Even though they both have the same overall length and rim diameter, the . 204 Ruger has a slightly greater case capacity than the . 223 Remington due to the slightly longer case length (1.85″ vs 1.76″) and steeper shoulder angle (30 degrees vs 23 degrees) of the . 204 Ruger.
Is a 204 good for deer?
204 Ruger is not efficient for deer. It's 32 to 34 grain bullets are frangible and would not give reliable, humane kills on deer, bucks or does. It is designed for varmints and does a good job on them.