New recoil springs will help... one of the design features of Len's semi conversion is the removal of the buffer... which would explain the violent recoil impacts against the rear of the lower slide... presuming that your recoil springs are weak or missing. I believe he did this to make more room for the striker to reciprocate. Adding a plug in the base of the return spring tube would add more "spring resistance to recoil." I have never had to do this but I've not put large volumes through my Brens. Another item might be that the barrel gas port has been opened up to much and is venting more gas than needed despite your low setting... another Len feature is that he has been known to drill out oversized the small hole in the gas regulator... so you might be using more gas than you would expect... you could obtain for little cost an additional gas regulator and determine if yours has been so altered or not. More gas was needed on some guns to overcome the resistance of the striker and the striker spring. I'm not proficient at posting pictures... if someone wants to... I can provide images of Len's design features for posting.