Plan on 40-80 fps, depending on ammo weight and pressure. Be aware that that’s within the normal variation for barrels that are the same lengths. The only two good reasons to choose 13.x is A)you want to use a 12.5” barrel, but have a 12” rail, which is also about 12.5”, or B)your only silencer is a Sandman S and you can’t/won’t SBR your gun.
As to the actual KAC gun, 13.7” is social-media popular lately, and that also happens to be the shortest barrel usable with their already existing 13” handguards. Its cutting it closely enough that it could interfere with silencer compatibility.
Rifleshooter.com has published a high-quality inch by inch test that you can reference. You’ll notice that lighter projectiles lose more velocity per inch than heavier ones. If you google “Labradar 1168”, you might find a thread here with some measured velocities in 12.5” and 14.5” guns.
Anything between 12.5” and 14.5” is pretty much the sweet spot for a general purpose carbine, with 16” being an outlying exception to that rule.
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