just purchased a cooper model 21 in 20 tactical. Now the fun(work) begins. I am wanting to use 223 brass that I have lots of. Is it necessary to use a forming die, or can I just run though my full length resizing die?

Provided that your .223 brass is only once- or maybe twice-fired, just run it through your .20 Tac FL die, trim, and chamfer.

If your brass has been fired multiple times then the neck area may be work hardened enough that it will give you difficulties like splits and cracks. Such brass will need annealing to soften it properly. This is a touchy process, so if you haven't done it before do some extensive research before diving in. It is very easy to get your brass too soft.

saubier.com is a good site for small bore info.

Hal

thanks to both of you for your help. will let you how how it goes.

finally getting to reforming my one fired 223 brass to 20 tac. seem to have a small but noticable ring at shoulder, cases chamber without effort. should I continue, or am I doing something wrong. using redding dies, and die just makes contact with the shell holder. I removed the decapping rod prior to sizing thinking it would be better, am I right.