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I'm a fan of, and own both. I've learned that I like my .300 shorter and 5.56 longer than 10"
Get a 5.56 can first. That setup you will get a lot more mileage out of, because it's cheaper to shoot. Effective to 100ish with nearly all ammo and well beyond that with good ammo that cost the same as cheap .300 ammo.
Why the 10" barrel for .300? The biggest benefit for .300 IMHO is that you can have very good ballistics, with supers, down to 7" or so. Combine that with a K can and you get something shorter and better ballistics than a 10" 5.56. Perfect for HD in my opinion. A 10" with most cans is right around the same oal as a standard 16" rifle. Not terrible, but a tad on the long side indoors.
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I've been eyeballing an 11.3" barrel.
I have 10.5", a 9", an 8.5", an 8.3", and a 7.5" in 300blk already.
For me, it is for the increased velocity and reduced muzzle blast/flash/recoil, along with more rail space.
As an example, 5 shots of Wilson Combat loaded 110gr Tac-tx chrono'd 2261 fps out of my brand new, never been fired 10.5" barrel. Since my 10.5" 300blk is my SHTF weapon, that velocity (coupled with that ammo) is good out to 200 yards (1735 fps, 735 ft.lb at 200y), and is still decent out to 300 yards (1501 fps, 566 ft.lb) where it within the full expansion thresholds. It is probably expanding out to 350 yards/+300 meters. That (for me) pushes the round beyond pure "HD/PDW" range.
My 9" is for HD (110gr V-max), while my 7.5" has a LAW Folder and is for a backpack (115gr Controlled Chaos).