Here's some online info on multigun rules:
http://www.brianenos.com/forums/index.php?showforum=224
http://www.uspsa.org/rules/2012_Multigun_Rules.pdf
There's a division, called Heavy Metal, where the minimum rifle caliber is .30.
So, you wouldn't be competing against .223.
But there are other conditions along with the rifle caliber, like 12 gauge pumps only, and 10 rds for handguns.
On the other hand, there's usually less people competing in that division and a better chance of scoring well.