Depends on your lens. This lens (150-600mm) has one of those single foot mounts (removable). I found the mating of the foot mount ring that attaches to the lens had a really small amount of slop even when tightened to the max (actually, upon reflection, it may have been the lens draw tube that had some free play when it was extended outwards). I saw someone did this with their lens (different) so I decided to try with my lens. This is the only lens I can do this with, none of my others have the 'free space' to do this.
Mine requires dissimilar ring sizes (one is 1/2" smaller) hence the funky 1/4" spacers top and bottom. Scopestuff will sell you an unmatched (if necessary for your lens) set for 2x cost of the more expensive ring if your lens has the available space to do this.
Ideally, the forward ring should be mounted all the way back towards the rear, but as is I am able to use the standard mounting positions on the dovetail, all it does it limit my low FL to 300mm (the lens tube can't retract all the way in)
Edited by rk2k2, 28 November 2019 - 11:15 PM.