Ready for some of my blah blah blah?
I sat in front of my Harbor Freight saw last night and chopped about 100 pieces which I had already run through the 300BO sizing die (decapping pin removed). There are (3) things I don't like about this particular method:
1) Occasionally (probably 1 in 10 or so), brass will have a nick/dent in the area where the new neck is being formed. Because the brass hasn't had an expander ball or mandrel/collet pass through it, the dent will remain. This will be felt when you chamfer/debur, or when you trim with a Giraud as the neck isn't a perfect circle at this point. (You would also need to adjust the position of the Giraud cutting blade if you switched to trimming another 308 caliber cartridge as the diameter of the neck in the other cartridge will be wider. Probably not a big deal for most, but I also trim a lot of 308 Win and getting that cutter exactly where it needs to be is a PITA for me.)
2) You have to run the brass through a sizing die twice using this method
3) You can't trim the brass using a Lee trimmer as the inner diameter of the neck is too small to fit over the case gauge
For me personally, it's better to cut the brass before running it through a forming die.
300BO Sizing Dies
You really don't want to use the Lee full length sizing die for forming 223/5.56 brass. The tolerances are too loose, and Lee admits this. I have the Hornady dies and the RBCS Small Base dies and they seem to produce similar results and measurements
Parent Brass Choices
5.56 brass works fine/great, but that appears to be restricted mostly to brass produced in the US. Once fired LC seems to be the brass of choice for a lot of people. A compilation of which 223/5.56 headstamps work and don't work can be found here: http://www.300blktalk.com/forum/viewtopic.php?t=88599
Some of My Personal Thoughts
300BO isn't famous for its accuracy, but I believe it certainly can be (and some of the commercially available ammo IS). Thanks to help from some board members, I gathered up a few rounds of factory ammo in order to obtain headspace measurements. I also measured some converted brass I have. The variations were surprising, to say the least. If you decide you want to go a little beyond just plinking ammo, I recommend watching the following video. It's very well done, and he does an excellent job of explaining why most people get disappointing results from their 300 BO ammo (as well as what can be done to improve it.
[media]https://www.youtube.com/watch?v=_ytfBI8ZJ_g[/media]
My final thought is there has to be an easier/faster way of chopping 223/5.56 brass than the HF saw and jigs. (An RT-1200 would be nice, but good luck getting your hands on one) There are a couple of guys out there using homemade jigs and a table saw/band saw, but the way they've designed them makes loading/unloading them cumbersome and slow. I think I know something that would work extremely well, but it would require either 3D printing, obtaining a chamber reamer, or milling down an EGW ammo checker. Unfortunately, I have access to nothing to accomplish any of that.
[/end of Old Jeff H's BS]