That is US military 7.62x51 NATO (army version of .308) that has been necked down to .243.
If it is heavier than standard, it has less volume which will run pressures up somewhat. The NRA used to say to reduce the powder charge by one grain for every 11 grains heavier brass. I think the current recommendation is even more cautious.
If the sized down necks are so thick that they cannot expand to release the bullet, pressure will be increased a LOT.
I don't think you who are saying "i am new to reloading" should be getting that experimental. Buy some real .243 brass.