In many cases the ULT does not have sufficient return force. I have never had any luck getting the ULTs to work in pneumags.
I have one of each and I will be extensively testing them. I can tell you that each rail Weighs about 1/2 oz
I read elsewhere that using a ult on a pneumag would allow one to use a lower lpr pressure. I, too, have zero experience with pneumags but am preparing