Most of the complaints about the FAS 6004 are the result of Chiappa's mass production. The original 604 was fitted and assembled by hand. The 6004 also appears to be hand assembled but the ‘fitting' part seems to be missing. Corrections can be made to undo the Chiappa errors. Not many shooters have machine tools, so I came up with simpler work-arounds.
The original 604 was designed to work with a unique, wide breech seal which was later replaced by a common o-ring that sits deeper in the seal’s groove.
IMO, the o-ring required a bit more finesse with barrel-to-port-face spacing during assembly which FAS accomplished but Chiappa has not. As I see it, to expedite assembly without the fitting steps Chiappa added an aluminum washer/spacer on the barrel to obtain a tight seal. The result of that genius, though, was ditching on the port face without creating a good seal.
Complaints about the pistol farting and/or failing to push some brands of pellets down the bore are partly related to the seal allowing compressed air to blow by it. The other part pertains to the breech chamfer. If you examine the breech of your 6004 with a magnifier and it looks like it was cut by a visually challenged person with a chisel, you have likely experienced the FAS flatulence and pellet sensitivity.
Chamfer roughness on the pistol used in this thread bit into the skirts of some pellets preventing them from seating sufficiently into the barrel with finger pressure. That situation, combined with seal leakage, occasionally failed to launch a pellet.
The first step to joy is to smooth the port face. Use oiled 400 grit wet/dry paper wrapped around a flat block of wood or piece of metal and sand the face smooth. Apply light pressure and circular motion to maintain flatness of the face.
Smoothing the breech chamfer is next after removing the barrel from the top frame. The barrel is retained in the top frame with two screws. Remove them, and the bracket, and the barrel will slide out so you can work on it.
To lap the chamfer you need something similar in shape to the chamfer bevel. The hardware store is your friend. I found an acorn nut that was a close match, and threaded it onto a pencil to make a lapping tool. Use powdered cleanser, Softscrub, whitening toothpaste, pumice powder, wood working ‘rotten stone’, or any other fine abrasive as a lapping compound. The dry abrasives will need to be moistened with oil. Take your time [it isn’t a fast process] and spin the nut back-and-forth in the breech. Add compound as needed, and check your progress frequently so you don’t overdo it.
You want this.
Lastly in this process, knock the sharp edges off the rifling lands with some 400 paper wrapped around a pencil.
Now that you have a smooth port face and refinished breech chamfer, it’s time to re-seat the barrel. The end goal is to close and latch the top without the barrel contacting and scarring the port face. Leave the factory aluminum spacer/band off the barrel and reinstall the barrel in the top frame. Position the barrel clamp and install the two screws. Don’t tighten the screws, just snug them so the barrel can’t move. Make a paper shim with two layers of printer paper and fold it over the port face. With the o-ring removed from the breech groove, close and latch the pistol’s top frame. Now, with a wood dowel and some form of striker like a small hammer, from the muzzle end tap the barrel back against the paper shims. A couple of light taps will get it done. Next, unlatch the top frame and tighten the two clamp screws. Don’t overdo the tightening, the screws thread into soft alloy. With the breech seal back in place, there should be no marking of the port face. If you do see a mark, use a third layer of paper, remove the o-ring and seat the barrel, again.
In my opinion, the breech o-ring is too small. You can compensate for its size by making a shim washer from paper card stock, or purchase an aluminum shim from McMaster-Carr. Their .010” shim works nicely, part 92661A575. Note that McMaster’s shim needs the inside diameter enlarged a bit to fit properly. 400 paper wrapped around a pencil taper with some wrist action will do it. For an improved seal with a much broader contact surface, try a square-profile Viton o-ring from McMaster, part 1170N21. This square o-ring used with the McMaster shim appears to be a very good setup.
If you haven’t paid attention to the compression piston’s felt washer, it can be seen through the small intake hole in the top of the chamber when you draw the piston back. Dampen the felt with 25wt silicone oil to lube piston’s o-ring and obtain maximum muzzle velocity. A good source of the oil is a hobby shop where it is sold as shock absorber oil for RC cars.
At this point, the 6004 should be transformed into the pistol that the original FAS engineers intended it to be.
addendum:
A rare problem that should be mentioned is what appears to be a sticking dump valve. I’ve found that lubing the piston’s felt washer every 500 shots with a few drops of the 25wt silicone oil seems to sufficiently lubricate the 6004’s valve. If I see more of that condition, I’ll tackle it and post something.
I should also comment that the wood shaping and surface finish of the adjustable grip that Chiappa offers for the 6004 is very nicely done. Unfortunately, they’ve made the grip too narrow which leaves the fingers dangling in space unsupported. Their ambidextrous grip is of the same finish quality and very much ergonomically superior. I recommend buying the pistol with the ambi and then, if you must have an adjustable grip, apply the money saved toward a Rink grip.
