I have a standard M4 clone in (I guess .223).

I want to change that.

I was thinking something with a little more "oomf" for hunting so that I don't have to purchase a whole other gun, thus I was thinking about 6.8 mm or 6.5 Grendel.

My question is, can I use a normal AR lower for either of these two calibers? If so, what else would I have to alter to get them to work in my M4?

Thanks for any info.

Any mil-spec lower will work.  The only items that need to be changed will be the barrel & bolt.  There are many strong supporters in both camps.  The concensus seems to be that both rounds are about equal out to 400 yards, but beyond that the Grendel starts to shine.  Ammo for each can be pretty spendy and availability of 6.5G ammo can be sparse, but the supply seems to be improving.  6.8 currently has more commercial ammunition manufacturers, but Grendel is expecting at least one more to come on line this year.  

Good luck in your decision,

T

6.8 you can get in a few weeks as far as I know.
My buddy ordered a 6.5 upper about six mounth
ago and still waiting for it.

Find the main thread on the "6.8 or 6.5" debate. This is dupe material.