MG4A6, if you haunt ranges for a while, and check in gargabe cans and the like, you will frequnetly find factory boxes stuffed full of nice, shiny, obviously once fired cases, disposed of by non-handloaders who are neat freaks about policing their range! You can also ask nimrods during sight-in day if you can have their empties. Non loaders will normally give them to you.
I have accumulated hundreds of cases for cartridges I do not have a rifle for, and sometimes give them to new hand loaders. They will be mixed headstamp, and mixed lots, obviously. If you are planning on taking this rifle out to a long range match, clearly you will need premium new brass, all of same lot, but for learing how to handload and to shoot your new rifle, mixed cases are seldom a challenge. The fact that they vary in capacity and therefore attendant pressure only re-inforces the need to exercise caution and to avoid max loads.
And, while economy can be realized by purchasing larger quantities, sometimes it is good to do a tad bit of experimenting -a box of these bullets, a box of those, this powder, that powder, full length re-size, neck size, neck turn, don't neck turn, seat near the lands, seat to the cannelure, yadda yadda, and then buy a bunch of what works.
Or not. They'll all shoot, and generally as well as non premium factory ammo, for some savings, and the satisfaction of rolling your own. Barrel life can be greatly extended by staying off max loads, and avoiding rapid fire.
And as usual, my advice is to go on major manufacturer's sites, read and watch all the info they offer, buy a new, good manual, get ABC's of Reloading, study, and give it a go.