I bought one of the dovetail guides, a waste of money in my opinion. It cuts way too deep of a dovetail as sent from the factory, something like .100. You have to shim it up to get a shallow dovetail in the.050 range. This is a pain on a straight barrel and a super pain on a swamped barrel because you need different sized shims on each end, keeping the shims in place is a pain as well.
The guide is made of super hard steel, it will wear out your hacksaw blade and a standard tri-corner fine in short order.
All that said; some people love these guides; I am not one of them.
In the picture I have the guide on a barrel cutoff trying to learn how to use it effectively. It works but I can do a better job with my hacksaw and tri-corner file.