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Knight's Armament Co URX 4 Free Float Rail System for M4 / M16 Series Airsoft AEG Rifles - 13" / Black

ID: 53768 (RIS-KAA-URX4-13)
4.6 stars, based on 5 reviews
Knight's Armament Co URX 4 Free Float Rail System for M4 / M16 Series Airsoft AEG Rifles - 13 / Black
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The manufacturer of this product enforces a strict minimum advertised price (MAP) policy which excludes this product from any additional discount coupon codes. This item, however, may qualify for free shipping.
The KAC URX 4 Rail is a fully modular design that utilizes the revolutionary KeyMod accessory mounting system. The URX 4 Rail is stronger and more stable than previous URX designs due to its one piece construction. It is also lighter due to its Integral Barrel Nut (IBN) system of attachment. The IBN system allows the rail itself to act as the barrel nut when torqued onto the rifle, completely eliminating the need for a separate steel barrel nut. KeyMod accessories may be mounted anywhere along the URX 4's length, making it one of the most user configurable designs on the market.


  • Fully licensed by Knight's Armament Company
  • Made from CNC milled aluminum
  • Matte black anodized finish
  • 20mm Picatinny top rail
  • Compatible with airsoft spec. KeyMod systems
Length: 13"
Compatibility: For Tokyo Marui, Matrix, G&P and other compatible M4 / M16 Series Airsoft AEG Rifles
Material: CNC Milled Aluminum

Manufacturer: Knight's Armament Co.

5 Customer Reviews

(Write a review)

by Grant W. on 11/22/2016
"Absolutely fantastic rail. I was a big skeptical at first because the whole rail screws on to the receiver, but it also has a little bar where the gas tube would normally go that can be pushed into the receiver's gas tube hole and then tightened down with hex wrench from the top. It's extremely sturdy and lightweight. Also, I have a 38mm diameter suppressor inside mine right now and it fits just fine, although I don't think there's enough space inside the rail for keymod attachments. I'd say the absolute maximum size suppressor that would fit inside the rail is around 40mm in diameter.

One con: The rail still needs to be tight in order to hold the outer barrel steady, but mine came with a few small shims/spacers and two larger ones. I needed a spacer that was thinner than the large ones but thicker than all the small ones put together. I ended up having to grind down one of the large ones till it was the right size.
by Joseph B. on 11/17/2016
"Great rail system. So simple in design it's milsim cool!

This rail is a bit challenging to install and it took 15 minutes to figure out which of the included shims to use to get the rail at the 11 o clock position. Then you can tap it to the 12 o clock position with a rubber mallet, or buy Knights URX wrench. Apply 100 mph tape to the rail system where you strike with mallet if you want to protect finish on the rail.

NOTE: For G&P, KWA and similar threaded upper receivers a MADBULL re-thread tool is required. That will change the thread pitch on those uppers and allow the URX4 to easily thread almost all the way on.
by Eric S. on 03/15/2016
"Fantastic rails, wonderful quality, I got these for a G&P WOC and they do not fit period, I attempted forcing them as another reviewer did but after a full turn I looked at the threads and they were being completely mangled (like I thought they might). Luckily, I stopped before the damage was total and can still use them on my Rec-7.

4 star review because they fit my madbull body perfectly on my Rec-7, no special tools required.

I'll re-review them after trying madbull's barrel thread die.

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This item is currently Out of Stock. Most out of stock items are restocked within 1-3 weeks. Some items may take longer. Please add this item to your wishlist to keep posted on its availability.