Hk thought it worthwhile to offer the G3K with a 12.8" barrel, and when they developed the 417, they had a 12.5" barreled version as well.
I have a G3K "clone", why? I dunno, I guess I just like the way it looks, it is a little lighter, and with the telescoping stock makes for a pretty small package that can still reach out with some authority, further than I typically practice. I've mostly shot the cheap ZQ1 in the rifle, and only notice muzzle flash if I'm shooting near dusk, perhaps flash depends as much on ammo as barrel length? I started doubling up on ear protection after I got this rifle, its a good habit with any firearm, but I won't shoot the G3K without doubling up.
I understand(have experienced) the cons of a short .308, especially 12", so I get a lot of folks wouldn't be interested, but I fail to understand why everyone freaks out to the extent they do every time a 16" .308 thread comes up. As they say, YMMV.