As mentioned in a previous thread, a Benelli M4 followed me home from a gun show. I was a little perturbed about the 5 shot magazine capacity as the military version can hold 7.
I have searched and found a magazine extender. HOWEVER, I also found out why Benelli artificially limited the magazine capacity to 5. It seems that because it contains imported parts (duh, from Italy), it violates an import ban unless the magazine is limited to 5. And because of ATF regulation 922r, I cannot legally add the extender because it has too many imported parts.
Here are some threads that explain it in more detail.
Benelli M4 & 922R
ATF Response to Legality of Full Length Mag Tube on Benelli M4
The gun is still NIB and I am seriously considering selling it because of this limitation. Had I known about this beforehand, I don't think I would have purchased this firearm. The other option is to keep it and just use/enjoy it with 5 rounds in the magazine. Ugh.