So, me and and my dad are getting into reloading and have everything we need to get started. Only problem, I'm deployed with the army. so for the time being I'm reading everything I can about the process and different capabilities with reloading. Everything from swaging my own projo's to fire forming and reshaping one caliber brass to another. The only thing I can't seem to find any information on is powder criteria. Such as, what does powder burn rates have to do with what caliber a powder would be good for? And also, what are things to consider when reloading for shotguns after you get the basics down. Like i see all these people playing around with different wads and shot/slug weights other then the load data "recipes" made by manufacturers, how do they decide what is Ok to use and what isnt? I don't plan on doing any of the more advanced stuff like playing with shotgun loads aside from the approved load data or making any "one off" caliber reloads for a while, I'm just in the pursuit of knowledge for the time being. Thanks in advance.

The Lyman manual has good information on metallic loads for you.

For shotgun loading, those shells are very dependent upon the specific hull/wad/powder/primer/shot used. They don't really have a flexible way of mixing components. Sticking to the exact data they provide is best.

Powder burn rates are very important. The optimum set up would be to have the powder just finish burning and building pressure as the bullet exits the muzzle.
Using a fast powder, it will be done burning and pressure will level off while the bullet is still in the barrel, the bullet then slows down. Sometimes it can even stop in the bore creating a blockage.
Using a very slow powder, the bullet will exit the barrel before it has burned completely creating a large muzzle flash. While large muzzle flashes are fun to see it is a waste of powder because it hasn't had time to burn all the way.

You also have to take into account semi-auto type firearms that use a gas system. You wouldn't want to have too much pressure in the gas system to blow the gun up so, you look for published data on powders that have been tested and approved to use.

Reloading manuals will have powder burn rate charts and listed powders and charges for specific weapons, calibers, bullets etc. That is a good source of info.

Loading for shotgun is pretty much the same. You want the powder to be fast enough to give the velocity you want but still be slow enough to just finish burning as the shot exits the muzzle. Wads do many different things. Some cushion the shot, some enclose it till it comes out the barrel or both. Again, a reloading manual will tell you all that.

You are doing yourself a big favor by reading as much as you can and asking questions before starting. You are way ahead of the guy who buys the stuff then starts loading without much knowledge of what he's doing.

started loading b\4 computers or the internet existed.

loading manuals and reading are what you need.

clown

Burn rates are determined by the powder manufacturers.  

Studying the chart isn't going to gain you much.  For example, look at any of the hundreds of threads in here that ask "what is the best/favorite powder for .223."  There is a spread of favorites and they are spread out on the chart.  

You may notice that some people also ask, "what's a good powder for .308," or "what's a good powder for both."  The favorite powders for both tend to be at the "fast" end of one caliber's range and the "slow" end of the other caliber's range.  For example, Lyman manual lists 114 powders in a relative burn rate chart.  Most of the powders people use for .223 are between about 60 and 84.  Most of the powders used for .308 are between 34 and 80.  (Both are rough ranges from a quick look.)

Within this chart, say for .223, I like TAC for pretty much every .223 projectile. I use it for 52gr through 77gr for high power shooting.  Another powder that high power shooters like is H-4895, which is near TAC on the chart.  I also like Win748, which is a couple spaces down (slower), but only for the lighter projectiles.  I don't get as much velocity as TAC, and it is harsher when it goes off.  So it's great for 53gr SMK, but not so great for 77gr SMK.  

On the other hand, lots of guys use only Varget for everything and it is even farther down the chart from TAC.  So you can see that it's about the whole combo of powder, primer, projo and how that combo pairs up with your particular rifle.

This gets even more irrelevant when you compare charges between the powder company, the bullet company, and the general manual.  For example, Lyman doesn't list TAC for 55gr .223, but TAC's manual shows 21.6gr to 24.0gr for one type and 22.2gr to 24.7gr for another (about 2200fps).  Sierra's manual shows 23.5gr to 26.7gr for all types of 55gr .223 with TAC(from 2800 to 3200fps).

Compare that to Varget, which Lyman lists at 25.0gr to 27.8gr for 55gr .223(2946fps and 3346fps), and Sierra that recommends 25.1gr to 27.5gr for the same(2700fps to 3000fps).  (More powder for similar velocity)

By the way, burn rate doesn't mean anything to me compared to how the powder feeds through my powder measure and how consistent the velocities are when it goes off.  That's why I like TAC and Win748, they meter like water.  I will use 8208XBR before I ever buy Varget again, since it's half the size and works just as well.

And all of this doesn't even matter if your favorite powder isn't even in stock..........

if just getting into reloading, or even one that has done it for some time but don't understand a lot of the variables that go into it intimately, stick to the manuals until you gain A LOT of insight and experience. certain powders don't like small variances from what the books call for (read: kaboom!) other times it can lead to a squib which is dangerous in itself if you don't notice it (read: kaboom!)

note: kabooms have a tendency of shooting shrapnel and burning gas towards flesh.

this isn't Lincoln logs we're playing with. combustion and projectiles have real consequences when safety procedures (ie loading instructions) aren't followed.