Sounds like the ar 15 platform is all you need,out of all those choices rr mentioned,the .243 wssm,6.5 grendel,6.8 spc,and with proper bullets the .223,would be all you ever need for a 300 yard max hog gun.If you do opt for the AR-10 platform,you have choice of .243,.260,7mm08,308,.338 federal,and others I must have missed in my life sustaining coffee depraved state. The only bad thing about buying an complete lower reciever is you'll likely want to change out the trigger and possibly the buffer. When I bought a couple dpms bull barreled ARs,I installed jard single stage adjustable triggers in them resulting in additional 105$ each. In my opinion,the m-4 type collapsable buttstock isn't needed for most hunting/varmit shooting-coolness look aside,so I kept the rigid buttstocks. The only other change I made wasopting for free float tubes (30$). Except for caliber (and selective fire of course),these rifles are sure different than the M-16 that spawned them that (we used) years ago. I would highly reccomend you stay with "flattop" and bull barrel.
One note of caution-a lot of AR barrels/uppers,such as dpms,are designed for civilian round .223 instead of the gvmt 5.56x45 nato and I would adhere to that. Also stay away from the cheap steel cased imported 5.56 ammo as the lacquer coating on the cases WILL melt and gum up your chamber IF you shoot at high rate of speed enough to where your chamber gets hot enough,and if you wait until later to clean it after it cools back down,that gummy buildup will solidify again,creating a monster to clean. Stick to brass cased ammo and you'll be ok. Since it's 5.56 you shouldn't use it anyway tho. Guess I'll shut up now and let the real AR gurus help you as my specialty is in bolt actions-just wanted to share that with you and not trying to confuse you further. I KNOW I'm missing some pointers/parts reccomendations.The only reasons I have these is for yotes and fun. Does this mean you're going to invite those of us out to assist in hog depletion that was of help to you? hint hint