IMHO... you could easily use a 9mm Carbine as a goto firearm... but remember, the 9mm, even out of a 16" barrel still has the trajectory ( basically ) of a 22LR round.
So right around 100yds is ( give or take 30yds ) it starts to drop fast... that is a limitation to me.
Muzzle blast from a 16" is so little that the Lantac brake made no noticeable difference in my 9mm carbine. Not enough gas to utilize it... again in a 16" barrel..( very curious if shorter barrels actually work better with that brake )
Also, some 9mm AR's can be fussy with SD ammo... like the wide HP on a HST 147gr.
I ended up polishing the feed ramps to get reliable HP feeding.
And as for more velocity from the longer barrel.... not really... just not enough slow powder in a 9mm to boost its speed much over a 5"ish barrel.
You get some... but not like a .357 Magnum does out of a 16" barrel.
http://www.ballisticsbytheinch.com/9luger.html
The 147gr hold their speed best, ( at 100yds ) and also show the least added velocity from a 16" barrel.
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