A good number of years ago, I experimented with Thompson SMG springs. They worked pretty well. I cannot remember who wanted to put a kit into production, but I had a bunch of Thompson springs and a timer/round counter that I used. I was trying to come up with a specific length of spring for a specific ROF, but I got tied up in other things. I did the same thing with the buffers that are available to change the ROF of Uzi SMG's. Get you some Thompson springs and work on determining the length that you need for a specific ROF. I had an M3A1, so the longer springs made it difficult to pull the bolt back with your finger. I like a ROF of around 750 RPM. I get that with my Uzi and I had that with my M3A1. I never took into consideration that this might beat out the back of the Greasegun. You can get away with it if you do not do a lot of shooting with the stronger springs in it, but my experimentation was back when Post May Sample M3A1's went for $200 each, so I was not too concerned about it. With the price of full auto guns now, beating up your weapon is definitely something to consider.
Charles Tatum
Alamo Professional Arms