Your question is to vague. What do you mean by "long range? Depenidng on that answer, barrel length could be from 24 inches on up.
You need a gunsmith.
A really good barrel is the beginning.
The effective range of an M16 assault rifle with a 20 inch barrel is 550 meters for a point target, and up to 800 meters for an area target.
No reference for a savillion rifle. If you mean civilian, same as for a military rifle- it will depend on the caliber and barrel. Military rifles in the US are made by civilian companies.
The 308 Norma Mag is beyond a hunting rifle. It is more of an AMW. (Anti-Material-Weapon) or a very long range sniper rifle. If the rifle is in good shape and still has the standard wood stock and medium barrel, I would say it is worth $1,000.00 American dollars.
By definition, a carbine is a short rifle. The standard M-16 has a 20" barrel, while the M-4 has a 16" barrel. A number of US Rifles have had carbine versions, with the notable exceptions being the M-1903 and the Rifle, .30 M-1.
I do not personally, but yes, there is a single shot rifle that can have the rifle barrel replaced with a shotgun barrel.
1892 Winchester caliber .44-40 carbine with a standard 20-inch barrel
That term refers to the outline shape of the outside of a rifle barrel.
A bayonet is the name of a knife attached to the barrel of a rifle.
Well, yes and no. Support should go under the forearm, not the barrel. When a support is placed under the barrel, it MAY change the point of impact. The pressure of the forearm on the barrel may change, and the normal vibration of the barrel is changed. For a hunting grade .22 rifle used to 30 yards, you probbably would not see a difference. For a long range target rifle used at 500 yards, you WILL see a difference.
The effective range of an M16 assault rifle with 20-inch barrel is 550 meters for a point target, and 800 meters for an area target.