H&R Handi-Rifle
The most commonly used sniper rifle in the Marines is the M-40A3. It is a standard bolt action rifle with a Remington 700 action that fires the 7.62X51 NATO round.
The NORINCO CQ5.56.
Not made as a standard item. It will be too long, and the rifle was not designed to support THAT much weight on the magazine catch.
The ruger rifle
If you meant come loose, it is because the rifle was not meant to use a 30 round magazine, and most were not meant to use a detachable mag. The standard is an attached, 10 round magazine. It is likely that yours fits poorly, feeds poorly, and will have a tendency to pop loose. Just my opinion, you are much better off with the original, since it works, and does not fall off the rifle.
M16A2 rifle
All of them are.
For WHO?
No- it is a better handgun round.
Yes, because the difference between a standard round and a hollow point is only the mushroom at impact.
Cheapest is probably Moldova but the cheapest for good quality of education and standard of living is Ukraine.