I fired off a .410 1/4 oz slug with my derringer a bit ago without any issues. There was a lot less of a "kick" than I expected and would have no problem firing this little gun several times in a row without the worry of a big palm bruise.
I don't see what all the negative complaints are all about
. There was even a degree of accuracy considering the short barrel length and the fact that only the last 3/4 inch of the barrel has a twist in it.
My thoughts and experience thus far is that this is a nice little project if you build as a kit or purchase the same factory assembled derringer...with that said I would highly recommend this firearm.