The FAB BM-4 Polymer 4-Rail Forearm
 | The Benelli M-4 is available in both 18" and 14" Barrel configuration. The stock forearm is actually 2 separate panels which cover the
2 short-stroke gas pistons. To start the installation, empty the magazine, and check the chamber. Physically and visually check the chamber again. |
 | Benelli Semi-Automatic Combat Shotguns all have a Cartridge Drop Lever on the right side of the trigger guard. Depress it until it clicks. Now grasp the bolt handle and pull back the bolt until it locks rearward. |
 | With the bolt locked back, unscrew the forearm/magazine end cap. Put it aside in a safe place. The barrel can now be removed by pulling it from the receiver. Once both barrel lugs have cleared the magazine tube, place the receiver to the side. |
 | Take a look at the chamber end of the barrel assembly. You will notice a ring, or shoulder on the rear of the barrel right where it meets up with the receiver. As you can see in the picture to the left, the outside diameter of this shoulder is 1.410". This is important to remember because later, you will need to modify the FAB BM-4 forearm so that it will custom fit, your particular Benelli M-4. |
 | The factory forearm is designed with a contour which matches the diameter of the barrel shoulder. This is what needs to be modified on the FAB BM-4. |
 | Remove the two halves of the factory forearm. Take a note of the inside of the rear barrel lug. You will see a spring inside that must be compressed fully when the Forearm/Barrel is fully seated and tight to the shotgun's receiver. |
 | The FAB BM-4 forearm has two holes in the forward end. Line up each half with both the barrel and the barrel lug's internal circumference. |
 | Note the mating between the inside diameter of the polymer forearm with the barrel shoulder. You will see that the forearm needs to be modified so that the forearm seam lines up with the centerline of the Benelli M-4 barrel. |
 | In this picture, you can see that the inside of the BM-4 needs just a little more cutting at the taper where it sits on the barrel shoulder. |
 | If you attempt to assemble the forearm without modifying it, the tension on the rear opening is so great that the forearm will spring and sit too high without it's rail lining up with your shotgun's rail. |
 | Here is a second look at the too tight back end of the forearm. You can see that the barrel will not position properly and the assembly will not fit with the mating areas of the receiver. |
 | This is what you end up with if you force the forearm without modifying it first. |
 | No matter how you force it, the forearm will not sit properly and because of the lack of clearance over the barrel shoulder, the whole assembly sits off-center and too high. |
 | Here is how you modify the FAB BM-4 to fit your shotgun correctly. The rear opening of the BM-4 needs to be relief-cut in the areas indicated. By cutting a larger taper at these points, the whole assembly will sit lower on the barrel and align better with the receiver. Cut a little at a time and check for fit as you go. |
 | Once you've cut the radius a little larger and are satisfied with the fit, install the BM-4 in it's correct position on the barrel assembly. Install the 5 supplied Allen Head Cap Screws. Just catch the threads - do not tighten everything down just yet. |
 | Slip the forearm / Barrel assembly over the magazine tube. Make sure you line up the mag tube with both barrel lugs. |
 | While holding your shotgun's receiver (with the bolt still locked back), line up the barrel extension and slide the 2 halves together. |
 | Note the cut-out at the rear of the bottom Picatinny rail and see it's relationship with the lip of the stamped steel receiver-mounting plate. |
 | You may be able to apply pressure in pushing the forearm / barrel assembly into the receiver by using a rubber mallet to slightly spring the forearm around the barrel shoulder. If it doesn't "POP" into place, move to the next step. |
 | In this picture, you will notice that the 2 forearm halves are still loosely screwed together. With your hand pressing down on the top of the BM-4 at the point where it meets with the shotgun receiver, and using constant pressure, use a large flat-bladed screwdriver to lift the bottom rail enough so that the forearm / barrel assembly slips into it's proper position. |
 | Fully seat the assembly and check your alignment along the top of the rails. If you performed the modification correctly, the rails will line up. Install the magazine cap and lock everything together. |
 | The original design intention was to have a tensioned fit to lock the forearm so tightly that there would be absolutely no forearm movement. The problem with U.S. Law Enforcement applications of the BM-4, disassembly and reassembly must be easy enough so that weapon cleaning and maintenance will not be avoided due to difficulty in assembling the weapon. This modification is NOT factory authorized and should not be attempted by anyone who is not Depot Level Maintenance-rated. |