A National Guard friend of mine, currently on active duty, tells me that the weight of the much-modified M-16 she is carrying (with red dot sight) is 9 pounds. This caused my semi-annual smile, because one of the selling points of the Armalite rifle from which the M-16 evolved was that it weighed only 7 pounds, and was much easier to lug though the rice paddies than the M-14 which was 9 pounds, or the M-1 which weighed 9.6 pounds (SIR!). So we are back to square one.
Actually, there is something to be said for heavy rifles. Some years ago in Texas, Craig Boddington loaned me an 8mm Remington magnum that had been built for him by John Rigby (in California, not London) and weighed 12 pounds. Because of its weight, the rifle had almost no recoil, and as fate had it, the shot I got was one where I had to jam my eye right against the scope. I resigned myself to a great scope cut, but nothing happened. The rifle hardly moved when I pulled the trigger.
The Thompson submachine gun weighed 12 pounds and was extremely effective because it recoiled very little. Ditto the Browning Automatic Rifle at just under 20 pounds. Civilian guns, too, profit by some weight. A skillful shot can hit with a 6-pound rifle (in a reasonable caliber) but an unskilled shot will have fits–the gun will be just too twitchy.
The real problem with a heavy gun comes in rough country, particularly mountain country. You not only have to carry the thing uphill, but because the footing is uneven, you’ll be fighting the rifle every step of the way as it does its best to pull you off balance.