Need help with current build

[HK SD9 Tactical]

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:

http://i4.photobucket.com/albums/y119/slessard/PSABarrel1_zps9ff26e8d.jpg

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.

[hotrodder636]

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

[HK SD9 Tactical]

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

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.

[polymorpheous]

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.

[markm]

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.

[HK SD9 Tactical]

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.

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

[hotrodder636]

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.

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

[polymorpheous]

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.

[HK SD9 Tactical]

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

http://i4.photobucket.com/albums/y119/slessard/DriversLicense018_zpsadb24554.jpg

http://i4.photobucket.com/albums/y119/slessard/DriversLicense017_zps2db39da8.jpg

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

http://i4.photobucket.com/albums/y119/slessard/DriversLicense015_zps54cada3e.jpg

[markm]

The rail is a UTG Pro quad rail.


I'm out!

[HK SD9 Tactical]

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.


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?

[polymorpheous]

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.

[HK SD9 Tactical]

I'm out!


Thanks so much for you help.

[polymorpheous]

Thanks so much for you help.

His 1st post was spot on.

[HK SD9 Tactical]

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.


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.

[polymorpheous]

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.

[HK SD9 Tactical]

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.


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.

[HK SD9 Tactical]

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.

[polymorpheous]

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.

[txbonds]

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

[munch520]

I'm confused. You come here asking for help. Help is given. You then refute that and want to argue.

http://i163.photobucket.com/albums/t294/myersma2/Lightweight%20Middy%20Build/a73db05c-0012-4324-9b62-11a62a32384c_zpsb27b7d49.jpg (http://s163.photobucket.com/user/myersma2/media/Lightweight%20Middy%20Build/a73db05c-0012-4324-9b62-11a62a32384c_zpsb27b7d49.jpg.html)

That is a PSA barrel. That has the notch/shoulder you speak of. A 'low profile' (ground down FSB) gas block fit on that barrel just fine, with a DD MFR over top.

As Mark and Poly said, your issue is not with the shoulder, it is with the parts. Scrap that rail, gas block, etc. and get a low profile gas block and a free float rail. Problem solved!

[polymorpheous]

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

Not if it is a standard profile barrel, which it is.

[HK SD9 Tactical]

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.

I thought of that but it is not a bad barrel for the money that I spent on it. I really have never seen a notched barrel like this until now.

Think that I will look at a different rail system and maybe a new front sight block or have an A2 F sight that I have laying around pinned to the barrel.

[munch520]

I really have never seen a notched barrel like this until now.

Here's an image from Spike's with a bunch of different barrels. All have the notch (shoulder) you're talking about
http://i135.photobucket.com/albums/q160/badazzar15/photo-34.jpg

[HK SD9 Tactical]

I'm confused. You come here asking for help. Help is given. You then refute that and want to argue.

http://i163.photobucket.com/albums/t294/myersma2/Lightweight%20Middy%20Build/a73db05c-0012-4324-9b62-11a62a32384c_zpsb27b7d49.jpg (http://s163.photobucket.com/user/myersma2/media/Lightweight%20Middy%20Build/a73db05c-0012-4324-9b62-11a62a32384c_zpsb27b7d49.jpg.html)

That is a PSA barrel. That has the notch/shoulder you speak of. A 'low profile' (ground down FSB) gas block fit on that barrel just fine, with a DD MFR over top.

As Mark and Poly said, your issue is not with the shoulder, it is with the parts. Scrap that rail, gas block, etc. and get a low profile gas block and a free float rail. Problem solved!


Not trying to argue, just trying to understand. As I indicated, I figured that I would have to scrap some parts and was only looking to see if there was an alternative that I did not consider.

[HK SD9 Tactical]

finding link


Thanks for the link. Bookmarked.

[Sean W.]

nevermind

[polymorpheous]

If it is a LW profile barrel you might need a .625 end cap. You should consider better quality parts for your AR.

Read the whole thread and look at the pictures posted.
The barrel is standard profile.

[MistWolf]

We all start out not knowing what we don't know. We're here to help, but only if you listen to what those who know what they're talking about. Ignore them and you're strictly on your own. There are several knowledgeable, experienced AR armorers on this site that will tell you that the handguard chosen for this build and the purpose that handguard is to be put to, are not compatible.

UTG handguards are not of the best quality. Use the https://www.m4carbine.net/nual/misc/nav_search.gif above to search for information on these handguards. As they are non-freefloat, any sights mounted to it will have a shifting point of aim, even if you were to get the cap fitted tightly.

What polymorpheous is driving at, is that to hold the handguard cap in place, a pinned front sight base should be used. That way, the FSB can be used to press the cap tightly against the shoulder of the barrel. However, as there is a step lathed in the barrel at that point, it probably won't work well.

Look into getting something like the Troy Battle Rail. It's lighter than the UTG and it's strong. It's cheaper than a quad rail and really, what are you going to do with all that rail estate anyway?
http://a248.e.akamai.net/origin-cdn.volusion.com/j4enh.r2en5/v/vspfiles/photos/VTAC-Troy-STRX-AVK-11BT-01-2T.jpg?1345706439

[HK SD9 Tactical]

We all start out not knowing what we don't know. We're here to help, but only if you listen to what those who know what they're talking about. Ignore them and you're strictly on your own. There are several knowledgeable, experienced AR armorers on this site that will tell you that the handguard chosen for this build and the purpose that handguard is to be put to, are not compatible.

UTG handguards are not of the best quality. Use the https://www.m4carbine.net/nual/misc/nav_search.gif above to search for information on these handguards. As they are non-freefloat, any sights mounted to it will have a shifting point of aim, even if you were to get the cap fitted tightly.

What polymorpheous is driving at, is that to hold the handguard cap in place, a pinned front sight base should be used. That way, the FSB can be used to press the cap tightly against the shoulder of the barrel. However, as there is a step lathed in the barrel at that point, it probably won't work well.

Look into getting something like the Troy Battle Rail. It's lighter than the UTG and it's strong. It's cheaper than a quad rail and really, what are you going to do with all that rail estate anyway?
http://a248.e.akamai.net/origin-cdn.volusion.com/j4enh.r2en5/v/vspfiles/photos/VTAC-Troy-STRX-AVK-11BT-01-2T.jpg?1345706439

You are correct. I do not need all that rail real estate. Am looking at different rails that have been recommended.

I also felt that even with the A2 F sight pinned to the barrel, it would not alleviate the potential movement of the hand guard cap in the "step" that was lathed into the barrel, especially using a rail system. Thanks for clarifying that. I guess though that a little movement in the standard hand guards would not be a problem though. I was pretty sure that was what polymorpheous was getting at. The only other alternative that I thought of was to have the front hand guard cap have some fins welded to them and use the two piece front sight that I had to "hold" the cap in place preventing movement in the planes, but that would require an exact fit to ensure the rail lined up, some welding, and a little ingenuity. I do not have the machine shop to do what would be necessary.

So it looks like I am looking for a free float rail, a gas block, and a front sight or just the free float rail and have one of the A2 F sights that I have pinned to the barrel. The last piece that I was going to mount on the firearm was a C More Tactical Red Dot Sight for my tired eyes, along with a flashlight and a laser system. So if I go with the gas block and FBUIS that is a flip up, I have to make sure that I stay on the same plane. I heard or read that aluminum for a gas block was not a good idea due to the variations in the metal heat dissipation that could cause problems and that steel was better. Considering the heat that the gas block that it would endure, I am going to have to look at a compatible sight for use as well.

I was originally thinking of using the A2 F pinned to the barrel but there is not a well versed GS/Armorer locally that I can utilize. The one recommended to me did not know the purpose of the tapered pins on the front sight and had only done two barrels. Made me iffy at best to utilize him.

Will have to look at finances to see if it will be more economical to go with the rail and gas block with a flip up sight or the rail and the A2 Sight with the cost of the postage to a reputable GS/Armorer and the cost of having the barrel drilled and the sight set for me.

I think that I am leaning toward the free float rail and the gas block and flip up sight though.

And I do appreciate all the feedback and help provided.

[Quentin]

I have that same PSA/FN 16" midlength barrel which is about 4 ounces lighter than a gov't profile and only 2 ounces heavier than a pencil barrel. Very nice design in that it allows the more common .750" gas block over the pencil's .625" yet adds little weight.

Mine came in a PSA barreled upper with the standard triangular end cap and F-marked FSB. With MOE handguards installed there is very little movement but I wouldn't install a front sight on the handguards, nor do I need to with the standard front sight. Anyway, just mentioning this to verify comments that the barrel would work fine with a proper end cap and FSB (or of course a FF rail).

[chasm]

No one has said it.... You don't put sights or lasers on drop in rails! Only on freefloat.

Lights, VFG, and canopeners are ok though.

[polymorpheous]

No one has said it.... You don't put sights or lasers on drop in rails! Only on freefloat.

Lights, VFG, and canopeners are ok though.

Check out post #4.

[chasm]

Check out post #4.

Ah, I stand corrected... I'll say it a different way... OP, instead of worrying about slots and debating parts, go back and reread the 1st sentence in post 4.

The slot is there for a reason

[discreet]

You are correct. I do not need all that rail real estate. Am looking at different rails that have been recommended.

I also felt that even with the A2 F sight pinned to the barrel, it would not alleviate the potential movement of the hand guard cap in the "step" that was lathed into the barrel, especially using a rail system. Thanks for clarifying that. I guess though that a little movement in the standard hand guards would not be a problem though. I was pretty sure that was what polymorpheous was getting at. The only other alternative that I thought of was to have the front hand guard cap have some fins welded to them and use the two piece front sight that I had to "hold" the cap in place preventing movement in the planes, but that would require an exact fit to ensure the rail lined up, some welding, and a little ingenuity. I do not have the machine shop to do what would be necessary.

So it looks like I am looking for a free float rail, a gas block, and a front sight or just the free float rail and have one of the A2 F sights that I have pinned to the barrel. The last piece that I was going to mount on the firearm was a C More Tactical Red Dot Sight for my tired eyes, along with a flashlight and a laser system. So if I go with the gas block and FBUIS that is a flip up, I have to make sure that I stay on the same plane. I heard or read that aluminum for a gas block was not a good idea due to the variations in the metal heat dissipation that could cause problems and that steel was better. Considering the heat that the gas block that it would endure, I am going to have to look at a compatible sight for use as well.

I was originally thinking of using the A2 F pinned to the barrel but there is not a well versed GS/Armorer locally that I can utilize. The one recommended to me did not know the purpose of the tapered pins on the front sight and had only done two barrels. Made me iffy at best to utilize him.

Will have to look at finances to see if it will be more economical to go with the rail and gas block with a flip up sight or the rail and the A2 Sight with the cost of the postage to a reputable GS/Armorer and the cost of having the barrel drilled and the sight set for me.

I think that I am leaning toward the free float rail and the gas block and flip up sight though.

And I do appreciate all the feedback and help provided.

It is a little disturbing how you keep mentioning welding in so many of your posts. Seems like it is meant to sound knowledgeable or technical. The only thing that would ever need to be welded on an ar is a comp or flash hider for a 14.5 barrel. Outside of that, having the idea about welding anything or ever needing to on an ar is a little out there.

You are making this way more complicated than it ever needed to be. No welding, grinding, epoxy, or space plasma light emitting panther mounting unicorn jel needed. Either run a standard non freefloat or a moe, or run a free float. either can run lights or grips fine. I dont see any reason someone would need to laser up etc for the avg shooter.

[HK SD9 Tactical]

It is a little disturbing how you keep mentioning welding in so many of your posts. Seems like it is meant to sound knowledgeable or technical. The only thing that would ever need to be welded on an ar is a comp or flash hider for a 14.5 barrel. Outside of that, having the idea about welding anything or ever needing to on an ar is a little out there.

Either run a standard non freefloat or a moe, or run a free float. either can run lights or grips fine. I dont see any reason someone would need to laser up etc for the avg shooter.

I think that I mentioned it initially and then indicated that I did not think that it was a viable idea.

Can someone tell me on the Troy industries 11" Alpha Battle Rail how it attaches to the barrel nut? I am assuming that it is done on a three point (screw) system 120 degrees apart, based on the literature. Is there a collar that comes with the attachment point that is inside the rail that is not visible on the PDF from their site or is it only three set screws that attach to the barrel nut? I am concerned that this "three point" attachment is not structurally sufficient to maintain no movement, especially because there is no forward hand guard cap that would isolate movement, being the free float system that it is. Not that I am looking at YH but the threaded nut system seems to be more sound as an attachment than the three screw points that Troy has.


It also appears that the DD Omega utilizes a proprietary threaded nut for the attachment of their free float rail system.

And it also appears that the URX also uses a proprietary screw nut system for the rail attachment.

So is the Troy lacking in this area concerning the attachment methodology?

[drx]

It mounts using these:

http://i.imgur.com/ZTAQB.jpg?1(Found on this site)

Mounts to the stock barrel nut. Easy installation. The troy alpha/vtac alpha rails are very popular and mount securely.

[HK SD9 Tactical]

Thanks, I appreciate that. Not quite the screw on system of the other manufacturers but appears to be an adequate system.

[drx]

Thanks, I appreciate that. Not quite the screw on system of the other manufacturers but appears to be an adequate system.

I was weary about the system myself but it appears to be pretty solid. Its popular with the 3 gun guys and a lot of members of this site (check the pic threads for build ideas).

I'll be getting rid of mine most likely just to shave off a couple ounces and the tube is a bit small.

[cthompson36]

try a true free float rail and it shouldn't wiggle at all. Hell, I use a YHM rail and it doesn't wiggle at all and has held a zero just fine with a RDS. UTG is known to be low quality and out of spec.

[HK SD9 Tactical]

try a true free float rail and it shouldn't wiggle at all. Hell, I use a YHM rail and it doesn't wiggle at all and has held a zero just fine with a RDS. UTG is known to be low quality and out of spec.


I know that UTG is not the best out there. However, their Pro line models are made in the USA (big plus there) and seem to be a good quality and as indicated (while not on the level of KAS or Troy, or MI), I have not had any issues with them in the past. But I need a free float system for this build and UTG is not my first choice even though the do make a mid length FF rail system. After all, KAS was once on the level of UTG (I know, this is blasphemy) and here KAC is today, one of the vaunted few.

I hate to spend $300 to $400 on a FF rail that from what I can tell is overpriced. Back in the 70's when I was in Germany, the same thing happened with stereo systems. Oh, buy a Marantz it's one of the best on the market. The coils are hand wound. The sound quality exceptional. It was good and you paid out the arse for the name but you could have gotten just as good quality from Sansui, Pioneer, Hitachi, Kenwood, and others. I still have my Pioneer system today and honestly, it has served me very well over the years. Today, it's Continuum Caliburn if you want to spend $112,000 on a turntable to get that wow and flutter down to immeasurable levels that only the most super sensitive computers can measure. Meanwhile the pops and hiss from the LP make that $112,000 you spent useless. Waste of hard earned money.

I don't have that type of money to burn. So yes, even though I was not considering it, I am looking at the Yankee Hill series, as well as MI, KAC, and others.

Thanks for helping me keep my eye on the ball.......

[Merle]

http://i192.photobucket.com/albums/z198/merlebrooks/IMG_20130408_162931_zps36ecebfe.jpg

I agree with everyone else. Ditch the Chinese and YHM junk. The Troy stuff is inexpensive and light. Looks good too.

[cthompson36]

the YHM ff rail I have was a little over a hundred. its nice and tight, and no issues. It's heavy but it works well.
as long as you aren't going to war i think the YHM will work well. Mine was installed professionally and hasnt budged and it's seen some knocks.

[polymorpheous]

After all, KAS was once on the level of UTG (I know, this is blasphemy) and here KAC is today, one of the vaunted few.


You have no idea what you are talking about.