At the end of the barrel there are a series of threads. First and foremost the threads have to match. If they do not, you could strip the threads and make the marker useless. From here simply screw the barrel in till it becomes snug.
If you have the money, time and a good gunsmith, yes.
yes
A hole. When you put a hole in the barrel, it reduces the weight of the barrel. But, when you remove the hole, the barrel becomes heavier.
holes
8.5 pounds or more depending on how much attachments you put on it.
You can put a hole in the barrel to make it weigh 8 kilograms.
Usually, liquor.
Helium.
No, do not put any lube in the barrel, but on the threads.
You could put a 1 kg weight inside the barrel to make it weigh 8 kg.
Pay your bills and it will never have a barrel put on it!
During a time which the US military was fielding a .30 weapon (M14 rifle), jungle warfare required a weapon that was lighter and could "put the lead out." The M16 could do this. Consequently, the M16 was sent to Vietnam as a jungle rifle; all other US forces (army, marines, navy, air force was the only exception) retained their M14 rifles around the world. ONLY troops in Vietnam received the jungle rifle (M16). Until after 1970...then the M16 began to replace the M14s.