I concur with SF Mini or AAC Mini4. The Mk18 upper rules out any sort of reflex cans, but you still want something compact for that bugger. Micro cans on MK18's are still kinda niche items, so it comes down to a comparison of price against POI shift.
As you already eluded to, especially on that 16" unit, barrel droop will be a factor, but in terms of minimizing POI shift I suspect you'd still be ahead with the SF, the question then becomes if it's really worth the added cost to you to go that route.
MagicSalad had solid suggestions, but price of the SF would suggest that a middle of the road can that does both jobs might be what makes the SF affordable enough to look at (admittedly I'm not up to speed on the street cost of the Mini4, but I know it's a not as expensive)
عندما تصبح الأسلحة محظورة, قد يملكون حظرون عندهم فقط
کله چی سلاح منع شوی دی، یوازي غلوونکۍ یی به درلود
Semper Fi
"Being able to do the basics, on demand, takes practice. " - Sinister
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