The study I read is probably over 30 years old now. I don't recall the barrel lengths used, but the 5.56mm was probably 55gr from a 16" or 20" barrel. I don't think the barrel length will be an issue for this question however. What they found was that 5.56mm bullets tended to upset when they entered the drywall and then fragment. Consequently, there were often fragments on the other side, but not always, and very rarely did a bullet make it through. 5.56mm bullets in FMJ configuration get their lethality by having an early yaw (all pointed rifle bullets will yaw, or tumble because the center of gravity is to the rear of center. When the yaw occurs is very relevant to lethality because if it doesn't yaw (overly stable), it makes a small, narrow permanent wound channel through most tissues. With 5.56mm, if there is enough velocity, when the bullet yaws, the stress on the bullet is too great and it breaks and fragments at the cannelure. That is why it creates the amount of tissue damage it is famous for, and why it breaks up going through walls. That threshold is about 2,500fps, so it is easily in excess of that, indoors, from a 10.3" or longer barrel.
That study was part of the reason we saw "SWAT" teams transitioning from 9mm SMGs to SBRs in 5.56mm. Less risk of an over penetration that could injure someone on the other side of a wall.