I spent this morning writing a very long post about comparing the weights, velocities, and kinetic energies of different common cartridges. Including the .223 Remington used in the ar15, compared with a 7.62x39 that is used in ak47 and SKS rifles, and compared with .308 Winchester / 7.62x51 Nato in both 150gr and 180gr weights.
Comparing their velocities how much more than the speed of sound they are, and using an equation from Chat GPT to calculate the perceived volume in Db of the sonic boom produced by each one, as dependent on their velocity. And then calculating the audible difference in volume, how many times louder it would sound than each one.
I'm not going to waste everybodies time with all those pages of calculations. But the point is that a 55 grain bullet going 3,200 feet per second from the ar15 is sounding between 41% and 94% louder volume than the 3 cartridges I compared it to.
The .223 Remington cartridge with a 55gr weight bullet is extremely loud compared to most other common cartridges, due to the velocity being so high. It is a very sharp and loud crack, compared to a relatively duller thud from a more common slower bullet. So the point is it would be very clear to anybody who is aware of the sound that this was a .223 Remington from an AR15.
It is possible it could have been something else like a .243 which is similarly a very small bullet going at a very high velocity, but this is not as commonly chambered in a semiautomatic rifle that is capable of repeatedly firing so quickly. This is commonly in a bolt action hunting rifle.