Need help with current build

AR/M4 AR General Discussion
1

Oh where to start. I am having problems mounting a quad rail on my mid length AR15 build but the problem is not the quad rail. Let me explain.

I purchased a barrel from PSA. It is this barrel: PSA 16" 5.56mm, 1:7, CMV CL MP, Mid-length, Premium Barrel - Lighter Profile and it was manufactured by FN and is chromed lined.

The problem that I am having, it that where the front hand guard cap goes (triangular for mid length) there is a notch machined which results in the barrel decreasing in size from .750 inches to .713 inches right at the “shoulder” that the front hand guard sits against. See here:

As a result of this “notch” when the quad rail is mounted, there is a 1/16" “play” or “movement” in both the y (left and right) and z (up and down) planes at the front of the hand guard. The gas tube hole in the triangular hand guard grip cap allows for this movement. There is a YHM flip up iron sight that sits in front of the hand guard cap that is held in place with 4 cap screws. The sight can be seen here: http://yhm.net/index.php?main_page=product_info&products_id=5 The front hand guard cap was obtained from Joe Bobs and the hole is .750" in diameter to fit over the barrel but obviously, it moves in the notch that is cut in the barrel at the shoulder.

Now because of this “play” of the 1/16 inch in the y and z planes at 12 inches forward of the barrel nut, if I mount a laser or red dot sight, when the rail moves, at 100 yards, I can have as much as 18 3/4 inches movement in point of aim in both the y and z planes due to the movement of the front hand guard cap.

I spoke with Alex at PSA where I purchased the barrel and he says that this is normal movement. I say it is not as it makes anything mounted on the rails useless for lasers, sights, etc… I have done other builds where there is NO notch at the shoulder of the barrels. Alex says that they will replace the barrel but all these barrels have these notches in them. I got a really good price on the barrel and do not want to have to fork our over $300 for another barrel in today’s market.

So how do I overcome this problem? Is there a special cap that I need to use? Do I have to “pin” the cap to the sight to prevent this movement? Do I have to put a weld bead around the barrel and machine it down to the proper .750"? What will that do to the barrel? I do not have the tools to do this and would have to send the barrel out for this. Will I have to go to a much more expensive free float rail that does not rely on the front hand guard cap for support?

I am really not very happy about this, and quite frankly am quite annoyed as PSA because there was no indications that there would be a problem mounting any type of rails onto this barrel. This has really trashed this build for me. What easy fix am I missing here? Any help appreciated.

2

Do you have a picture of the rail system mounted? That may help with finding an answer.

3

Thanks for the reply. Can you specify exactly what you are looking for? Picture of the rail mounted in the barrel nut? At the front in the hand guard cap? Showing the movement in the y and z planes? I do not know what you are asking for.

4

Unless you use a free floated rail, you will not be able to hold zero with a laser or red dot sight.

You also don’t mention the make of this rail.
This would be helpful information.

Also, do a site search on YHM.
You will find their quality lacking.

Personally, I would ditch the YHM gas block and install a low profile gas block.
Then I’d run a free floated rail over the gas block and mount a flip up BUIS.
Troy makes good, economical free floats.

5

It sounds to me like you have the wrong rail for your build. If your rail requires a front end cap, then it should be mounted on a barrel that has a real, pinned front sight base.

With a propper free float rail, the end cap is not needed. Without a pic of the set up, I too am a little unclear.

6

The rail is a UTG Pro quad rail. I have had success with 7 builds using these rails and have never had a problem with them. I have a couple of other builds with low profile gas blocks and this one just happens to have this sight on it.

So from your response, it seems that because of the design of the barrel, ie, the cut at the shoulder, there is NO hand guard cap that I can use that will alleviate this problem and only going to a free float rail system will fix the problem because it will not use the front hand guard cap for support? Or, the alternative is a new barrel where there is no notch cut into it?

Are these my easiest two options only?

Thanks

7

Specifically a picture showing the front of the rail where it is interfacing with the hand guard cap.

8

Or purchase a front sight base and send it out to get pinned.

You got lucky on your other builds.
But it sounds to me that you really lack understanding about how these rifles are put together.
A handguard retaining cap requires a pinned front sight base to hold it in place. (As MarkM stated.)

Parts are not parts.
You can’t throw a bunch of substandard parts, (at best), and expect there to be no problems.

9

http://i4.photobucket.com/albums/y119/slessard/DriversLicense016_zps6d418e44.jpg

10

I’m out!

11

I disagree. I also disagree with your use of the term sub-standard. I also have the A2 F sights that I can have pinned to the barrel but do not see how that would alleviate the problem of the cap in this instance. There is no pining of the cap to the A2 F sight so how would that help?

12

Again, you do not understand what goes into building an upper receiver group.
The FSB pins the handguard retainer tightly against the shoulder.
I’ve had barrels where the retainer was pinned so tightly, the FSB taper pins had to be removed in order to rotate it.

There is no welding involved, nor should there ever be.
Placing a weld around the barrel will likely distort it.

UTG and YHM parts are bottom of the barrel as far as build parts go.
You got lucky in your previous builds.

13

Thanks so much for you help.

14

His 1st post was spot on.

15

I still do not see how this would help if the cap is not pinned. A good whack of the rail and even under the most strenuous friction caused by the pining of the front A2 F sight, the rail cap would still move within the notch, regardless of whether I was using a rail system or just plain hand guards, although the use of hand guards would not require the immobility that the rail system requires.

I also figured that a weld bead to alleviate the notch in the barrel was not a possible alternative.

I figured that I was going to have to go to a free float rail system and was just looking to see if I had missed a possible alternative.

16

How does the KAC RIS stay in place so well then?

Once you install a gas tube, the retainer isn’t going to rotate.

You have 2 options:
Get a FSB pinned.
Install free float rail.

17

I am assuming that the barrel used does NOT have the notch in it that allows the hand guard cap to move in both the y and z planes. This is the first barrel that I have utilized that has that notch in it right at the shoulder where the front hand guard cap goes. Like I said, this is my 7th build and the first time that I have utilized or seen a barrel with this notch. I have not used a free float rail system before and thought that this was a unusual find with the notched barrel and was looking for assistance in overcoming this problem.

So if the barrel is .750 inches in diameter, and the hand guard cap has a .750" hole in it to slide over the barrel, there would not be any measurable movement of the hand guard cap in the y or z planes. This would also support the front of the quad rail and prevent movement in both of these planes. Is my thinking wrong here?

It is not a bad build. It sits on a Aero Precision lower, has a BCM BCG, a milspec A4 Upper, a what I though to be a good barrel for a light weight design, a ALG Gisele trigger, a MOE stock and upgraded buffer tube assembly, and the rail and front sight is what I had lying around.

I figured the rail was a loss as I was trying to work around this issue but was not aware of the low esteem of the YHM front sight.

This is just a shooter for me as I have long passed the age of playing army as my 20 years was enough so this is going to be a low maintenance low impact rifle for target shooting, nothing more. It will certainly not be my “go to” as I can’t even carry the rifle with the basic load out ammo right now do to my back that I willingly gave to the military and have been fighting the VA for ever since.

18

And I really mean thank you to all for your help. I was not being sarcastic. I have a problem and thought that I came to the experts for assistance and do appreciate all the input.

19

I don’t know who made your previous barrels, but all of my BCM barrels have this notch.

I can almost guarantee that my Colt upper will too.

You could always sell the barrel if your not happy with it.

20

Could it be that this barrel requires a hand guard cap with a smaller opening like a light weight or pencil profile barrel?