The reason we are allowed to own such deadly weapons is because the 2nd Amendment. The 2nd-A is about citizens being able to resist or overthrow a corrupt government. It is a very important and equally misunderstood concept even in our own country.

That said; I love old milspec rifles. All of the WWI and WWII rifles are great. Lee Enfield, Mauser, 1903 Springfield, M1 Garand, Schmidt Rubin and Moisin Nagant; all great reliable rifles.
The Lee Enfields, Nagants, some Schmidt Rubins and Mausers of all kinds of orgin can be had for $100-$300.
You can even get a filthy Mauser(Yugo, Turk...) or Nagant for less than $100 but you will have to clean all the cosmoline off.

All of these rifles are proven in battle and are as useful today as when they were made and are even useful for hunting larger game. Not as gnarly as a semi auto / auto of todays standards but still a reliable weapon. Some collect them for their nostalgia and some rifles with original markings and serial numbers can go for much more even thousands in the case of 1903's and Garands. Others buy them because they are inexpensive.

No special license to own and in fact in many staes you do not have to do the sale through a dealer as you do for non-C&R (curio relic).

The only "licenses" are for full auto and concealed carry in many states.