Custom build SBR or the AR upper equivalent of building a ship in a bottle

[nightchief]

I’ve really liked the KAC URX 4 hand guard for quite a while, and after shooting a friends SR15, I was sold on having the URX 4 on my next upper. I decided to integrate the URX 4 into my current SBR, requiring a rebuild of my current 11.5” upper. This is a mixed use (50/50 suppressed/unsuppressed) SBR so I wanted certain items to reduce gas flow and pressure, but still cycle reliably unsuppressed. This build is a sort of “jack of all trades, but master of none” so to speak (I am convinced the only true way to have a “soft” shooting, suppressed SBR is to tune the gas flow for the suppressor, but I digress…)

This was a fairly straight forward build until the hand guard installation. I have read about some of the adventures installing the URX 4, primarily installing the gas block as one of the set screws is obscured by the hand guard.

One might ask why KAC would design the hand guard in this manner. I too questioned this until discussing the gas block design of the KAC rifles with my SR15 owning friend. The KAC rifles have a castle nut to secure the gas block, so bottom mount set screws are not required. It would have been nicer if KAC considered other users in the design, however I understand why they didn’t worry about it.

Friend of Nightchief, who has the SR15, wanted to observe the URX4 installation, which turned out to be a good thing! A couple of hours noodling on the “how” as well a couple of failed ideas left us with no solution. A search of the internet (including M4C) found several hits. A video of Failure2Stop installing the gas block after the hand guard was secured didn’t discuss an obscured gas block set screw issue. Several others talked about drilling holes or cutting away part of the hand guard. This was not something I wanted to do.

Friend of Nightchief suggested installing the gas block before torquing the hand guard down, then threading the gas tube into the upper thru one of the MLOK slots. Huzzah! We did a dry run to be sure we could build this way proved successful.

So the order for installation we used for the URX install was as follows:

1. Add URX4 hand guard and tighten to the 11 o’clock position (using the necessary supplied shims per the instructions), with the URX4 between 1030 and 11 o’clock, the gas block set screws are accessible thru the angled MLOK slots.
2. Set gas block on barrel and secure set screws with torque driver and Rocksett. No red (272) Locktite as it will be difficult to remove with heat, if necessary later.
3. Torque hand guard to the required 65-120 ft lbs to line up the top rail. Mine only required 90 lbs fortunately as my torque wrench only goes to 100 lbs.
4. Thread gas tube into the hand guard, thru the the upper receiver, then set into gas block and secure with spring pin. The spring pin was taped to a standard punch as a starter punch would not hold the pin at the angle dictated by the MLOK slot.

Here are the specs:
Upper

• Sionics Reduced Gas Port (.070) 11.5 medium contour barrel (CMV, chrome lined, 1:7 twist), with BCM gas block and gas tube
• Sionics stripped forged upper receiver
• LMT Enhanced Carrier and Sionics NP3 bolt
• Knight’s Armament 10 3/4” MLOK URX4 hand guard
• Geissele Super Charging Handle
• Surefire 556-212a closed tine flash hider/suppressor mount

Lower

• BCM4 complete lower
• Colt supplied H3 buffer and Sprinco Blue action spring
• Battle Arms Development BAD-CASS selector
• Geissele S3G trigger with 4# spring

Accessories

• Magpul MOE-SL stock
• BCM pistol grip, fore grip, and hand guard covers
• Aimpoint H-2 RDS with Scalar Works mount
• Surefire 556-212 suppressor
• Ferro Concepts Slinsgster 2 pt adj sling.

Attachment 50212 Attachment 50213 Attachment 50214

How does it shoot? Well, the barrel has somewhere around 2500-3000 rounds thru it using my old set up (Hodge upper and wedge lock hand guard) and now this one. After some initial teething pains related to the feed ramps between the barrel extension and receiver, it has shot well. I can hold under 2 MOA at 100 yards when shooting supported.

[Vegas]

I had fun and games installing my URX4 rail. I ended up grinding down an Allen wrench because one set screw was obscured. Not ideal but got the job done. Reading your method is definitely a Eureka! moment I wish I had when installing my rail.

Nice looking build btw!

[officerX]

Looks nice but some may give you a hard time since you didn’t build the lower yourself. (And you placed this in the custom build section)

[nightchief]

I had fun and games installing my URX4 rail. I ended up grinding down an Allen wrench because one set screw was obscured. Not ideal but got the job done. Reading your method is definitely a Eureka! moment I wish I had when installing my rail.

Nice looking build btw!

Thank you...I actually tried grinding down an Allen wrench first but it didn’t work for me...How did you mod the wrench to make it work?