Those videos of people firing M1 Garands and MG-42s. Being that we dont really use that kind of ammo anymore, wouldn't we eventually run out, or do we still manufacture .303, and 7.92 ammo for these old girls?

Simple they still make ammo that is compatible with those rifles you just have to do some preliminary research on what to buy and you are good to go.

7327805
Oh thank God. I was worried that eventually the ammo for these guns would be gone and they'd be useless.

.30-6 springfield has been used in almost all US guns from the winchester lever action rifle to the m1 garand, and so they are still popular today for things like hunting and target shooting, even if some shells are not manufactured anymore a person might be able to hire a skilled gunsmith to maufacture a shell or 50.
my favorite shell is the french 8mm Label as it has a really funny look to it

Once you get into the world of exotic ammunition types, you've descended into the rabbit hole and there's no way out. If nothing else, reloaders can produce practically every round imaginable, from the Puckle Gun to the Dreyse Needle Gun. Replicas abound, because the originals can sometimes be too expensive (and dangerous) to shoot. (but too darned fun not to)

7327848
Is it true if you pour too much gunpowder in a Flintlock it will explode?

7327874 If you are shooting a period black powder weapon and you put the *right* amount of black powder into it, you run the risk of explosion. Remember, not only is the weapon getting close or over the century mark, but the process of making a barrel and breach at that point in time was problematic, and you can have invisible hairline cracks propagating through various impurities and welds from all the other times it has been fired. Age, expansion/contraction cycles, moisture, strain, and impact from firing can make old weapons more dangerous to the user than the target as they are used.

Hence, replicas. Modern metalworking gives Hollywood an endless stream of weaponry that people of that time period would drool over. Just look at the arsenal that Lord of the Rings put out in cutting and chopping stuff. (And people like me add to our collections) If it ever came down to a war where firearms did not function, companies like Cold Steel would make out like bandits.

7328053
Oh, like old medieval swords heavily corroded? I have heard the heat from MG's can wear out the barrel. In the 2010 Medal of honor video game, theres a mission where your flying AH-64 Apache longbows, and when you run out of ammo, the pilot says "Wouldnt matter. We just about melted the barrel". And in some WW2 games, when theres an MG-42, someone will yell "Wait til he changes the barrel"!

7328061
Yes, the MG-42 was an incredibly unreliable weapon that often melted its own barrel due to the heat from its air-cooling system. The stupid fire-rate was so high that often MG-42 gunners could not aim well and it was mainly used to suppress enemy forces rather than actually kill anyone. Thus, MG-42 gunners and their assistants often had to carry 1-2 extra barrels in case they overheat. Light LMGs such as the BAR or the DP-28 were far more effective, and this contributed to the ineffectiveness of German small-infantry tactics compared to either American or Soviet tactics.