I find slip on slings kind of a pain to use. Although permanently mounted swivels are unsightly, I much prefer them. I use a sling with uncle Mike's quick release swivels extensively on my French boxlock, and find it extremely helpful when handling the dog or scrambling up talus slopes after chukars. And, you don't have to worry about the sling interfering with the line of sight as you do with slip on slings. I do have a nylon slip on that I use with my Lefever, but I tend to avoid it because it's such a pita to put on.
