John3921, would you mind telling us the thought process that led you, while in high school, to purchase that 300 win mag?There is also an outfit that makes a .243 WSSM upper for an AR platform. http://www.dtechuppers.com/index.html
I don't think .223/5.56 are good choices for deer. Maybe small whitetail in certain situations and in the right hands. But in general - no.
.243 / .250-3000 / .257 Roberts would be about as light a round a I'd suggest as being responsible. Even these are going to require good shot placement.
I'd also avoid the 'man-up' approach. I bought a .300 win mag when I was in high school. I developed a hell of a flinch. About the only way I broke it was to not shoot it for about 10 years. Even now I limit bench time with it.
Consider getting a lead sled for the range. You can shoot without the recoil.
Were you a big kid, or did you have a lot of experience shooting centerfire rifles? Were you and your buddies into a caliber race?
The social and practical aspects of your decision might help others when their turn comes up to buy their first rifle.