Especially for the Witness, if you have one. When my Witness gets dirty, it will occasionally fail to feed, hanging up with the round part way into the chamber. This problem only manifests when it's dirty but I'd like it to work as well as my Glock, XD, and 1911. I have been using 1.255" for my COL and the problem exists with a couple different profiles of JHP. I haven't shot enough factory ammo to determine whether it exists with the factory ammo. I plan to try 1.250" but haven't had a chance yet. What produces the best reliability for you?

My 1911 is finicky about COAL length with most bullet profiles.  It does OK with my 200 grain cast bullets at any length, but for most factory JHP bullets it wants no longer than 1.250".

Both my Witness guns handle anything fine, dirty or clean.

I load most everything for myself to 1.250", but I have seen some factory ammo at 1.2405 (usually WNFP & 200gr XTP from DT was doing that to insure more reliable feeds) but also some stuff is 1.2400" and out to 1.2600".

Some of Underwoods loads are at 1.2520"

Tighter chambers will show failure to return to full battery quicker as chambers get dirty...

Would you say that, generally speaking, shorter COL should be more reliable?

Theoretically, a tight chamber should tolerate higher pressure and be slightly more accurate, right?

While pistols are accurate to a point, they are not so much like a bolt action rifle where putting the bullet against the lands or a few thousands off is going to make that much difference.  However bullet profile in relation to the seating depth can improve feeding in most guns.  Just remember that a semiauto is very dynamic process and subjecting it to varying impulse ammo does change these dynamics.  If the carbon and soot build up from the use of dirty ammo we can expect some issues.

Therefore I try holding everything to 1.250" as a personal standard for 10mm and beings I load & shoot 40S&W's from a conversion barrels I hold those to 1.125" and loaded to perform in any 40S&W pistol.

While it is nice to have some higher performance ammo, it isn't worth a hill of beans if it doesn't run like a sewing machine with out missing a beat! 

I reload, and the COL I use on 10mm is 1.260", with a 185 grain bullet and HP-38 powder.