I have a few questions (and might have more as the thread progresses). I'm currently pricing and speccing out parts to build my first AR-15. I discovered I can get it in the standard 5.56/.223 caliber as well as 7.62x39mm. I already own an SKS and have reloading equipment for 7.62 so this is good news for me.

I'd like to ultimately build an upper for both. It is my assumption (based upon my research) that I can use the same lower for both calibers. Is that correct?

Also, when speccing out individual parts for the upper I see that I can buy a bolt assembly for different calibers. That leads to the second question. What do I need to do to convert from one caliber to another?

If I build it in 5.56, what do I need to change to convert it to 7.62? Do I just need a new barrel and bolt assembly? Or is the upper receiver itself different?

For right now I'd like to buy as few parts as possible that let me fire both 5.56 and 7.62. If I have to buy a completely new upper for 7.62 that's fine, but I'd like to know that ahead of time.

Thanks!