I was feeling "robbed" that my 46M was only shooting at 400 to 430 FPS, rather than the rated 500 FPS. So, I decided to increase it compression ratio, by utilizing the piston stroke "TDC" adjustment mechanism.
The forward most position of the piston in the compression cylinder is adjustable in 4 increments per rev on the 1 mm pitch thread. This is a factory adjustment, but can be repeated to compensate for linkage pivot wear, or to detune the pistol.
First, I decided to shoot some other pellets that are close to 7 grain (the assumed pellet to achieve 500 FPS): The 7 grain Meisterkugeln I have, shoot at only 390 FPS out of my 46M, but seem "slow" in every platform
. This, while 8.2 grain Meisterkugeln shoot at 430 FPS, and 10.34 JSB domes at 400 FPS, out of this SSP pistol.
I have some HN Excite 7.4 grain wadcutter in unopened cans. On shooting them over the crony, I discovered that they behaved much better, at 465 FPS. However, this is still not "fast enough" to convince me that my 46M was shooting up to par.
I went looking for info on taking the pistol apart to access the piston stroke position adjuster. The most useful was a slideshow in exe format. Now, I am normally leery about clicking on any free exe, but the file opened fine. I wanted to share this "how to guide" with you, but can't remember or find which site I got it from. So, as GTA does not allow such large files, or exe format attachment, I copied, compressed and attached each image; and posted it to this thread as a separate entity: https://www.gatewaytoairguns.org/GTA/index.php?topic=153388.msg155687765#msg155687765
The red arrow in the image below shows the locking screw and the four increments per rev on this 1 mm pitch thread.
I figured that shifting the piston's TDC position by one increment might not be enough, so I tried two; or 0.5 mm more air compression. The result was that the HN 7.4 grain wadcutters now shoot at 495 FPS. This is close enough to 500 FPS with 7 grain pellets
Now, I noticed that closing the cocking lever with the valve open has it come to a cushioned stop, with the lever about an inch from the parallel to the air tube rest state, when locked closed. Pushing the lever closed then takes about 5 lb on the lever (much more at the piston), indicating that the seal is running into the end of the cylinder and being compressed.
As this may damage the seal and actually make it leak in the future, I think I will back off the adjustment by one click. That way, the piston TDC would be 0.25 mm closer to the end of the cylinder from "out of the box"; rather than 0.5 mm further, with obvious interference. If I can get the piston seal to just meet the end of the cylinder, I may not lose any velocity...
Other pellets also gained around 30 FPS. If this seems like "nothing", consider that it represents a 20+% increase in FPE...
Anyway; once you have the pistol stripped this far, it becomes apparent that it really is beautifully simple in design and construction; and quite easy to work on.