Firstly, you don't need 1/2in brass. If you're going to get brass, 17/32 is the typical choice for stefan darts, and will sometimes be too tight for elite darts. This can be switched out for 9/16 brass which can fit for elite darts. Typically all you need is 17/32 and 9/16. For a brass breech, you have one of two options, 9/16 as your barrel, or 17/32 as your barrel. If 17/32 is your barrel, you can use the 9/16 on the bolt itself and it will sleeve over the 17/32 as a sheath, and when you prime forward, the dart will sit theoretically 50/50 in 9/16 and 17/32. If you're using 9/16 as your barrel, you have more options. you can use the 17/32 as the dart pusher, and push darts into your barrel and create a seal, or do the same thing as you would with the 17/32 barrel and 9/16 sheath, but instead with 9/16 barrel and 19/32 sheath.
So basically my suggestion is get 17/32 and 9/16 brass. don't really need anything else.
Okay Thank you!