New brass? Converted 223 brass? Once fired head stamped 300 BLK brass? One of the biggest issues with 300 BLK is converting 223 brass, and not annealing it. The cut for the conversion is made on the unannealed portion of the brass, and annealing is the key to neck tension. Annealing softens the brass and reduces spring back. Unannealed 300 blk, if you have any neck tension guages, you will see that even when you size it, it springs back to very nearly the unsized point immediately. I've seen some that was so bad, a proper guage literally fell out. That means the only support for that bullet is your crimp. Some will make up for that with a heavy crimp, but that's just covering up the problem (crimping is NOT a replacement for poor neck tension). If you are using actual head stamped 300 BLK, then you could have other issues going on. In the great scheme of things, unless you are using big bullets that are close to the OAL for 300 BLK (basically AR mag length), the issue is if it jumps out enough to hang in the mag......OR it sets back enough to cause higher than anticipated pressure. The other issue with poor neck tension is that if you have a round that does not go all the way into battery (for whatever reason), and you attempt to clear it...depending on the bullet type and weight, it **could** leave the bullet in the chamber, which will require you to rod it out.
One of the worst offenders is folks selling converted brass, who've simply done the trimming and sizing, but did not anneal it (bulk annealing equipment is expensive). Folks buy this converted brass, and then have all kinds of problems and never figure it out. There are soooo many brass peddlers out there selling converted brass like this, it's amazing.
You can bust out a torch, socket, and cordless drill and anneal a couple yourself to test this, super easy to do, and lots of youtube tutorials on "poor man annealing" out there.