If you are new to reloading please read a reloading manual and familiarize yourself with the process and risks inherent. Reloading is a great way to save money and a fun hobby that the whole family can enjoy. But reloading can also be dangerous if you do not follow basic safety procedures and use good, solid common sense.
Palmetto Projectiles' 300 Blackout 203gr subsonic optimized projectiles need to be loaded as cast bullets would. This requires belling or flaring the case mouth of your 300 Blackout brass so it will not shave lead off of the bullet, just as you would with any other cast bullet. We highly recommend the Lee Universal Neck Expanding Die for this as it is inexpensive and can suit all of your cast bullet loading needs for both rifle and pistol. You can bell the case neck before or after adding powder (or during with some presses).
After inserting the projectile into the flared case mouth you will need to seat your bullet. We highly recommend the Forster Micrometer Seating Die for 300 Blackout. This die is easy to dial in back and forth between your supersonic and subsonic loads. It is also the best die in our testing to minimize run-out. We have recommendations for case overall length in the load data section.
The last step on your press after seating the bullet is crimping the case. The Lee Factory Crimp Die is the best tool for this job. You can taper crimp in either through a standalone die or as a part of your bullet seating die if you so wish, but you will see the best performance with the Lee FCD. The Lee FCD is a collet type die that tightens the case mouth horizontally and will not force the bullet into the case changing COL or shaving lead. You need to make sure that you do not crimp hard enough to damage the bullet. When properly crimped it should look like it was barely crimped at all.
Your final step is to check your loaded round in a case gauge. We use Sheridan Engineering Ammunition Gauges and wouldn't do it any other way.
As a rule of thumb you can use published load data from subsonic loads with similar weight projectiles. Many reloaders are using load data from Hornaday 208 grain A-MAX subsonic loads with excellent results. As any loading manual will recommend, start at a 10% reduced load and work your way up until you achieve desirable results.