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How to Install the Magpul PTS UBR stock to your LM4

UBRMagpul LM4 GBBR How Install PTS

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#1 ky2970

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Posted 14 November 2012 - 11:44 PM

In order to install a Magpul UBR stock, some basic modifications are needed. This thread will show you everything I did to get mine to properly fitted. Note that this will require some permanent modification.

Tools:
Dremel Rotary Tool
-Cutting Wheel
-Sanding Wheel
-Wire Brush Wheel
AR-15 Wrench (not required but highly advised)
Standard Flathead Screwdriver

Step 1: Basic Disassembly
Remove Magazine.
Disassemble Upper and Lower.
Remove Buffer and Buffer Spring.

Step 2: Standard Stock Disassembly
Pull down on Length-of-Pull Adjustment lever.
While holding the LOP lever down, pull back on the stock.
Use an AR-15 wrench or a makeshift tool to move the Castle Nut aft. (Turn Counter Clockwise)
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Pull back on the endplate so it separates from the lower receiver.
At this point, the buffer tube should freely rotate. Turn Counter Clockwise to remove.

Step 3: UBR Buffer Tube Install
The UBR buffer tube will freely spin into the threads until about halfway. Follow these steps to fix that.

Install the Wire Brush wheel to the Dremel and begin sanding the threading inside of your receiver.

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Sand down the UBR buffer tube threading.

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NOTE: Do this in increments. Sand and check to see if the UBR buffer tube will fit. Although it's unlikely you'll destroy your threads with a wire brush wheel, it's good tool etiquette.

When you're done, simply thread on the buffer tube. Don't worry if it spins a bit too freely, the design of the UBR compensates for this.

Step 4: Buffer Tube Cover Install

Separate your Buffer Tube Cover from the lower half of the UBR.

As your slide the cover on, the first thing you will notice is that it will not fully seal against the receiver. Follow the steps below to fix this.

The Buffer Tube Cover's Endplate protrusion is slightly too large.
Sand down the protrusion edges by using the sanding wheel on your Dremel tool.
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NOTE: Be sure you are sanding down the protrusion and not the endplate itself. Sand and check fitment until you are satisfied.

After you are complete with the fitting of your Buffer Tube cover, use a flathead to tighten the screw in the aft of the cover into the buffer tube itself. Don't overtighten.

Step 5: Buffer Truncation

Alternate Step: A Brownells CAR Buffer Assembly is a perfect fit for the UBR. No mods required.

This is the most "iffy" part of the install. The UBR buffer tube is shorter than the original buffer tube. At this point, your LM4 will short cycle. This means the bolt will not lock to the rear and you cannot actuate the bolt catch. To remedy this, we will cut the buffer. I have test fired after doing so and have not encountered any issues. Be  aware that doing this will VOID YOUR WARRANTY.

You can choose to skip this step. I didn't want to risk dry firing.

The part of the buffer tube you will be cutting is that rubber section at the end. Use a Dremel with a cutting wheel. Note that the center of the buffer is metal. Be patient and steady with the Dremel and you will be able to cut through it. Don't apply too much pressure at once to avoid shattering the wheel and possibly hurting yourself.

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NOTE: I absolutely recommend wearing eye pro for this step (if you haven't already). In addition, be sure you are in a well ventilated space; outdoors if you can. The noxious fumes of burning rubber is dangerous.

Reinstall the buffer and buffer spring into the tube just as you would with your normal buffer tube.

Reattach the lower half of the UBR.

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Edited by ky2970, 21 February 2013 - 11:08 AM.


#2 gcw360

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Posted 17 November 2012 - 05:35 AM

Thank you for the writeup.  Why not post the actual pictures?  I think it would add to the thread.

#3 ky2970

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Posted 17 November 2012 - 07:24 PM

Hm.. Not sure how to do that. When I click on the image button it just tells me to add a link.

#4 gcw360

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Posted 17 November 2012 - 07:31 PM

Save the photo to Photobuck (it's free) then cut and past the link for the picture into the required area.

#5 Chuck S

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Posted 17 November 2012 - 07:37 PM

Since the photos are already in Photobucket just copy the image code and post it.  The image shows to the right of your photo.

Samples (yours! :) )

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-- Chuck

#6 ProtectionPerfection

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Posted 21 December 2012 - 08:50 AM

rather than cutting down the factory buffer, could you shave off the spacer on this to get it to fit and cycle?
http://shop.ehobbyas...e-m4a1-gbb.html
I really want the UBR but I dont want to have to permanently modify my factory parts.

#7 ky2970

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Posted 22 December 2012 - 02:37 AM

I don't see why it wouldn't... But it'd be cheaper to just buy a spare KWA buffer.

#8 ttsupra98

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Posted 02 February 2013 - 11:57 PM

This might seem like a dumb question, but couldn't you also cut the buffer spring instead of the spring buffer itself?
Slap me if its a dumb one haha

#9 BeachEMT

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Posted 03 February 2013 - 02:24 AM

No, cutting the buffer spring would not help. The problem is the buffer over-travels and so you need to cut it down to allow the bolt to cycle completely.

#10 akiraspeedstar

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Posted 03 February 2013 - 06:20 AM

View Postttsupra98, on 02 February 2013 - 11:57 PM, said:

This might seem like a dumb question, but couldn't you also cut the buffer spring instead of the spring buffer itself?
Slap me if its a dumb one haha

Like beach mentioned that wouldn't help, you could pull the entire spring out and it would still hinder the bolts travel. The spring itself can coil completely over the buffer.





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