M4 Receiver Optic Rail - Why it sucks with Picatinny 1913 spec Optic Mounts (Pics)

[Duggan]

I've said this a bunch of times, but never really took pictures or explained my position until just now, when I was prompted by another thread to show exactly what the problem is.

The problem is that the m4 rail is slightly "shorter" than a 1913 rail when you measure it's height from the receiver, but more importantly it is made up of "stubs" instead of a solid "bar" of rail.

The result is that optics mounts (most all of which are made for 1913 picatinny spec) have 2 small, shallow "stubs" to grab onto in order to anchor the optic, as opposed to a full height, full width "bar" that lets it fully grab on as was intended.

The end result of this is that your optic is not mounted to the receiver rail nearly as strongly as it would be the m4 used a proper 1913 rail. You can and will twist/scrap/contort/ruin your mount/rail if you use your weapon roughly, shoot heavy rounds consistently or drop/snag/smash your m4. It happens, rail damage occurs as a result in most cases.

I personally botched up up a Larue Tactical EOTech mount a few years ago in this manner, as I did not realize that the m4's bootleg rail would cause an issue. I shot my weapon quite a bit and handled it roughly, and I have an optic mount that has been "pushed" out of shape as a result. You can see it in the last picture ... other members on this forum and on other boards have had similar things occur.

As I was saying in the other thread, this is reason alone to consider getting a replacement top optic rail for your m4 ... but upon further inspection it seems that both Mesa Tactical and Sidearmor (the 2 companies I was going to suggest) both use the Benelli "stub" style rail, and they both claim it to be 1913 picatinny ... which it is not.

Behold, the 1913 picatinny specification:





As you can see, the spec calls for full "bars" of rail, not "stubs" as Benelli and all the benelli accessory makers seem to be copying.

[Duggan]

Now, to compare the m4 optic rail to a legitimate 1913 picatinny rail, as seen on my LMT MRP:

(The MRP is always on the left, the M4 is always on the right)

[Duggan]

In these 2 pictures you can kind of tell the height difference ... it is minimal but it is there (it's hard to capture on camera, the shadows make things look funny)

In the second picture you can clearly see the problem with the rail, look at how small of an area the optic mount has to grab onto ... just 2 shallow stubs at either side instead of a big tall full length bar of rail.





^^See how small of an area the optic mount has to grab on to? If you catch that optic mount on something and put it under stress, it will likely cause problems, as it only has a tiny bit of rail to hold onto ... likely the following picture will happen.

[Duggan]

^^That's the underside of my eotech mount after I dremeled off the metal that had been smooshed to eitehr side by the m4's stubs (you can see where the stubs grabbed on ... it's where the metal starts to be smooshed). The mount still works, but it isn't pretty, may have an altered return to zero when removed, and it shouldn't have been put under that type of stress. The fault lies 100% in the rail design, the optics mount is strong as an oxe and made to spec, while the rail is not.

So, my point is that while you CAN use most optics mounts with the m4's stock rail, it is a pretty weak system that will fail if put under stress. If you have a really nice optic mount, plan on handling your weapon roughly, or plan on putting your life in your weapons hands, this may be something to consider.

I'd link to a solution to the problem ... but I don't think there is one at the moment. I thought Mesa Tactical and Sidearmor both made replacement rails that used full bars instead of stubs, but upon closer inspection they both imitate Benelli and use the inferior stub design.

This is just a heads up, and a post I will use later on to answer inevitable questions that will arise about the m4 and mounting optics.

PS - Perhaps I can convince Carriercomp to make this simple replacement part to the proper specs

Update - 9/28/10 - Justice has arrived.

The replacement, vastly superior rail from Carriercomp is now completed and shipping.

http://shop.carriercomp.com/product....6&categoryId=5

One of the M4s few weak links has now been fixed!

Thanks to Carriercomp for making this possible.

There also now is another alternative to the stock rail made available by Sidearmor ... it is a much larger rail, with options for forward light mounting above the handguard and 2 different shell carriers.

This rail is available with and without the center cut rail groove ... be sure to order without if you order!

http://www.sidearmor.com/index.php?m...products_id=70

Either option should be good to go and vast improvement over the stock option. Personal preferences are personal, etc.

[Test Hawk 13]

I certainly don't intend this to sharpshoot you in any way Duggan, but it looks to me like the damage to your lever mount may have been caused by the deflection of the rail stubs as the mount was closed, along with a misalignment of the mount tab that you dremeled off. Both conditions could be a result of the rail and moount designs. Was there any damage to your rail? I'd look closely for cracks under the rail stubs. The 1913 rail would not be as susceptible to this problem because of the full width bar. I discovered this myself while mounting accessories to my M4 and my SIG 556. The SIG has both types of rails on it, stubs on the upper receiver rail and full width bars on the plastic foregrips. My EOTech sight and M3 Surefire light both have thumbscrew mounts which cannot be misaligned because the screw must rest between the bar/stubs. I have considered the throw lever mounts, but after seeing your pic, I see the potential for trouble down the road, especially with possible damage to the plastic rails on my SIG.

Again, not sharpshooting you here, especially since all I have to go on is your pics. I just wanted to give my thoughts.

[Unobtanium]

Duggan's only fault was putting a 1913 spec mount on a crap rail. Been there, done that.

[Northern Lights]

PS - Perhaps I can convince Carriercomp to make this simple replacement part to the proper specs Last edited by Duggan; Yesterday at 07:31 PM.


I would love to change my m4 rail for that reason alone and install Carriercomp's rail instead.

Then i would gladly put my Eotech on it with no fear of breaking something.


Kip,if you decide to produce this item when your inventory as stabilized,i would definetly purchase those rail for my weapon.


P.S. Duggan,i was so impressed and happy about your thread that i forgot to thank you for that brilliant explanation of the problem when i wrote this post.Great job with all the pictures and detailed explanations.

[tactical1]

Kip - put me in for one please if you ramp up.

[M1014]

I certainly don't intend this to sharpshoot you in any way Duggan, but it looks to me like the damage to your lever mount may have been caused by the deflection of the rail stubs as the mount was closed, along with a misalignment of the mount tab that you dremeled off. Both conditions could be a result of the rail and moount designs. Was there any damage to your rail? I'd look closely for cracks under the rail stubs. The 1913 rail would not be as susceptible to this problem because of the full width bar. I discovered this myself while mounting accessories to my M4 and my SIG 556. The SIG has both types of rails on it, stubs on the upper receiver rail and full width bars on the plastic foregrips. My EOTech sight and M3 Surefire light both have thumbscrew mounts which cannot be misaligned because the screw must rest between the bar/stubs. I have considered the throw lever mounts, but after seeing your pic, I see the potential for trouble down the road, especially with possible damage to the plastic rails on my SIG.

Again, not sharpshooting you here, especially since all I have to go on is your pics. I just wanted to give my thoughts.

Test Hawk 13 makes sense,,Compatibility for every market name brand was not considered when they made the rail for the Benelli m4,,,if it fits fine if it don't,,change the rail out,,as far as sharpshooting goes,,a touch tempted but duggan is very helpful around here and cough cough does know his stuff,,,kind ovvvvvvvvvvvvvvvvvvvvvvvvvv

[Fett Tracking]

lulz. yea hes got it. the rail is ******.