As I mentioned in my review, the Air Tech 4000 is a pretty average blaster for how large it is. It does however have a whole boatload of potential to be something amazing like it's smaller counterparts, the Air Tech 3000 and Air Tech 2000. Of course, being practically twice as large as an AT3k, it's going to require roughly twice the work. Basic premise of this mod is a full re-barrel with rear loading capability and a pump replacement. Time to get messy.
Materials
Twelve, 1' pieces of 9/16 Brass Tubing(or whatever barrel material you want)
1' of 17/32 Brass Tubing(if you plan on using stock darts)
Hornet AS-6 or Magstrike Air Pump
About 2' of Polyurethane Tubing(air hose)
Super Glue
Electrical Tape
Craft Foam(or something else suitable for a turret seal)
Barrel Spacer(optional)
Tools
Vise or Clamps(recommended for drilling)
Power Drill or Drill Press with a 9/16 Drill Bit
Hot Glue Gun
Dremel with Cutting Wheel and Sanding Bit
Pipe Cutters
Phillips Screwdriver
Flathead Screwdriver
Hammer(I'm serious)
Xacto Knife
Sandpaper
Pot of near boiling water(if all else fails)
Go ahead and open up your AT4k and pull the screws out of the shell. Take note of where they go as this thing is a mishmash of screw sizes so remembering where they go is important. Now take a long, hard look at those internals. Basically the only pieces we care about retaining here is the air tank and part of the rotation mechanism. Since we're replacing the pump and adding rear loading, auto rotate will no longer be possible. Assuming it wasn't already broken to begin with of course.
Gut the shell. Leave nothing behind except the screws holding the cosmetic trim on and the glued in portion of the rotation mech. At this point you can go ahead and disconnect the goo gauge from the air line. As much as I'd like to leave it alone, you can risk blowing it up with a plugged pump so it'll just be there for looks now.
Take your air tank/rotation mech combo and take the whole thing apart. I'd advise taking pictures so you remember how it goes back together. Once it's apart, you can remove the turret. Now this is the fun part. I mean, it might be fun depending how good of job they did solvent welding the damn thing. So we'll see.
Start by removing the screw holding the central retaining piece in place. Take your hammer and flathead and start splitting apart the solvent weld holding the barrels together. Then turn the turret upside down, with the barrel you want to extract hanging over the edge of your workbench. Stick the flathead into the hole in the rear and start hammering out the barrel. Do this for all of them. In my case they all came out remarkably easy but your mileage may vary. If you start seeing stress marks in the plastic, stop hammering and bust out a pot of near boiling water. Dip the turret in for about 30 seconds or so and then try again.
Now your turret should hopefully be clear of it's stock barrels. Later pictures don't show this but you also need to grind down the ridges on the back of the turret, otherwise these will interfere with your seal later. Then take your drill and the 9/16 bit and bore out the holes in the back. These will be your rear loading holes. Clean it all up with an Xacto and sandpaper if need be.
Now to start building your new barrels. Take your foot of 17/32 brass and cut it with the pipe cutters into inch long pieces. For each barrel, stick one of these pieces into the back of the foot long 9/16 piece and then put two to three tightening rings with the pipe cutter to secure it. It doesn't take much so don't crank down on the cutters and slice a hole into your barrel. The section of 17/32 is simply there to provide a tight fit for stock darts while the looser 9/16 will reduce friction when you fire.
Wrap the ends, the ones with the tightening rings with electrical tape and ram them into the turret. You probably won't need adhesive if they're tight enough but you also don't want any air leaks. My barrels came out pretty straight so I'm not going to use a spacer but if you happened to use PETG or something more flexible for your barrel, a spacer might be necessary.
Swapping the pump is fairly easy provided you aren't too bad with a Dremel. The stock pump kind of blows so I'm replacing it with a Hornet pump which will be better suited for higher pressure. Since the stock air hose is too short after getting rid of the goo gauge, you'll need to replace the whole line. Line up the Hornet pump at a slight downward angle and cut your slot in both sides of the shell. This might take some trial and error so start your cuts small and gradually work your way up until the pump fits.
One the pump is in, it will unfortunately leave you with a crap load of dead space where the old pump was, so I decided to improvise. Take the stock pump tube, chop off the nub for the air hose and then bore out a hole in the back. Then cut a length of CPVC or whatever will fit in the old pump to stop the Hornet pump from shifting back. Run the air hose through all this along with the trigger and it should keep everything nice and tight. You may have to put a dab of super glue on the hose connectors so they don't pop off under pressure. Also, plug the Hornet pump if you wish.
Go ahead and partially re-assemble your blaster to see if everything fits and feels sturdy. Test your air system and see if it holds pressure and discharges. If you plugged your pump, I tested mine up to six pumps with no issues. If all things check out you're basically done with the hard part of the mod.
Now for the rear loading slot which will go on the other half of the shell. This can be a little nerve wracking but it's far easier to work from inside the shell so you know exactly what's being cut. Take a pencil or marker and draw an outline of your slot on the inside of the shell like the picture above. Don't Dremel along these lines, cut well inside them to give you room for error and then take a sanding bit on lower speed and start evenly grinding down to your cut marks. You'll also have to cut a piece of plastic out linked to the screw hole too as it's obstructing part of the turret. Finish it off with sandpaper and you should hopefully have clean cut loading slot.
The tank assembly will also need some work. Lop off the support pieces for part of the rotation mechanism as they'll get in the way of the loading slot and finish it with sandpaper. Then take your craft foam, in my case I just used a slice out of a Big Bad Bow arrow since it was a good fit and start padding around the outlet of the tank with super glue. A bit over a quarter inch did me fine but you want a little bit of compression in the foam for a good seal. Also, cover the spring on the firing pin in hot glue, you'll get a much nicer trigger feel as a result.
Attach the turret and rebuild your rotation mechanism. Just leave the long gray piece off to the side. Line your turret up and then cover the extension spring for the lever in hot glue to keep it from moving as seen in the picture. I'd also advise covering the screw inside of the white cap of the rotation mech with hot glue just to be sure it won't unthread itself. From here you can reassemble your blaster and enjoy your new professional grade welt maker!
No official word on performance yet but with stock Elites at six pumps, you basically pull the trigger and the dart is already where you're aiming. Heavier tips like Accustrike darts improve accuracy immensely and having rear loading is god send on something that would otherwise be extremely inconvenient to load with barrels this long. Three pumps will get you a quick shot off with respectable power where six pumps is the most I'm willing to charge this tank with. Really genuinely pleased with how well this turned out though and it's among one of the most powerful blasters in my arsenal now. Not sure if I'd want to be shot with it though.
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