Quote from Reloader on August 21, 2020, 1:21 am

Congratulations on the 550. I've owned one for thirty years and continue to be well satisfied. With respect to loading 300 blackout, I first begin by saying that I do not use this cartridge. Not that there is any problem, I just don't have one. But consulting the loading manuals lead me to recommend that you obtain: (1) a .223/556 shellplate with locator pins; and (2) 300 blackout dies; and (3) the proper funnel (that you may need to call Dillon about; and (4) a good reloading manual. Consult the manual and you may also utilize the better reloading websites like the Hodgdon reloading site for recipes.
With respect to resize 5.56 brass, the Dillon shellplate with locator pins will be your first step. You will also need a good resizing die. Some believe if you plan on loading for a semi-automatic like an AR, that your die should be a "small base" die. These are the same price as a regular die.
When you begin, I first clean the brass to remove the "guck" and then spread the cases out on a cookie sheet. Spray Dillon Case Lube and then let it set for 15 minutes or so for the alcohol carrier to evaporate before you begin. A case gauge is also essential as is a set of good calipers. When you adjust your die, consult the Dillon Youtube video which will instruct you to turn the die down until it makes contact with the shell plate, and then turn it another quarter turn. This has the effect of causing the press to "cam over" a bit but ensures the brass is fully sized. What happens if you fail to correctly size your brass in an AR? You may have seen a frustrated shooter at your local range with an AR that has "locked up" and most often with a loaded round in the chamber and no ability to set the safety. With that in mind, when I think I have my first case correctly sized, I check it first using the case gauge and again consulting the Dillon video. If it looks good at that point, I check it again by chambering the empty newly resized brass in my rifle. Yes, you can still lock it up, but if you do, it's much easier and safer to remove the incorrectly sized (and empty) brass from your rifle at home. Additionally, when bottle shaped rifle cartridges are resized, they tend to stretch a bit. This brings into play another operation called "case trimming". Most of us began with fairly inexpensive tools that were more labor intensive. Several years ago, I acquired a Dillon case trimmer and the trim die (which also resizes) and it speeds up the operation. True, this is not the device to set up when sizing 100 pieces or less. I last used mine during the c-19 quarantine and did about five thousand (mine and also a few friends).
Lastly, the Dillon forum is replete with questions, answers and tips from your fellow reloaders and I can't think of any topic that has not been covered at least once.

Best,

Terry