Marushin FN 5-7 Five-Seven CO2 Repair and Maintenance

This time, I'll be repairing a Marushin CO2 gas blowback FN Five-seveN that I was asked to repair.

First, let me describe the extent of the damage.

Damage status and type of damage

Last fall, I went airsofting with a client, and when he returned from the shooting range, he found that the hammer wouldn't down when he pulled the slide of his Five-seveN, and no matter how many times he pulled the trigger, the airsoft gun wouldn't work.

First, I disassembled it as much as I could on-site, and the cause was quickly identified.

The sear that holds the hammer was broken.

When I asked him how many rounds he'd fired so far, it seemed like it was less than 2000.

Upon closer examination of the fracture surface, it appears the casting quality is not very good, and it looks like low-cycle fatigue fracture, which makes me suspect a design flaw (this is just my personal opinion).

I realized that the repair was impossible without the necessary parts, so I played with other airsoft guns that day and went home.

I felt bad for the client, but a little while later I ordered the sear and hammer assembly from Marushin.

The ordering process involves checking stock availability by phone, then paying via registered mail with shipping costs included, and finally receiving the parts.

The registered mail fee is a bit of a pain.

Also, the fact that it takes over a week from sending the registered letter to receiving the parts is a bit disappointing for someone like me who's used to modern online shopping. (Online services like KSC's are much better.)

I bought two more because the shears seem like they'll break again in the future.

Unfortunately, as of the time of writing (February 2021), Marushin's re-release has included a change in the sear component. Research suggests it's now made from machined zinc or aluminum, but the one I received had the older cast sear (it seems they were aware of the problem).

Did they prioritize re-released items? That's a shame.

Repair

We'll start by disassembling it to begin the repair.

First, remove the slide as usual.

Remove the two pins at the rear of the frame.

Signet Pin Punch Set
It's not a luxury brand, but it's a recommended mid-range brand in the automotive industry that offers good value for money. Plus, it's inexpensive.

Eagle Force Maintenance Bench Block for Handguns
It's incredibly convenient to have. Moreover, it's designed for handguns but can also handle longer firearms, making it very useful. In fact, I personally didn't see the need for their M4 version.

Vessel Plastic Hammer
It's inexpensive, sturdy, and long-lasting. For a good balance of control and power, the 1-pound size is recommended.

Points to note:The top pin has an E-clip to prevent it from coming loose, so you have to remove that first before you can remove it.

These are the E-clips I removed. Since these clips don't do much work, I'll leave them off for easier maintenance in case they break again.

Once the pin is removed, the hammer unit can be detached.

The shear is broken and hanging down.

This is the starting point of the damage.

The part in the red frame is the damaged shear. This can be removed by removing the negative bolt shown in the photo.

For reference, I've included a photo of the hammer area.

What's interesting is that, unlike Tokyo Marui's models, the knocker return mechanism is located inside the hammer unit.

The knocker return mechanism has retracted.

A top-down view of the entire hammer. The hammer spring appears to be quite strong to withstand the pressure of the CO2.

All that's left to do is replace the hammer assembly and sear that I procured here, but since you asked me to do it, I'll do some simple maintenance.

First, we'll disassemble the slide.

The slide can be disassembled by removing the three screws at the rear of the slide and the rear sight, as shown in the next photo.

Two screws on the blowback engine side

• Screwdriver set
While cheaper options are fine, for affordable yet high-quality products, I recommend brands like Hazet, Tone, KTC, and Vessel.

This will allow you to remove the inner frame.

Remove the plate that holds the slide stop in place.

This was the limit of what you could reveal to me.

The bolts holding the blowback engine in place were incredibly tight and wouldn't come off.

Since there's no other option, I'll just lightly clean it with silicone spray and assemble it.

This time, I tried applying Bellhammer No. 2 extreme pressure grease to the sliding parts of the components.

Bellhammer No. 2 Gold Grease
This is a classic, top-quality grease. No. 2 has the same ingredients as No. 0 but with a higher viscosity. I primarily use it for metal-to-metal connections.

Next, pull back the feeding nozzle and take a look inside the blowback engine.

It's brass.

I sprayed the inside of the blowback unit with silicone spray until no more black paint ran out (it was quite dirty).

All that's left is to assemble the frame and it's done.

I tilted the hammer and confirmed that the shear was engaged.

Shear is released.

The sear is holding the hammer.

When you pull the trigger, the sear is released, the hammer falls, and the valve knocker is exposed.

Despite the hammer spring being quite stiff, the sear's wall thickness is quite thin, so I suspect many people have experienced breakage with this.

Damaged and new shears (damaged on the left, new on the right)

I haven't tampered with anything that affects performance, but I measured it out of habit. (Marui Bio 0.2g, hop-up setting 0)

If you're interested, please also check out my review of the chronograph.

Live firing video (room temperature around 12℃)

Marushin Five-Seven CO2

It works so well that it's made me want a CO2 gas blowback gun myself.

・Marushin Five-Seven CO2 Aluminum Piston ver.2
I think it's generally well done.
Version 2 should have a machined shear to address the issue.

・Marushin Five-Seven CO2 Brass Piston ver.2
The brass piston makes it slightly heavier, which increases the weight of the blowback. Version 2 should address this issue.

・予備マガジン
Its highly safe structure provides peace of mind.
Some from bad manufacturers will burst.

Marushin CO2 Gas, 30-pack
I think Marushin offers the best balance of price and quality.

Summary and advice

You could certainly ask me if it breaks, but it seems that sending it to the manufacturer will fix it cheaper. (I don't know how long it will take, though.)

Looking at the parts' shape, the sear is so weak that it will break even with minimal firing. Therefore, it's a good idea to keep a spare.

If you have a punch and needle-nose pliers, you can fix it on the spot.

Furthermore, for those planning to purchase or order parts, it seems that the sear has changed from a zinc casting to a machined aluminum or zinc part, so if you're ordering online, check the production date, and if you're buying in a store, it's a good idea to visually inspect it before purchasing.

You can tell whether it's the old product or the new one by looking at the sheer surface and its smoothness. The new product is shiny, while the old product has a slightly cloudy appearance.

If you're interested in the mechanism of gas blowback guns, I've also provided an explanation, so please take a look.

CO2 Magazine Strength CalculationI've tried it out, so please take a look if you can.