What are these lens used for?
They make your foresight picture look bigger.
They are legal in some competitions, but not in ISSF.
Like a lot of magnification they can help, but they can also magnify problems as well.
I use one for 7.62 target rifle. It magnifies the image of the target and fits on the front of the foresight tunnel ie in front (target side!) of the foresight element. This means you need a bigger foresight element. As it is only one lens it does not make the image clearer, just bigger. I think you can put them on so that they magnify both the element and the target picture, but I have never tried.
I started using one for long range(900 and 1000yds) first and now use it all the way through from 300yds as my eyesight continues to degrade!
It works very well, but does introduce something else to go wrong or forget, plus if they get wet ...disaster, you can't see a thing!
You come into this world with nothing, go out in debt and you've made a profit