What is the advantage of the M-4 barrel over just a 16" barrel?
Why does the M-4 barrel have a step down in it?

Thanks in advance.

The step down is for mounting a grenade launcher.  

Thank you KD, I figured it was for mounting something fun.

Anyone know if there is advantage between the 16" and the M-4 barrels?

Quoted: Thank you KD, I figured it was for mounting something fun. Anyone know if there is advantage between the 16" and the M-4 barrels?

The 16" barrel has little more velocity than the 14.5" M4 barrel.

You could get the best of both by getting a 16" M4 profile barrel (with M4 feedramps like Colt 6920, LMT, Sabre Defence, CMMG etc.).

The M4 barrel looks better than the 16" because the barrel doesn't stick out so much from the handguards.

You could also mount a standard bayonet on the M4 but a special bayonet with longer handles is the only way for the 16".