I chose 16in and 12.5in as I feel these are the best lengths for the cartridge. The 16in for me is the most versatile. Its the minimum legal length with out giving up the ability to change flash hiders and breaks and try new cans ect. I feel that a 16in give me what I need in performance from a 5.56 rifle if I need more that what that offers I’m in an environment that its time to step up to a 7.62. The Recce rifle that later turned into the MK12 program started as a 16in precision rifle and does a great job as a precision weapon in 5.56. If I wanted a carbine as an assaulter or as a general purpose carbine then I’m a fan of the 12.5in barrel its the shortest you can get and still have good terminal performance out of a large variety of ammunition that is on the market. The 12.5 also gives you enough dwell time to be reliable with a wider variety of ammo and in a wider variety of environments it just has a wider operation envelope then the shorter barrels and is a GREAT all around length.
YVK- There are a few little tricks you can do to figure out the gas port size 
I’m going to reserve any accuracy guarantee for right now on the cut barrels not because of the chrome but because of the taper bore and the fact that you are removing some of it. The Recce will hold its accuracy better during heavier strings of fire being heavier it will absorb the heat better also a heavier rifle is much easier to shoot. The chamber in the Recce is designed for SMK ogive bullet set to magazine length and have minimum jump into the lands and grooves. It will shoot better with those bullets then the hammer forged barrels but for practical accuracy its kinda splitting hairs.