Steve Rose

That is the old style M2 20ga latch. It will work in the 12 ga M2 but it has 2 downsides. (1)The release post (that sticks though the receiver) is hollow and contains a spring. You cannot drill/tap it for an oversize button (2)The forward section of the latch is just a little longer so you have to push shells even farther into the mag tube to get them to clear the latch (farther in compared to the standard 12ga latch)

If my math is correct your barrel will need an OD of 0.840" to accept Mobil choke threading. On a related note I am looking for a set of 12ga Mobil choke installation tools (piloted reamer and tap). If anyone has a lead on a set of the tooling (Manson or PTG) in good shape there will be a “finder's fee” coming your way

Rose Action Sports can thread the factory length (long or short) M4 barrels for either Rem-Choke or Mossberg 500 / Invector choke tube threads.

Does the SBE 3 12ga use a similar (removable) top cover piece? Do the optic base tapped holes extend into the actual (steel) barrel extension?

That is the conclusion I've come to also. I have found one example of a small dot sight being mounted back on the receiver itself, but not much real estate back there for a longer optic base.

Turkey hunters (tight chokes, small patterns) are big fans of a dot sight on their shotgun. I should have a 20ga SBE in the shop soon to look over.

The SBE 3 20ga differ from the 12ga model in that the receiver top cover that slips over the barrel extension is a removable aluminum part (see the picture above). The 12ga barrel extension is one piece and all steel.

Is the top cover (over the barrel extension) on the SBE 3 20 gauge secure enough, and thick enough, to hold a small red dot sight if the top cover is drilled/tapped for a mount? Think Burris FastFire, RMR, Holosun size sight.

The sight is soldered on and will take A LOT of heat to remove. I have cut/ground off one of those sights before. Obviously this leaves a bare spot on the metal that will want some type of finish applied.

This riveted style is the way to go.

I can't answer on the BEST, but I've bagged several deer and even more steel targets with the Remington Low Recoil (1200 fps) 1oz slugs and an IC choke using my M2. I can say that combo was good enough for the task at hand.

When you close the bolt are you pressing the bolt release button and letting the bolt go forward under its own power, or are you holding the bolt handle and easing the bolt forward? If the Benelli bolt is not fully closed (in battery) the shell carrier will not move up quite far enough to allow shells to pass into the magazine tube. To illustrate the issue fully unload your shotgun. Open and close the bolt (letting the bolt fly forward on the empty chamber). Turn the gun upside down so you are looking in to the bottom the receiver (where you load shells into the magazine tube). Press the shell carrier all the way down and hold it down. Now slightly retract the bolt. You will feel the bolt start to ride against the shell carrier and slightly raise it up.

I've never seen any M4 barrels any longer than the standard 18.5" barrel.

Moving the M4 front sight and magazine tube ring back for the shorter barrel configuration presents challenges, not only in the remove/reattach processes, but also due to the smaller barrel OD in the mid section of the 18” barrel which affects sight regulation and mag tube alignment if not properly compensated for before reattachment, not too mention keeping everything properly aligned via a fixture while reattaching the parts. That smaller barrel OD at the new muzzle may also limit choke tube threading possibilities. They may be a shop out there doing the conversion and the folks here on the forum would like to know if there is. We get asked about converting the M4 barrel to Entry length and always decline based on the above reasons. The factory Entry barrel is relatively expensive and often hard to source but in the case of the M4 it is the better route.

Did you take the stock off to remove the Speed Bead? Was the action spring tube loose in the receiver, or was the stock nut overly tight and imparting a bend or bind on the tube?

The M2 barrels are interchangeable, same forearm and mag tube on all the M2 versions. If your M2 has a ghost ring rear sight keep in mind that the GR rear sight will not align with a vent rib sight bead.

Yes

From a legality standpoint that model can be used for duck hunting (as long as the magazine is plugged so the gun hold no more than 3 shells total). From a performance standpoint the GR sights, short barrel and lack of choke constriction will severely limit its usability for what most hunters consider an ideal duck gun.

Functioning / cycling will not be affected by the shorter barrel. Having the cut down barrel threaded for the original choke tubes might be an issue.....measure the OD of the barrel at the proposed“new muzzle” and see if the OD is considerably less than the OD at the factory muzzle. If you can post the new muzzle OD (to 3 decimal places) I can tell you more about the choke tube threading options.

As you've probably found in your research steel shot is a poor substitute for lead shot that only came about due to concerns and regulations with lead in the environment (ie, it started with waterfowl hunting). Steel is the less expense substitute for when lead shot is not allowed, but pales in performance compared to the other “non toxic” types of shot we now have available on the market.

Conventional thinking has always been (1) Back off one choke constriction with steel to achieve patterns similar to lead. In other words steel shot through a Improved Cylinder constriction will pattern like lead shot through a Modified choke and (2) Do not shoot steel through a Full or tighter choke constriction. The steel shot column is less fluid therefore it will not compress as easily in a tight choke resulting in erratic patterns and possible choke tube/barrel damage.

Briley has the bolt release parts for the SBE3

Yes it is.... Does you M4 cycle well with a standard velocity loads and the new “breacher” tube in place? (I cannot see any reason why that breacher tube affect reliability.) @SolidBuilt Those 1600 fps slugs are a very “hot” load and no doubt recoil is significant. Any chance you are anticipating the recoil after firing a few rounds and letting the gun move too much?

We can shorten and crown those slightly long M4 barrels, we just don't remove/reattach that front sight tower on the Benelli barrels.