An SBS barrelled (14.5" entry barrel) Benelli M4 utilizes the commonly available steel OEM short mag tube, which is the correct component for the SBS configuration.
A few customers asked to be notified when "carriercomp" produced more SBS length mag tubes in titanium.
We completed a run a few weeks ago, but have not had time to list them in the store.
If intersted please let us know and we will promptly add the items to the category.
Being standard length they retain the OEM (shorter with reduced coil count) magazine spring.