A firearm is a system. If you want a fast running MP5 style of gun, just get a MP5K clone. The barrel is shorter so there is less of an impulse from the round. The receiver is 3/4" shorter and the bolt group is shorter so it does need to move as far to cycle. So it takes less time to cycle. So the cyclic rate is is higher. To make the cyclic rate higher, the carrier in the standard MP5 would need to be accelerated or lightened. The problem is, if the carrier moves faster, it will hit the back end of the receiver hard and cause damage. So the spring resistance would need to increase. But that would decrease the cyclic rate. So you are back to the slower cyclic rate, maybe. I'm not a firearms designer, but it would be difficult to safely change the cyclic system.
I've got a reverse stretch. The cyclic rate is a little higher than my full size. It has a K carrier in a full size receiver, so the carrier is lighter, but it goes further in a full size receiver. My K cycles faster, but I've still been able to get singles out of it, but it does take a little more effort. To me the easiest thing to do for higher cyclic rate is get a K clone. YMMV.
Scott
