Here is a brief written review of Air Venturi's "Rail Lock" spring compressor.
In a nutshell, it works. But not without a few issues.
Firstly the construction is solid and durable. It should last a lifetime as long as you keep it clean and free of rust. Made in the USA that's a plus.
The instructions are brief and easy to follow. Pretty much fit one end to your scope rail Dovetail or Picatinny (more on this later) then center the drive bolt using the best feature the 'quick release' button. Then tighten the other screws. One smaller grub screw is supposed to help keep the drive bolt in position but the large quick release assembly has a detent drilled into it and more often than not, for me the 'alignment' grub screw would push it off center to one side. But if you tighten both screws well enough it stays pretty well centered.
Speaking of centering. When I first attached the device to my Benjamin titan using that gun's built in dovetail, the drive bolt was so low it was striking the end of the compression tube and could not be used. I ended up having to put a dovetail to picatinny rail adaptor on to 'raise' it some. Thing is now it is nearly too high and really pushes off to one side of the edge of the spring block. Some sort of way to adjust the 'height' of the drive bolt is going to be needed for many guns. Just a heads up. The device itself is NOT adjustable.
Also when using the picitanny adaptors one end of the tool is much wider than the other. It ends up in sort of a shallow V and can be quite frustrating to get on the picatinny rail correctly and still have the other end lined up with the assembly. See my comment about the 'centering' grub screw above.
The drive bolt has a nylon tip on it which is a nice touch, but it often tends to grab onto the spring block and rotate it while tightening the drive screw. If it was better centered this may not be as much as an issue. Also perhaps using a washer or something might help but as I said I was alredy way off center and there would be no practicle way to use a washer. I had to constantly put screwdrivers, punches or something in there to keep it lined up for the keeper pin. It was a bit fiddly but as I said in the long run it works.
To tighten the drive screw Venturi tells you to use one of the allen keys placed in the hole in the end of the drive bolt. While this seems perfectly acceptable and it works, it leads to alot of frustration from constantly having the darn allen key fall out of the drive bolt onto the floor. I probably have to pick it up at least 3 or 4 times every time I use this device! With my bad back not much fun. Not provididing a captive lever like one finds on a vise is a real oversite!
So there you have it. As I said it works, but not without some adaptation and patience!