Other air rifles with the same seal diameter produce more velocity with even a slightly longer stroke. The R9/R10/HW95 and the Beeman Kodiak/Webley Patriot all owe their high performance to a high diameter-to-stroke compression chamber ratio. Still, when you add a Maccari kit to the FWB 124 the performance is noticeably increased--it adds a nice amount of knockdown power and more yardage to its effective range.
For those who have yet to add an ARH piston seal: The fit of the piston seal will probably seem a bit on the tight side when you first install it compared to some other rifles, but if you retest the velocity in a couple of days you'll see an increase because the seal will have fit itself to the ID of the chamber. The circular valley in the face of the seal is a little wider than on some other seal designs so it takes a little time for it to reduce to the ideal size. It's not a defect--the OEM seal was/is the same way.
Sometimes it pays to turn down the ARH piston seal a few thou if the fit is VERY tight in the chamber when you try to install it. As with some other models out there, even though FWB was a high quality manufacturer, the ID of the chamber in their air rifles can vary from one specimen to the next. However, if in doubt it's better to leave the seal a little too-tight than too-loose. Like I said, it will adjust itself after a little shooting, or even after a week or so of sitting undisturbed in the closet. If you're going to adjust the OD of a piston seal, always leave the leading edge of the piston seal a little larger than the trailing edge to guarantee an airtight seal in the chamber.
Safe and Happy Shooting!
Ed Krzynowek, The Airgun Tune-Meister
"We can rebuild the squirrel. Make him stronger, faster...We have the technology"---Skyler M.