For the record...
Here are the weights of the different uppers on an M-11/9 SMG:
MAX-11k 5 pounds, 3.1 Ounces
MAX-11Sk with CF-W Bolt 5 pounds, 11.6 Ounces
MAX-11 mk2 5 pounds, 12.7 Ounces
Practical Solutions M-11/9 Side Cocking Upper with CF-W Bolt 6 pounds, 0.1 Ounces
MAX-31 mk2 6 pounds, 2.9 Ounces
All of these weights are with the Lage Mfg K-Stock attached and without a loaded magazine in the gun. Also, no sights or accessories added to the upper.
For a fair comparison, I measured the unloaded weight because adding a 30 round mag' to any of these guns will increase the weight the same. Obviously adding a 50 round magazine or 72 round drum will increase the weight disproportionately.
Surprisingly, there is only about a one pound difference between the lightest and the heaviest guns and a negligible difference between the MAX-31 mk2 and the Practical Solutions M-11/9 Side Cocking Upper with CF-W Bolt.