I guess this is kind-of a stupid question.. But, what happens if the neck splits or something when it is fired? I don't think it will blow up, but what is the most likely thig that it will do?
How many loadings are usually obtained for 300 win mag cases?

The only "bad" thing that happens when a neck splits is that the case is no longer reloadable. It is also certainly possible that the point of impact of the shot is affected, but not usually to a large degree. But there is no danger from a split neck.

How many times can a .300 Win Mag case be reloaded? How many angels can dance on the head of a pin? The loading methods (FL or neck-only sizing) dimensions of the chamber as compared to the dimensions of the reloading dies, pressure of loads, "stretchiness" of the action, hardness and thickness of the case web, anneal of the shoulder and neck, and any number of other factors can have a huge impact on case life.

For maximum case life:

Resize the minimum amount

Load to mild pressures

Start with cases with a hard head and thick web

Properly (not easy) anneal the necks after they have been work-hardened from several loadings

If a neck splits on a specific cartridge case during firing, then the neck tension of that case is different than the others fired in the string.

At a 100 yards, there porbably would not be a major point of impact difference. However, as the distance grows, the difference in neck tension will show up as a shot outside of the group.

It doesn't do anything to the gun, etc..

Gurrie

Most everyone who has reloaded for some time will experience a split neck or at least a cracked one. Anealing the necks after a few reloading (about 4 or so) will do wonders to keep this from happening. Some cases have harder brass and it doesn't streach like a softer brass and it can split. I had my first split neck from a second time fired case not to long ago. All the rest did fine.

As a rule the hotter the load you produce the worse the case streches because it makes the brass flow,if you keep trimming the chance is you will get case head seperation which is very unhandy in a hunting situation.As for how many times can you reload a case is hard to answer I generally experiance enlarged primer pockets before i experiance over length cases because I tend to load on the warm side.Chamber pressure is what kills brass if your chamber headspace is not up to par. Lot of things contribute.