I recommend them. I have the 21gr and the 28gr in .22 and they are well made and shoot amazing out of my Gamo Magnum.
Very accurate (hole in hole) and they fit well. I think on NSA's website, they have diameter measurements for the 28 and 21gr. Something like .217 if I recall correctly.
The 28gr load like a standard diabolo (push in the last half) and the 21gr drop right in.
The weight of the slugs and the power of my platform limit them to about 25FPE...but I'm going to assume that since they are a soft lead, they're definitely going to mess up the internals of those pesky rats.