Did a bit of playing with them . Worth the effort in my opinion - because it takes very little effort .
The shims came from Umarex and I was surprised to see the description of the 48 shims : Ring F controlled velocity . Measurement below .
breech seal/shim experiment -
2shims= +20fps no real change in es or sd .
3 shims= additional 15 fps . +35 fps and still smooth and consistant ...gotta love it .
This was done on the .22 Panther .
Added a new seal afterwards . Nice cushioning affect when closing - is that "too" high now ?
24-350 shim dimensions .365" id x .483" od and .004" thick
48/52/54 .0787" (2 mm ) id x .401" od and .012" thick .
For the 48 shims : the F port shim :
5/64" is a bit loose so I figure it's a 2mm . Measures 1.89 mm with my caliper so yeah 2mm for all intents and purposes. That is choked FOUR standard bit sizes!! fwiw a #30 drill will yield a close to optimal "stock" configuration .1285" port. A straight through measurement of the port is in the .160" range - Way too big! Going with a straight through would more than likely give some harsh firing cycles. RussB and EdK and others have discussed this openly in other forum posts (search ,it's there) . Possibilties : sleeving the port with thin wall tubing , drill/tapping /plugging and drilling proper sized port, fill with material and drill or a simple shim ? hmmmm.
Just my observations .ymmv.