.20 practical questions

I had someone ask me to build a rifle in .20 practical.

What's the difference in a .20Br, .20 Varget and .20 practical?

Also will a practical headspace on .223 gauges?

20 practical is a necked down 223 and yes 223 gauge works, 20vartarg is a 221fb necked to 20cal with a little different shoulder i believe, 20br is a necked down 6mmbr then there is 20tac and thats a necked down and different shoulder 223

Thanks for the good info. I have to confess, I'm only familiar with the old standard chamberings so you've been very helpful.

the vartarg is loved but hard to get brass, the tac and pract are very close to the same thing, I shoot a tac but have reamers for all 3