The .222 was introduced by Remington in 1950 as a varmint cartridge and quickly became very popular. It is basically the parent of a number of rounds that followed: .222 Mag, .223, .221 Fireball, 7x47 etc. It was used in some of the early work on small caliber military rifles but it did not have the case capacity to drive the bullets as fast as the developers and the Army wanted. It was lengthened a little, blown out a little and this became the .223.
The .222 has a reputation for great accuracy and was the choice of the benchrest community until the 6mmPPC came along. It's popularity has diminished since the very similar .223 is the military cartridge and does offer a little more performance. It is still a very useful round though and a number of very fine rifles have been made in .222. It is popular in some foreign countries where the people are allowed to have guns but military ammo is forbidden.
The .223 does have an advantage in the availability of cheap military ammo. If you handload, the ammo cost differential is non existent. The .223 has a little more power but this is probably not an issue in the field.
What is the .222 Model 7 worth? Hell if I know. New Model 7s sell for around $550 here so a used one in good shape might be $350-$400. The .222 Model 7 might be a collector's item though so who knows. It would be a neat little rifle in a fine caliber.
my $0.02
Drue