Im new at this and Im probably going to build my first ar soon. While looking through deltons site I saw 762, 300, 10mm full conversion uppers. My question is will they fit on a regular ar15 lower or do you need to buy a special lower reciever. I ask because I noticed delton didnt sell anything but ar15 recievers? Also is the 300 a easily obtained round, I never heard of it before? If they do match up this seems like the perfict gun. 223 for tactical and 762 or 300 for hunting.

Thanks in advance for not ripping apart the new guy.

It's all in the mags, bolt, bolt carrier and barrel.  Depending on who's building it, there might be a spacer in the lower receiver to use magazines for another firearm that uses 'X' caliber.

You don't have to change parts on the lower.  Now, as soon as I say that, I'm probably wrong in some way, but thankfully folks here will correct it.  

Hope that helped.

SPC Richard A. White, Senior Medic
249th MP Detachment (EACF)
Camp Humphreys, ROK