edit: also for very close work, it was always preferred to have something around 11" in length, with the 11.5" being a minimum without a 24/7 suppressor and useful out of the box without special tuning other than an H2 buffer. the 10.3 and various CAR-15's and CQBR type rifles and M16 shorties that used a suppressor could get down to that with a heavy buffer to increase dwell time with mass (since there's no length), or is used with a suppressor all the time and so the suppressor itself acts to increase velocity and barrel length. M855 and to a lesser extent M193 tends to work best above 2400, and 11.5 with no suppressor gets there (barely) reliably. under that threshold and it's less likely to fragment or even tumble significantly. this is why you don't tend to see this length used without a suppressor in professional hands. the exception is if you use boutique ammo that's very heavy (mk 318 for example which is sadly discontinued now) or bespoke 77gr SMK's at 2k f/s.
ive had a lot of good experience with an 11.5" and just basic M855 or M193 (and if you have M196 or M995 it still works great but has a bit of a hold over). we'd also get some neat stuff like 75gr BTHP from AAC back in day which i think you can just buy now rather than have special batches made. it worked great in shorty CAR-15 types. you can also "extend" a 10.5 with a phantom or troy medieval muzzle device too, or maybe try a blast diffuser or linear comp.
