I have used them in my FX Independence and other air rifles and c This is especially important for springers to maintain compression and prevent damage.
I have loaded two felt cleaning pellets without a backup lead pellet in PCP type rifles and then recovered them in the yard to verify that there are dark stripes on them from removed gunk. I usually load a cleaning pellet and then a lead pellet behind it. I think it would be easier to shoot these into a bucket of water to recover and see how dirty the felt pellets are.
Again, for the spring piston guns, absolutely do not use a felt cleaning pellet without a backup lead pellet behind it to prevent damage.