What is the m27 on cod4

The M27 is an assualt shotgun (gold plated) which is unlocked by completing the following tasks;
Unlocking the golden desert eagle (reach lv55)
Unlock the golden dragunov (finish all weapon challenges for sniper rifles)
Unlock the golden AK47 (finish all weapon challenges for assault rifles)
Unlock the golden Mini-Uzi (finish all weapon challenges for SMGs)

The M27 is largely agreed to be the best weapon on the game as its superlative range and high power make it perfect for clearing buildings and also firing shots down range, much like the mp44, weapon attachments are not available for the M27 and so the iron sights will have to be used and the weapon is unsuppressable.
The M27 fits under the category of shotgun after unlocking it.