Several 16" carbines from Olympic have 1 in 10" twist.
Mid-weight bullets from 50 grains to 69 grains seem to work well from 1 in 7" to 1 in 9" twist.
My humble advice, from an idiot like me who knows nothing,
I don't care if the other members may bash me for my idiocy:
My percentages may be off a bit, but generally
One-third of accuracy is the barrel, 1/3 the shooter, 1/3 ammo.
Generally, factory ammo accuracy is about the same for 16, 18, 20 and 24 inch barrel,
true, shorter barrel gives up a couple hundred feet per second, but still accurate.
Premium ammunition shoots better than bargain cellar ammunition.
Premium ammunition costs a lot, oh yes they are very proud of it.
There is no dirt cheap bargain-basement ammo that drives tacks.
Military surplus is seldom highly accurate, unless it was made for match/sniper use.
Premium hunting ammo is often surprisingly accurate, and often easily available.
Standard soft point hunting ammo is generally a little better than FMJ.
Nearly all FMJ rifle ammo is good for fun plinking, but hardly ever match-grade accurate.
In addition to the good advice from members on this thread, what I found personally:
Every Day Ammo bargain: Any 55fmj will help you learn skills.
Bargain accuracy ammo: Remington 55 grain CoreLokt soft point is often on sale.
Costs a little more but better: Federal Premium PowerShok soft point (not fmj).
Better accuracy ammo somewhat higher priced: Federal Gold Medal Match, Remington Premier Match, PRVI Partizan PPU Match.
[Disclaimer: I reload large quantities, so I very seldom buy factory ammo.]
If I were helping a shooter improve with his 16" AR15 carbine,
I say buy 500 rounds of cheap bulk ammo and use it to develop shooting skills.
Sling, stance, grip, breath control, trigger control, etc.
Go to an Appleseed shoot if you need instruction, those are FABULOUS.
After that, buy the more expensive stuff to see what you got.