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Review: JBU M4 Kurz conversion kit (4.5/5)



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Old October 12th, 2007, 15:38   #1
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Join Date: Jan 2002
Thumbs up Review: JBU Olympic Arms M4 Kurz conversion kit (4.5/5)

Review - JB Unicorn "Olympic Arms M4 Kurz" conversion kit

I purchased this set through Garlic City Airsoft (, for $112 - final total was $131.10 after shipping by FedEx Express.

The Inspiration:

My main intention was to make a shits-and-giggles CQB buzzsaw. Nothing “serious”, nothing “MilSim” – I just wanted something cool to play around with as our local outdoor season ends and our indoor CQB season begins. I decided an M4-based ultrashorty was what I wanted – a gun whose operation and handling I was familiar with, but made as small and light as possible, with an unrealistically-fast rate of fire. It’s an ego gun, plain and simple .

I looked at a number of kits, including the Patriot kits by Madbull and PDI, as well as the Classic Army shorty kits. However, the ones that really caught my eye were from a company that I had never heard of: Taiwan’s JB Unicorn, also known as JBU.

I was intrigued by the kits - firstly, they offered a licensed product with accurate trademarks. Secondly, they offered a wide variety of parts and configurations, allowing me to mix and match part in the future. Finally, the kits seemed fairly complete - meaning, I would not need to purchase a bunch of parts from a series of vendors and from a variety of manufacturers to complete my project. The various JBU kits had handguards, silencers, front sights, and inner barrels....all the major parts were included, except flash hiders.

On to the review:

The Packaging:

The kit comes neatly packed in a small cardboard box approximately 8 inches long, 3 inches wide, and 3 inches deep. The box is marked with a plain white sticker with the item number, item name, and the website of the manufacturer, JB Unicorn (

The first thing that struck me was how efficiently everything was packed. Pieces neatly packed within pieces to minimize wasted space. The package itself was quite light - less than 1 kg.

Inside, the individual components are wrapped in plastic sleeves to protect their finish during shipping.

The JBU kit, next to the donor TM M4 wired to the rear and awaiting a G&P 416 stock.

The Parts:

A collection of parts.

I opened the packaging and removed the individual components. Included in the set are:

1 X 3.75" replica Olympic Arms-branded tubular 3.625” FIRSH handguard, with barrel nut.
1 X one piece 3.5" metal outer barrel, which is cut to allow for front wiring. It does not have slots cut for mounting a front sight or gas block.
1 X 3" "barrel shroud" - it has no internal baffling, so it's not a silencer. It looks like it's made simply to cover up the extended inner barrel.
1 X 6" polished aluminum 6.03mm inner barrel
1 X handguard adjustment tool for the barrel nut
1 X allen key

All the parts appear to be of finely milled aluminum, and a anodized a matte black very similar to the current Classic Army M4 body finishes. The finish on all parts is free of blemishes. A closer examination shows the cuts on the handguard rails to be sharp, straight, and well-defined. As well, the threading of the handguard and barrel nut, as well as the silencer end caps and the barrel, is very fine and free of detritus and leftover milling material.

The handguard tube is straight, well milled, and engraved.

The handguard is a simple tube that threads to a barrel nut, and the barrel nut threads to the gun’s body. There is a pair of screws at the handguards’ 6 o’clock position right against the magwell that can be tightened to lock the barrel nut and handguard tube together tightly. The handguard appears way too short to house a battery of any kind (despite the inlets for wiring on the outer barrel), so running a rear stock, AN/PEQ or RediMag system is a must. The handguard has crisp, deeply inscribed Olympic Arms trades.

The outer barrel, mounted.

The outer barrel is a solid, one piece design that appears machined from a single piece of aluminum. The anodizing matches the silencer and handguard. The barrel threading is excellent – sharp, even threads. It’s important to note that the barrel does not have any channels for mounting a front sight or gas block. The outer barrel has the common 14mm CW threading, so any muzzle break pr silencer that normally threads on a M16 or M4 will fit.

The "silencer" is just a 3” hollow tube with a threaded cap at each end, contains no baffles or foam to act as a silencer. It is obviously designed as a barrel shroud to protect the extended inner barrel. It is marked on the left side:

CAL 5.56MM SER NO. 00827"

The markings on the silencer appear painted on in white - I can find no evidence of laser engraving. JBU advertises that the silencers are individually numbered. Only one of the endcaps is threaded to accept a 14mm CW threaded barrel – a regular M4/M16 barrel.

The tools appear to be made of steel. The handguard mounting wrench is simple bent steel wire of a heavy gauge – strong enough and suitable for the job. The handle has a plastic slipcover for grip. The allen key is also steel, but unmarked as to size.

The inner barrel is finely cut and has no visible imperfections. It is intended to extend right through the mock silencer and terminates neatly at the muzzle end of the silencer without protruding. The bore looks smooth and well polished, and the muzzle end is crater-cut. The cuts for the hopup sleeve are very sharp. As I do not own a micrometer, I will take JBU at their claim that it is 6.03mm inner diameter.

Fitting The Kit: Problems

After removing the outer barrel, NITRO rail, and RIS from my current setup, I inserted the JBU outer barrel and attempted to tighten the barrel nut. Here’s where I ran into my first and only issue.

If you are trying tom mount the JBU front end on a TM plastic body, the fake gas tube on the front of the upper receiver sticks out so fat that the barrel nut cannot be screwed down all the way. The JBU outer barrel will flop around and have several millimeters of movement in all directions.

The fake gas tube prevents the barrel nut from being screwed in tightly.

To solve the issue, I grabbed by handy-dandy dremel tool (something no airsoft-smith should be without) and grinding wheel and in 10 seconds the fake gas tube was gone, ground flat to the receiver. This shouldn’t be an issue with most aftermarket metal receiver sets, but anything that is based on the Marui (JG, A&K, Palco, etc) may have to do this.

After the fake gas tube was ground off, the outer barrel, barrel nut, and handguard mounted easily.

The rest of the mounting went flawlessly. With the fake gas tube gone, I used the JBU-supplied wrench to torque down the barrel nut, and this tightened the outer barrel right up, with no movement whatsoever,

The inner barrel mounted easily to the hopup unit, and extends through the mock silencer right to the tip.

The Verdict:

The kit, installed.

The view from the rear.

All dressed up and no Zombies to shoot

Overall, this JBU conversion kit is flawless. Everything inside the kit went together perfectly. Beautiful finish, excellent machining, great fit.

My only complaint would be the lack of mounting instructions. It’s not so hard, but it would have been ice to know ahead of time that I needed to modify the fake gas tube.

I’d give this 4.5 out of 5, for 2 reasons only

1) Lack of any way to mount a fake gas block on the outer barrel (for the sake of realism)
2) Lack of instructions

I’d definitely buy more JBU products. A great value for the price, IMHO.
Originally Posted by TokyoSeven View Post
That was a very bad move on your behalf. Sort of like cutting off your foot for money, but not getting the money first and then letting the person with the money run away.
Originally Posted by MadMorbius View Post
Liberals rely on emotion. Conservatives rely on evidence, and the Socialists rely on everyone else.

Last edited by Skruface; October 12th, 2007 at 15:46..
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