VTAC Rail Issue, or iron sights? Updated w/ SOLUTION

[Gunzilla]

Got a response from BCM regarding my problem with zeroing my new BFH upper with the 13" VTAC hand guard (still hitting way too high).

They said: "Many times we recommend not using this type of setup because of the sight issues." Can someone explain this? I guess I'm thinking that someone had to find this problem before now, so why are they still having a problem?

This brings up a ton of other questions but all I want to know is who has 13" VTAC hand guard on their BCM (that came that way from BCM) that is using folding iron sights that are zeroed......and can you post a pic of your front sight from the front and rear? Thank you in advance.

My set up is: 16" BCM w/ 13" VTAC and Troy folding sights, zero is impossible to achieve, shots are hitting 4 inches high.....and I can see daylight under the front sight detent collar.

[William B.]

I had a similar issue with my 11" Troy TRX after over-tightening the screws that fix the rail to the barrel nut. Like Grant alluded to in your last thread, when these screws become over-tightened they can cause some funny things to happen at the muzzle-end of the rail. Try removing your sights and optic and run a straight-edge along the 12:00 rail from the rear of the upper to the muzzle-end of the rail. Then you can see if everything is lining up properly.

[Gunzilla]

I tried the straight edge with a 1' square and noticed a tiny gap with the receiver being slightly higher and for that 1' it seemed to be a consistent gap, so no angle was visibly noticeable. Looking down the empty rail everything looks aligned and squared up, and this is why I stumped.

I understood Grant's comment but thought that he was referring to some of the early model VTAC rails, mine is only a month old. The hand guard was removed twice for mounting attachment rails so it is possible that I over tightened the cap screws...especially if there is a torque spec for that.

I've contacted BCM and I'm sending it back for inspection, hopefully they'll find the problem quickly....and it won't cost me.

This really blows huge chunks:

[MitchK]

Try shimming between the hand guard and the barrel nut. I used soda can parts. You can "drive" the front end where you need it to go.

[C4IGrant]

I tried the straight edge with a 1' square and noticed a tiny gap with the receiver being slightly higher and for that 1' it seemed to be a consistent gap, so no angle was visibly noticeable. Looking down the empty rail everything looks aligned and squared up, and this is why I stumped.

I understood Grant's comment but thought that he was referring to some of the early model VTAC rails, mine is only a month old. The hand guard was removed twice for mounting attachment rails so it is possible that I over tightened the cap screws...especially if there is a torque spec for that.

I've contacted BCM and I'm sending it back for inspection, hopefully they'll find the problem quickly....and it won't cost me.

This really blows huge chunks:

Really any rail that is this long CAN have issues with the the irons zeroing. We used to see it a lot with LaRue Tactical 12.0 rails as the two top screws would pull the rail upwards.

So yes, the TRX rail (no matter if it is old or new) can have this issue.



C4

[Gunzilla]

Grant, thank you for clearing that up for me.

Mitch, how many times did you have to shim your upper before you got it lined up? Can you advise me on the shape you cut your shims to? I'm picturing a crescent shape that I would install at the bottom 1/2 or 1/3 of the barrel nut.

This sounds like a viable idea that may make it so I don't have to ship my upper back.

If there is another chance of getting my upper back only to have this happen again it may be better to try the shim method as a starter. Thanks for your input, it may have saved the day. I'll let you know how it works out.

[Colt guy]

It sounds like when the rail is installed and the set screws are tightened down it pulls the end of the rail downward. You stated it placed the shots 4 inches high but at what range 100 yards? Rather then adding shims which could fall out loosing your Zero. I would suggest getting a taller front site post. Colt A3 Removable Carry Handle Sights are taller than A1& A2 Rear Sights, and require an .040” taller Front Sight Post. This would lower the point of impact.

[MitchK]

Grant, thank you for clearing that up for me.

Mitch, how many times did you have to shim your upper before you got it lined up? Can you advise me on the shape you cut your shims to? I'm picturing a crescent shape that I would install at the bottom 1/2 or 1/3 of the barrel nut.

This sounds like a viable idea that may make it so I don't have to ship my upper back.

If there is another chance of getting my upper back only to have this happen again it may be better to try the shim method as a starter. Thanks for your input, it may have saved the day. I'll let you know how it works out.

I just cut a rectangular piece of coke can that would wrap around about half way. If youre familiar with the Burris posi align scope rings, this is the same principle. It will take a few tries to get it in the right place. Once you get it tightened down it is secure and won't move until you loosen the screws. Make sure you're not torquing the screws too much. Hth

[Gunzilla]

It sounds like when the rail is installed and the set screws are tightened down it pulls the end of the rail downward. You stated it placed the shots 4 inches high but at what range 100 yards? Rather then adding shims which could fall out loosing your Zero. I would suggest getting a taller front site post. Colt A3 Removable Carry Handle Sights are taller than A1& A2 Rear Sights, and require an .040” taller Front Sight Post. This would lower the point of impact.

The range was 25 yards, I was doing my initial zero with a new upper when I found this issue.

There is no shortage of front sight post (excuse the pun), as this pic will show. That is the tallest post that comes with the KNS front sight post kit. At the current setting it's close to a 50 yard zero. Needless to say that post is GARBAGE, it's too thin to see...or should I say: It's too thin for ME to see.




It's obvious that I need the end of the hand guard to be higher than it's current position for a normal sight post to work in it's normal position.

Since there are no mechanical parts moving in the area the 'shim' idea seems viable. I've used them a lot when I worked in industry as a mechanic and they do work great for dialing in and coupler alignments.

With what I was told by BCM (and Grant) it seems this is a common problem with the longer hand guards and BCM can't promise they can fix it. So, that may be all the motivation I need to try a fix that doesn't include shipping my upper back.

Worst case: I revert to an 'optic only' rifle, ...but I'm optimistic I can do better than that.

Thanks for responding.

[Gunzilla]

UPDATE: I think I found my issue.

Looks like the gas block is rubbing up against the backing for the attachment rail mounted right under the gas block......and this is what is keeping the rail from being able to move into it's proper position.

I'll grind or cut the rail backing piece as a starter to see what I get.