Those won't work, so it's not worth buying them.
There are two sounds from the gun that you hear.
1. The piston head hitting the cylinder head.
2. The gears' sound. And this is usually the louder.
So first, do as Monty said. Very good shimming (and it must be very-very good, not just the normal methods you usually read about). Stick with factory gears because they are matched. If you for example put a new motor in, which you should, the motor's gear might not match perfectly the bevel, and of course this is the loudest connection.
High torque motor also helps, because it will have faster first shot so the sound of it is shorter.
If you have space, you can wrap the gearbox in some foam. P90, AUG, maybe the M14 has some space in the body for this.
As for the piston head: forget the Element one, it doesn't work. I don't know, maybe CA is a bit better. However the best I found is a soft rubber bumper glued to the cylinder head and piston head. If you can shape them that they contact gradually instead of at once then even better. Shaping them is risky I got a few GB breaks until I figured out everything and unfortunately you can't get those exact rubber pads anywhere now, that I used. Try soft sorbo pads, they are also good.
With this on the piston head, the stroke is much shorter so you'll loose fps, and setting the AOE is essential.
The next best thing to do is to swap the cylinder for a shorter one. I mean if you have a full cylinder, put a 4/5 in, or if you have a 4/5 put a 3/4 in, etc. Presuming you have the correct one for the barrel length now. This will mean that the BB is still in the barrel when the piston hits, and the sound waves of the impact can't escape the barrel. This makes a huge difference in the impact sound you hear. Also it will further lessen the fps a bit.
Next, you put a replica suppressor on the gun with foam inside. This will change the sound to a deeper muffled thud instead of the snap you usually hear.
Use only single shots. The longer you fire, the easier they hear it.
