Mid length rail on a 6920

[intrepidagent]

Hi....I'm a newbie awaiting delivery of my 6920 this coming week. I have a few technical questions. I'm considering putting a mid length free float rail on it instead of a carbine length RAS. The Knight's mid length is a bit over 9". I presume I'll have to remove the front sight / gas block and replace it with a low profile one. Is removing the old / replacing with a new gas block an easy procedure? Any suggestions regarding manufacturer choices for the new block? I'm planning on putting a set of YHM / Troy / or GG&G BUIS on the top rail when it's installed.

Thanks!

[Robb Jensen]

Easy is relative to your mechanical abilities. I don't replace the FSB with a low profile gas block, I mill/grind them down to fit under a rail/tube. That way it's still pinned and can't move.

This is an old pic when my 10" LMT had a FSB and a KAC RASII rail.


Then I 'shaved' the FSB and installed a 9" DD Lite Rail.



Nowadays it's looks like this.

[rob_s]

I'm with Robb, I greatly prefer shaving down the FSB. I've paid to have it done once and I've done it myself once. Wasn't too difficult at all. When I was done I just sprayed it with hi-temp grill paint and it's still chugging along just fine. If it was going to be exposed to bumps and scratches I might have opted for a tougher finish, but since it's protected by the rail I'm not concerned.

If you don't feel comfortable doing it yourself, ADCO will do it for you. When they did mine I bought the DD 9.0 rail from them and they didn't charge me to install the rail, just for the price of the rail and the price to shave the FSB. Don't know if that's still their deal or not.

Here's some pics of my shave job (some in-progress and some complete)


















and the final products

[Robb Jensen]

I forgot to mention since you said this was going on a 16" barrel. DD makes 10" Lite Rail now too which will be just a little longer which is nice. A 9, 10 or 12 rail will fit. DD is now milling out a section of the back end of the rail which will allow it to slide over a gas block during installation for easier less tricky installation. It's similar to what ADCO was doing.

Here's my 12.5" SBR that started with a 7" Lite Rail, then I shaved the FSB and added a 9.5" Daniel Defense RISII MK18 rail.

Before:


After:




Then it's latest pic, just yesterday:

[intrepidagent]

Thanks for the quick replies. .....great pics too. I'm not adverse to cutting it down and or milling it myself. There's a shop that I have access to which has a bandsaw and enough grinders for me to do the work myself. Any suggestions for removing the front handguard cap?

[rob_s]

The handguard cap will come off when you take the FSB off.

In case Robb and I weren't clear, I wouldn't (and I don't think he would either) advocate doing this work with the FSB still on the barrel. You can see from my pics that it's removed and clamped in a vice.

[Robb Jensen]

Originally Posted by rob_s

The handguard cap will come off when you take the FSB off.

In case Robb and I weren't clear, I wouldn't (and I don't think he would either) advocate doing this work with the FSB still on the barrel. You can see from my pics that it's removed and clamped in a vice.

Correct. I'd only do it on the barrel if you had a non removable muzzle device. Since your adding a DD Lite Rail you'll have to take it off anyway since the DD has it's own proprietary barrel nut.

[lindertw]

The more I see gotm4's rail/light setup, the more I like it. I photoshopped this using LaRue's weapon profiler to get an idea of how the 9" rails look with a 16" barrel. It doesn't look as bad as I thought it might...

[Robb Jensen]

Originally Posted by lindertw

The more I see gotm4's rail/light setup, the more I like it. I photoshopped this using LaRue's weapon profiler to get an idea of how the 9" rails look with a 16" barrel. It doesn't look as bad as I thought it might...

It works very well. I used my 10" SBR set up this way for the mods/staff Vickers class and it worked very well for the low-light night stuff we did.

Here's some pics of a shaved gas block and barrel, the gas block has been sandblasted after shaving and refinished with flat black Norrells moly before reassembly.





Here's one I recently did while it was still attached to the barrel since I was installing a 10" Troy rail (which clamps onto the stock/OEM barrel nut) and the muzzle device was perm. mounted.

After shaving and sandblasting:



after refinish w/Norrells:

[rob_s]

Originally Posted by lindertw

The more I see gotm4's rail/light setup, the more I like it. I photoshopped this using LaRue's weapon profiler to get an idea of how the 9" rails look with a 16" barrel. It doesn't look as bad as I thought it might...

IMHO if you're going that route I'd go with a 10.0 or 11.0 depending on your preference and what each maker has available.

[lindertw]

Originally Posted by gotm4

...Here's one I recently did while it was still attached to the barrel since I was installing a 10" Troy rail (which clamps onto the stock/OEM barrel nut) and the muzzle device was perm. mounted.

I got a peek at that the other day in the shop, nice. I considered pricing out a separate upper built in this fashion, but it'll probably be cheaper to just modify the one(s) I have. I'll stop in this week to pick your brain!

[lindertw]

Originally Posted by rob_s

IMHO if you're going that route I'd go with a 10.0 or 11.0 depending on your preference and what each maker has available.

thanks rob, I'm going to 'shop up a couple more examples (with longer rails) to see if it looks a little less obnoxious with the longer barrel.

I'll put up more pics in a bit...

[rob_s]

FWIW, 9.0 on top, 10.0 on bottom



Also, not to confuse you even more, but if you're going to run a fixed FSB you could go with something like the Daniel Defense AR15 Lite Rail 12.0 FSP

[lindertw]

Originally Posted by rob_s

...Also, not to confuse you even more, but if you're going to run a fixed FSB you could go with something like the Daniel Defense AR15 Lite Rail 12.0 FSP

thanks for the profile shot of the 9.0 v 10.0 rob_s. I forgot about DD's option that allows me to keep my current FSB; I'll check into that as well.

[Razorhunter]

Man do those DD RIS II Mk18 rails look GOOD.
I love the look of the area right up near the bolt up plate. On the regular LITE Rails, you see THREE bolts on each side (I think), but with that RIS II Mk18, you only see TWO bolts on each side, and then there is that extra "FLAT" right there, on the bolt up plate, similar to how a Troy rail looks on the side right there. That bolt up plate just looks SO much better than a regular LITE rail.
I wonder if DD would be willing to make me on in BLACK, and even better yet, I wonder if they'd make me one for an AR10???
I'd skip the Noveske/SWS rail in a second, and put one of those on my AR10 build. Thanks for the pics gotm4 and robs!

[intrepidagent]

Thanks all of you for the pics and the input. Not having my 6920 infront of me (yet) is it just two knockout pins at the bottom of the fsb that clamp it onto the barrel? Would I have to remove the stock flashider to then slide it down the barrel to get it off? Sorry about such basic questions but suffering from being a noob.

[lindertw]

apologies to the OP for jumping in on this thread

here's a quick 'shop of 10" / 11" / 12" rails (LaRue were used as they are the only models available on their profiler tool):



The 12" rail seems a little long for my tastes; I'll have to mull it over. Again, sorry if this is diverting too much from the OP.

[Robb Jensen]

Originally Posted by intrepidagent

Thanks all of you for the pics and the input. Not having my 6920 infront of me (yet) is it just two knockout pins at the bottom of the fsb that clamp it onto the barrel? Would I have to remove the stock flashider to then slide it down the barrel to get it off? Sorry about such basic questions but suffering from being a noob.

Yes to get the FSB, handguard cap or barrel nut off the muzzle device must be removed. On Colt barrel your taper pins will be knocked out from left to right (muzzle facing away from you).

[Razorhunter]

Yes bro,
You must tap out the FSB taper pins, and MAKE SURE you tap the out in the CORRECT direction, as they are tapered. You will need to tap them out from the small diameter, TOWARDS the large diameter tip of the pins. I'm sure you understand that.
Yes, you will also have to take the flash hider off, for the FSB to slip off the bbl.
Make sure you put a 2x4 block of wood under the FSB when you are tapping out the pins. Start the pins with SHARP whacks, using a good punch, and then finish tapping them out with a proper sized punch that will fit through the hole. Starrett makes a NAIL SET, with a CUPPED/DOMED point, which fits the round conical shape of the taper pins. It's not totally necessary, but is what most armorers use for this procedure. You can use a flat faced punch/nail set if you know what you are doing though.
You might also want a "gas tube roll pin punch" to remove the FSB from the gas tube. You COULD skip this step, and just leave the gas tube on the FSB, but this might only be good for experienced armorers to do.
It's up to you, and your comfort level.

[Razorhunter]

My bad,
Looks like gotm4 beat me to to "punch" (pun intended).