17, for me a 23" is about the max, and I really prefer the feel of a 20-21" bbl in handling, but with a stiff sporter, like a 3, 4 or Light Palma contour for steadiness.
M7 never floated my boat in precision, had a few, rebarreled one, cured it. Not a flyweight rifle fan, but like a portable rifle that handles well, short bbls have advantages in certain uses. Most of my uses do not require more bbl or speed, though I detest blast.