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The traditional M4 weighs in at 7.8 pounds unloaded. This new model with its rail and different stock weigh in at 8.4 lbs. Considering most of the new stock is polymer, my guess is most of the gain lives in that rail. I'd assume this rail is either 6065 or 7075 aluminum. So its a bit of a chonker. Looking at the rail closer, I don't think those are MLOK/Keymod sockets. It appears to have threaded holes for attaching Picatinny rail sections. In two years, I'll be at 20 years on this platform. I've been riding the struggle bus chasing a proper rail system all that time for the sole purpose of mounting a weapon light in an efficient manner. Nothing beats the ergonomics of the OEM polymer handguards, extra points if they're stippled professionally. Every rail system out there for the Benelli M4 has been poorly designed. Poor installation systems that require the use of tools to service the firearm. Terrible ergonomics where you're holding onto a Pringles can. Stupidly positioned Picatinny/MLOK/Keymod rail positions that have little to no thought process on why they are there. Failure inducing designs where the rail makes contact with the gas pistons. All add substantial weight to the firearm. All rail systems abandon the enclosed ARGO piston design that keeps debris out and gases trapped inside. This is why the IWC weapon light mount is so popular for the M4. It gets you a serviceable light mounting solution without ruining the firearm with stupid crap. Unfortunately the mounting solution isn't perfect either on 18.5" M4's. Most of these rail systems require that you remove all of your accessories from the rail in order to slide the thing off of the barrel assembly. So after you've locked all your screws in with thread locker to keep them from being shaken loose, you get to do it over and over again for general maintenance of the ARGO system.

Recently picked up my very first shotgun: SuperNova tactical. Ordered mag extension for it even before I picked up the gun. Shortly after that I discovered the whole 922(r), and the fact that it applies to all imported shotguns. Installed US made stock, hammer, and follower. There were only stocks with the riser available, so I put on rail for optics just so it looks like it belongs - no point for riser without at least an option to put red dot. The triple rail adapter on the mag extension is machined to match factory extension precisely, and is handy for putting a light on. With these slight modifications the gun does not look anything like what I picked up at the store, and it feels awesome.

I hate the attachment system for the rail, but it does offer excellent positioning for a weapon light. The fact that you have to remove your mounted items in order to separate the rail from the barrel assembly to access the ARGO system is retarded. Ergonomics are meh compared to the OEM polymer handguard.

Where did you end up on this endeavor? I think someone like Steve Rose is your only option on trimming the barrel. Unfortunately Benelli only offers a entry option in European venues for the M3. https://www.midwestgunworks.com/page/mgwi/prod/60045 this company Offers M3 barrels in both rifled and ghost ring configurations. The trick is going to be finding an applicable front sight in a proper height for the configuration. I was pretty hot to trot on a sbs’d M3 after seeing the factory configuration, with collapsing stock. Very adaptable considering you can pull the pin and have a pistol grip only configuration for breaching or a pack. curious if anyone knows what parts are needed on the newer style receiver to install the collapsing stock, that converts to pistol grip, if I were to buy one with a fixed stock. if I were to purchase the firearm with the applicable collapsing stock, I would have to source the shorter, standard magazine cap, with 5 shot extension. These seem difficult to source.

My M3 Tactical has no chokes.

Benelli factory barrels as I'm sure you know have "flared" muzzles, they get bigger for the Mobil or Crio chokes. If you cut one and re-choke, you'll be stuck with thin walled Tru-chokes or something similar. (only had it done with SBE, end result isn't awful, but certainly not as good as the factory chokes.)

Maybe I'm just, "once bitten, twice shy" but I'd hate to spend money on something only to find out I ruined a key feature. The M3 is one gun I personally would not SBS.

Benelli does make a 14.5 M3 http://www.benellidefense.it/products/m3-145-telescopic-stock But I have no clue if there are any additional considerations, like you mentioned, for the semi-auto cycling when the barrel is shortened. I wouldn't go past the 14.5 at least for a SBS

*cough*