Rebuilding the FN303

The FN303 is relatively easy to rebuild with the right parts and a few basic tools.  Unfortunately the parts are ridiculously expensive from through FN’s primary dealer, Midwest Gun Works (MGW), and in many cases are only for sale to LE/Mil.  Fortunately the FN303 is very similar to a paintball gun called the Automag, many of the essential parts are interchangeable and are readily available from Airgun Designs for reasonable prices.  Links to the required parts are at the bottom of this page. In this write up I will cover the replacement of seals in the FN303 along with some of the common issues I have seen. The key parts you will need are the Automag O-Ring replacement kit and the “On/Off (small) O-ring retro”, both are available from Airgun designs. While you are at it get an extra pack of the Power Tube O-Rings, it’s the most common seal in the whole gun and it’s nice to have extras.

One of the most commonly missing or rusty items, in my experience, have been the clamshell screw and nut. There is nothing special about these black 6-32 socket head cap screws however MGW wants $3 apiece which I felt was a bit steep. Neither my local ACE Hardware or Lowe’s offered any suitable replacement parts. The 1″ black 6-32 screws had a different head type which would not fit in the clamshell and the only nuts they had were stainless. Amazon to the rescue, the links to hardware are at the bottom of the page. In
any case you will want a T15 bit to remove the screws and disassembly of the clamshell is quite simple.

Once you have the “receiver” out you will want to remove the field strip screw from the left side which will allow for the removal of what FN calls the “operating system” which is the operational core of the gun.  Before accomplishing this step it is important to ensure there is no pressure on the gun. Even if you have leaky seals its entirely possible for the mechanism to retain some pressure. Residual pressure will, in my experience, cause a small brass assembly called the on/off switch to go flying across the room. It can have some good pressure so watch out that you don’t shoot your eye out. If your trigger mechanism is non functional the easiest way to ensure any residual pressure is off, in my experience, is to push the bolt forward with a screwdriver.

This on/off switch, as it is called is called in the 303 and Automag, appears, to me, to be analogous to the trigger mechanism/sear in a firearm and it pulls directly out of the operating system.  When you put pressure on the gun a small stainless steel pin extends out the bottom of the brass assembly as part of the automatic cocking process. It is possible for this pin to break off and though it’s not available from FN it can be purchased from Airgun Designs as the .75 Retro On/Off pin for $8. Only one of my 303s has had a broken pin so you probably won’t need one, but if you do see the link at the bottom. You should see two O rings at the “top” end of the On/Off assembly however sometimes they will stick inside the body of the Operating System. In a couple of my 303s these O-Rings were degraded to the point that they were in pieces and I had to use a can of compressed air to blow the little o-ring bits out. The On/Off assembly unscrews allows the replacement of the tiny little On/Off (small) O-Ring inside and makes it much simpler to replace the O-Ring on the outside of the assembly body. With the exception of the On/Off (small) which you ordered separately all these o-rings are in the Automag rebuild kit and it shouldn’t be tough to figure out.

There isn’t any particular order this all has to go in but where I go next is to remove the bolt, the thingy in front with a big spring, which reveals the “power tube tip” which is the brass piece at the end. It should unscrew fairly easily if you put a flat glad screwdriver across the notches in the face of it. The one pictured here is a one piece power tip however I have seen some that include a brass spacer. In any case you should see an O-Ring down inside the stainless steel power tube. Pick that O-Ring out and replace it with the appropriately sized one from the kit. As far as I have been able to determine the O-Ring on the brass tip itself is really just to provide some grip so that it stays in place. As long as it’s not falling apart you probably don’t need to replace it. The final item to replace on this end of the operating system is the bolt bumper, the flat rubber washer that goes around the power tube. All of them on my FN303s have been cruddy and falling apart.

Next up is to remove the regulator assembly on the rear of the operating system. Because I’m a retard who can’t have nice things I just clamp onto the thing with a pair of pliers and unscrew it. Then remove the regulator nut and piston assembly, the “spring” thingy behind the regulator piston should be greasy and you should not clean it off. In my experience these seals are often in very rough shape, fortunately they are all included in the Automag O-Ring Kit and it’s fairly straightforward to figure out. They can be a little tight however I have found that a small flat blade screwdriver carefully applied is just the thing. When you put the regulator screw back into the regulator assembly only tighten it to the point where it makes light contact with the spring and then tighten the nut. In some later write up I will go over adjusting the velocity of the FN303. The O-Ring on the front of the regulator assembly will be loose around the tip of the regulator piston before you screw it back into the operating system body. As best as I can tell that’s normal, it gets compressed and deformed to make a tight seal when assembled. Don’t over tighten the regulator assembly, all that really matters is that the seals make good contact and the “Z lock pin(s)” that stick out the right side are aligned to slide into the receiver.

If you have a bad interconnect hose now is the time to replace it. I have seen several FN303s with bad hoses and every time it seems to be from the hose being bent/kinked back against the side of the gun. If you have a 303 with a bent hose it might be worth replacing it while you have the thing apart. Originals are $65 but don’t worry too much because MGW won’t sell you one. After a little bit of searching online I found similar 8″ braided stainless hoses available both on Amazon and on various online paintball shops for $10-15. After ordering a few from various places and receiving identical appearing pieces I have come to the conclusion that they all come from China and it doesn’t really matter which one you buy as long as it’s for high pressure air. I received the suggestion after my previous FN303 post that cheap hoses were a waste of money however in the absence of any suggestions of where to find “better” hoses I really don’t have any alternatives. To remove the hose from the operating system I simply clamp the hose end nut in a vise and turn the operating system by hand. They appear to use LocTite at the factory but it’s not that difficult to break loose. When installing the new hose I have been using Teflon tape rather than LocTite as it seems to give as good a seal and is easier to remove. Depending on what hose you bought you may need to also swap out the female quick disconnect. If you are using the original disconnect the o-ring in it will likely need to be replaced as the majority of mine have been leaky.

FN oil = Generic Oil?

Once you have the whole operating system back together slide it back into the receiver and secure it with the field strip screw and power it up. Be sure to put a couple drops of oil down the air tube to help keep all those fresh seals nice and lubed. It’s much easier to troubleshoot any leaks with the clamshell halves off. I have rebuilt a few of my FN303s and with the Automag O-Ring Kit installed I have had 100% success.

Want a clean 303?

In rebuilding my various FN303s I have noticed a number of small variations.  Most seem to be largely cosmetic, such as the color of the regulator housing, and presumably come from different parts runs.  One difference that may make a difference to some people is that the majority of the “operating system” chambers are welded shut.  For the people who repurpose the FN303 for paintball use this presents a challenge as they must carefully cut the welds in order unscrew it and “devolumize” the chamber.  I’m not interested in paintball but from what I gather this involves fitting a round chunk of plastic in order to reduce volume which keeps the velocity down.  On two of my FN303s I have discovered that the chamber is not welded shut and is sealed with a unique red o-ring.  I haven’t gone hunting for a replacement o-rings as mine seemed in good shape.  Presumably for folks looking to convert to paintball these “unsealed” chambers would be preferable however I cannot tell any way to determine this without full disassembling the launcher, there does not appear any correlation with the other small difference such as regulator color.



Automag O-Ring Kit

On/Off Small O-Ring

Power Tube O-Ring

On/Off Pin – .75

HPA line

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