Finally got my work bench together and I can now do stuff.

This is for 300blk.

I had bought a batch of reloads from a shop a few years ago.  500 rounds with 100 not going to go into battery.  I had one jam in there when I was at an indoor range and didn't have the time to deal with it so I ended up cutting that trip short.  First time at an indoor, short trip wasn't a big deal, someone next to me had a flame thrower and was roasting my box.  I soon ordered one of those Sheridan gauges and went through that whole batch and found around 100 of them were no go, some were several millimeters sticking out the back, most were just a millimeter out but I wasn't interested in trying to see if those would seat, had a hard enough time removing that one that got stuck in battery at the range, and the gauge said if it stuck out past the end at all, it was out of spec and don't use it.

I pulled the bullets, intend to measure the cases to see what is up with them but that is a later project.  I have plenty of brass that is usable and would like to use those bullets, I need to measure them just to make sure they are good to go.  Most should be fine, most of the case problems were too much case but some of them looked like they ballooned at the bullet in the case so I will be measuring them just to make sure.

I don't know specifically who made them but I need a starting point.  I can find stuff with dimensions and design just like what I have so I'm wondering if I can use data from bullets with same dimensions as what I have but start on the low side of the powder range and work at it from there.

Your first mistake was buying reloaded ammunition. Never buy or shoot other people's reloads as a rule of thumb. That's a good way to destroy your rig or worse you.

If you are looking for load data pull a bullet and weigh it then look at bullets in that weight to figure out what you have. Start low and work up.

This. You also might want to measure those bullets to see that they are .309 (jacketed bullets).

Then use the .25 rule to set your bullet depth. The point where the bullet ogive measures .25 should line up with the magazine rib when the cartridge is in the magazine.

You did the right thing getting the Sheridan,

300 BLK is touchy to work, the shoulder gets mushed easy.

I'd resize, check case with the sheridan at every step, load according to the bullet weight.

Do you have one of the main reloading books?  Lyman, Hornady, Speer, rcbs???  If not buy one.  Hell at a minimum ask someone to take pictures of the pages of the ammo you care about.

Do you have a scale?  Do you have a caliper?

Get those so you know what it should be and you can determine what you have.

The case gauge is a good start but you need a caliper to measure what ever you want

Lyman book, several type of gauges to measure length and even case wall thickness.  Got a Lee single stage press kit and then some like a digital scale, Lyman case prep express, so I have everything needed to reload except powder which I haven't decided on(I'm looking at a different weight bullet for after these so I'm looking at a few powders before I decide on which one I'm trying first.)  Bought all the equipment over a year ago, waited for my work bench to be finished.  I don't have the tools for building that but my uncle does but he had overtime so it took a while before the bench got done.

Doing research before I purchase powder which is why I'm checking on these bullets.  Hate to see them go into the trash if I can use them, especially if I can learn by using them before I get something that I really want to shoot.  I have a methodical approach to making these, rechecking some of the research I did when we started to build the bench before I knew how long it was going to be before it got finished, over a year later.  I just want to make sure of things before I start to use these bullets.

The brass that came from the no go rounds are part of that learning experience I'm looking at.  This is something that can potentially be dangerous to myself so I'm doing this slow and making sure I have everything and even though these were defective, I can learn something from them.  When I get into a hobby, I go all in, no half-assing for me, besides that, this is actually fun.