Could you legally plug an extended tube with a dowel so that the capacity was limited to 5 shots and be legal?

Bomber

First of all, there is nothing illegal about having an extended, >5 round magazine tube on a Benelli.  The sporting purpose/922r naysayers have a non-issue on their hands.  Benellis are imported, sold to distributors, then resold to the public with pistol grips AND extended mag tube AND in the case of the M1014 a collapsible stock.  That alone should clue you in that there is nothing wrong.  But to add, Benelli is a Type 8 importer that also has a Type 7 manufacturing license, the combination of which, when further combined with the fact they they have bonafide contracts with state and federal LE and gov't organizations for the products they import makes them exempt from the "sporting purposes" clause of the GCA, so the 5 round shotgun limit does not apply.  922r is about restoring an imported firearm to unimportable configuration OR domestic assembling an unimportable firearm out of imported parts. When a firearm is permitted to enter the country on  Form 6 in that configuration and actually enters, as Benellis are and do, neither applies.  This also applies to the FABARM shotguns that HK imports.  At a glance they do not meet GCA import requirements, but they do qualify under GCA exemptions because of who is importing them.

You may have a state law to contend with, so I'll answer to that:  Regarding modifying any firearm to comply with any federal law (import restrictions, barrel minimum length requirements, etc) regulations require the modification to be "permanent" meaning "not readily restorable."    If you wanted to modify a mag tube to meet a legal requirement as you say, you would need to do it in such a way that it could not be readily restored to hold more than the target capacity.  A dowel would be fine as long as you were to seal the mag tube to keep the dowel from being later removed.   Sealing threaded attachments permanently has very specific requirements like silver solder of a specified melting temperature applied to the threads, a blind pin or set screw that is inserted 90 degrees through the attachment and the threads to a certain depth and welded in place, etc.  There is no space-age adhesive that meets the "not readily restored" requirement.  Loctite is not even close as the melting temperature is way too low.  The toughest adhesive out there for metal work is "Rocksett" and it is also far from having an acceptable melting temperature .  Molten metal is the only type of "adhesive" that would qualify.

Hope this helps.

It does.

I'm confused because of everything I have read. I purchased the Benelli NIB from a dealer and it came with an extended tube and the pistol grip. It sounds like you are telling me this is a legal configuration. I always wondered about it because I bought it new in that configuration.

Bomber

oops

It is a perfectly legal configuration.  Not to worry.  They were Form 6'd and imported in that configuration.  I bought mu Benelli M1 Tactical with an extended mag and a pistol grip years ago.  Look at the Benelli website where they offer many shotguns that have all of these features.  FABARM shotguns too.