I recently bought a Ruger Ar556 MPR in .223 and absolutely love it. However, I would like to upgrade and get another upper for it dedicated to deer hunting. I almost never have a shot over 150 yards and usually hunt in thick areas.
I assume when you say that you've never shot any game over 150 yards that 90% of your shots are probably 80-90 yards or less; am I right?
Do you have any restrictions for certain times of year? Straight walled, pistol only, etc.?
Do you reload?
Personally, it sounds like a 16" 7.62x39 would do fantastic for what you want. If you more reliably took game at the 150 yards and further I would say a 6.8SPCII (or the 6.5 Grendel or .277 Wolverine; I have no personal experience with these in parenthesis but the ballistics are close to the 6.8 SPCII).
Given that I already have a 7.62x39; my next bigger bore AR15 caliber will likely be the .358 Yeti, if you study out the ballistics and energies on this one it looks like a powerhouse for the type of hunting you are looking at. But given the price of 7.62x39 ammo and availability of good commercial ammo for the x39 in the form of Federal Fusion and Hornady SST ammo this would be a definite plus over something like the 358 Yeti if you don't reload.
I mostly am looking for something to handle shooting into light brush if needed.
...I have looked at pretty much all of the ballistics of the calibers but no one ever talks about capabilities in the brush. I want to be able to shoot through a few small branches and not have to worry about it. I have no experience with any of these calibers.
There isn't a whole lot of difference between performance when a bullet hits brush on the way to target. The greatest effect on accuracy on intended target (in my opinion) is how far away from the intended target did the bullet strike brush/object; meaning if the deer is standing immediately behind the brush that the bullet strikes the brush will have little effect on accuracy. However, if the bullets strikes brush/object at more distance away from the intended target it will have a larger impact on accuracy. I do feel there is a minute amount of accuracy gain through brush (based on my research) using larger meplat, rounder and heavier bullets; but I don't think its enough to get caught up chasing by any means. It is far better to wait for a better shot, or move to a better shot than count on a bullet going through brush to the intended target.
I have basically narrowed it to 300 blackout, 6.5 grendel, 6.8 sp and 450 bushmaster. Im leaning towards the grendel and blackout. I love how little the .223 kicks and want to stay as close to that as possible but am willing to budge on that for performance.
If you want to stay close to the recoil of the .223 then I would think the .458 Socom, 450 Bushmaster, 358 Yeti, 50 Beowulf would not be in the mix. I however, would challenge you on expecting the same recoil, there has not been any AR15 calibers that I've tried that are uncomfortable; but I'm used to more powerful rounds in my bolt actions, 308 Win, 30-06, 7mm, 300RUM, etc.
Cartridges that are similar enough to the .223 would be 7.62x39, 300BO, .277Wolverine, 6.8SPCII, 6.5 Grendel.
Obviously cheaper ammo is better but i wont be plinking with it so thats not a big deal. I will be buying a whole new upper for whatever i bought. Any thoughts? Thanks in advance.
As far as cheaper ammo goes, along with your recoil requirements, and the distances you posted of 150 yards or less, the 7.62x39 is the clear winner in my book. Any greater distance and I would suggest 6.8SPCII wholeheartedly (or 6.5 Grendel).