That sounds about right. Here are some dB figures:
http://www.freehearingtest.com/hia_gunfirenoise.shtml
Table 2. CENTERFIRE RIFLE DATA
.223, 55GR. Commercial load 18 _" barrel.....155.5dB
.243 in 22" barrel...........................155.9dB
.30-30 in 20" barrel.........................156.0dB
7mm Magnum in 20" barrel.....................157.5dB
.308 in 24" barrel...........................156.2dB
.30-06 in 24" barrel.........................158.5dB
.30-06 in 18 _" barrel.......................163.2dB
.375 — 18" barrel with muzzle brake...........170 dB
That ranks an 18" .223 as slightly quieter than a 24" .30-06, so if you lop off that 18" tube to the 14.5" of an M4, it will likely be at least as loud as the 24" .30-06 (but perhaps not as loud as the 18" .30-06).