The following is an excerpt from a military review of the M1014 specifications. It shows the M4 carries 6-3 inch or 7-2.75 inch shells. Some have been commenting on how they just were able to insert 7 shells.
The characteristics of the M1014 Combat Shotgun
Manufacturer: Benelli U.S.A. Corporation.
Caliber: 12 gauge, accepts 23/4" and 3" standard and magnum loads.
Length: 39.8" w/stock extended, 34.9" collapsed.
Weight: 8.44 lbs. empty.
Safety: Ambidextrous manual cross bolt.
Magazine Capacity: Six 3" shells seven 23/4", plus one chambered round can be unloaded without cycling through the action.
Trigger Pull: 5.5 to 7.28 lbs.
Buttstock: Modular telescopic with removable pistol grip.
Sights: Adjustable aperture rear and fixed post front, radius 23.7"
Maximum Effective Range: 40 yards with 00 buckshot and in excess of 100 yards with the rifled slug.