I use the first one pictured (left).
It does work, but there is a word of caution to go with that. Be careful, particularly with carbines that have a spring tube. Because of the shape, it is easy to crack the receiver with it. While, I haven't done that, I know others who have. Most recommend the other type.
A second note here, don't depend on the witness marks when lining up a replacement barrel. Instead, place a straight edge on the bottom of the receiver and a second one on the flat on the bottom of the barrel. The two should be parallel to one another. This will assure proper slide movement.
It does work, but there is a word of caution to go with that. Be careful, particularly with carbines that have a spring tube. Because of the shape, it is easy to crack the receiver with it. While, I haven't done that, I know others who have. Most recommend the other type.
A second note here, don't depend on the witness marks when lining up a replacement barrel. Instead, place a straight edge on the bottom of the receiver and a second one on the flat on the bottom of the barrel. The two should be parallel to one another. This will assure proper slide movement.