I need information on making 300 BO cases from 223? To what length do you cut the cases before running them into the die to shape the shoulder and neck? Any other data I need? I have an upper on the way but can't find the ammo. No cases in stock either. I have once fired 223 and can pick the best so I can get at least 100 finished cases to load. I think it will be Federal cases unless I have enough Remington. Some are already polished just waiting to be loaded. TIA. Be Well, RZ.

RZ, I formed mine and then cut them to trim length. If you need to borrow a saw and jig let me know.

I really wanted to know the optimum length to cut them off. I am going to buy a 6" cutoff saw and will make the jig? There must be a better length to cut off so I don't have quite as much to trim off later for the final length. Do you have a specification for yourself? Just wondering. Or where is your jig set to cut? Thanks and Be Well, RZ.

I use the Zep 300 black out jig with the cheap harbor freight 2 inch chop off saw, it works great. As far as cutting length prior to running them through the 300BO die. They recommend putting a properly sized 300BO case in the jig and then putting a piece of heavy paper card stock at the mouth and then pushing the jig to the blade and tighten. So, just cut a little longer than what you want for you final case length. Another tip, if making hundreds of them is to invest in the little crow gunworks trimmer. I have them for 300 and 223 and they save tons of time trimming mass quantities of brass.
https://www.littlecrowgunworks.com/store1/300-Blackout-p148723756