1911's designed specifically by Colt. But honestly it was probably a pretty old rerun.
Honestly, most of the mainstream calibers will do you just fine.