.204 barrel or 6.5 Grendel upper

[Dust]

My first attempt at tying this got really long, so I will try to break it down better.
Purpose: long range shooting and 500 yard coyote and below hunting
Problems: I do not reload at this time. Have .223 magazines, so the .204 would be cheaper buy-in. Have an extra .223 upper.
Reason: Saw an R-15 takeoff in .204, and keep going back to the .204, even though it seems the Grendel is better in the numbers game.

It also seems that from reading some threads that the Grendel would make for a decent deer or even larger game caliber. I have a 7.62x39 and a .308 that take that role now.
It seems that a Grendel can be built much lighter than a .204. Anyone have any clarifying thoughts on the subject?

[Warg]

If you're looking for "coyote and below" varmint round, the 204 is a good choice, but so are varmint-weight .223s. If you're looking to tag deer too, naturally the 204 is not a caliber to consider. Since you've a .308, however, bear in mind that you're going to have a 1,000+ ft lb energy at the muzzle and 500+ at 500 yds advantage compared to the 6.5.

You certainly can build a lightweight 204 upper, but it may not be a light as a 6.5 off-the-shelf lightweight offerings from some vendors. Relatively light 204 Ruger barrels (2.5 lbs for 20") are available from White Oak (perhaps others) and can be re-contoured, fluted, etc. if you really want to save weight. This is the route I'm taking and will be as light as any other AR-15 upper employing a similar barrel length.

Since you don't reload the major advantage the 204 is going to give you over the 6.5 is ammo availability. I can walk into any WalMart or Joe Schmoe ammo retailer and pick up a box of .204. Not so for 6.5.

The AR platform is reliable in 204 configuration as well and you can use standard 223/5.56 mags, and bolts. Cycling is no problem either and buffer selection can be same as .223/5.56.

[Dust]

So if I put in Texas whitetails, and pigs, then it would obviously side with the 6.5, correct? What is the max range of lethality for the 6.5 and .204 for canine-down targets? I know I don't want to get out that far right now, but if the rangefinder and drop chart are handy...

Gfelber, about availability, I own a .17hm2, so I know a little about buying online. It's simple with the hm2 because 95% of them shoot the best with Eley, so you buy it. With the Grendel, it seems that buying a box of each and seeing what happens would be the easiest, and then buying in bulk thereafter. With the amount of ammo I have put down range in the last 7 years living in Japan, I don't think that most would label me an "active" shooter.