Ok I'm starting a new build on a savage long action and I am stuck between these two calibers. Wil be used for longrange rocks/targets and maybe goats/deer. Please help me decide and let me know why. Thanks.
Brass might be a little easier to acquire for the 260 (even if you were necking up .243 win cases)...
Three guys who shoot our long range course here in SW Virginia are using 260's, and all three shoot the same load: 142 grain Sierra Matchking and 38.0 grains of.... Varget. No kidding. That load rocks in the 260. 2800 fps from 24 inch barrels, so it's not too hot... just an accurate, easy load that works.
Bullet selection between the two cartridges is about a draw... great long range bullets in both 6.5 and 7mm.
Barrel life... I'd give the edge to the 260 there, for sure. But the .284 would not be bad either.
The 6.5/284 gives on average about 100 fps over what you can get from a 260, maybe a little more... but not a lot. The 260 for all intents and purposes will do what the 6.5/284 will do.
For taking down game past 600 yards, however... the 284's heavier 7mm bullets would be better, having more energy for the job.
Either way you go, you'll have a winner. Both are great cartridges.
If most of your shooting is going to be rocks & targets I would suggest the 260. If you are going to be shooting deer etc a fair bit I would opt for the 284. Huge selection of target and hunting bullets in 7mm.
While I agree that the 260 will do what a 6.5x284 will do (I have a 260 longrange rig), the extra 100fps of the 6.5x284 make a material difference when the range gets longer than 900 yards. The 6.5x284 will shoot the 140grain pills at 3100+. Because your plans for the rifle are primarily low volume targets and hunting, barrel life is not much of an issue.
284 is a nice cartridge and will shoot the 180grain bullets (the 180grain berger hybrid is simply badass). These heavier bullets will also work well for you if you decide to hunt anything larger than deer.
So my recommendation is 6.5x284 or 284win. Either way, you have a winner. Given that it does not sound like you are leaning towards hunting big critters like elk and deer, I would favor the 6.5x284.
One more advantage of the 6.5x284 is that you can get Lapua brass!
You can use 6.5-284 Lapua brass necked up for the 284 win or 260 Lapua brass so there is no advantage there. The 260 would have less case prep and being a bit shorter will fit better in a standard box magagine.
Personally I see no advantage in using the 180 over a 168 Berger in the 284. The BC of the 180's downrange performance is just about equaled by the higher velocity of the lighter 168. Crunch some numbers through a ballistics program and you will see that the wind drift out to 1000 is almost identical. Sure the 180 edges out the 168 but the barrel must have a faster twist. Factory barrels will not be twisted that fast.
As for barrel life who knows with the 260 vs the 284 it will be close. The 6.5-284 will wear out faster than the two you originally suggested.
I've done the 6x284,6.5x284 (long action) and now I shoot a 284 on a long action with 27" long barrel and it's one of my deer/elk rifles. I had that rifle build same time did the 280AI forget which one was done first.
I've always like the 284 case and with that long barrel I get in the low 3000fps with 160gr AB and very easy to tune a load. I've hunted with it few time mostly been used a back-up rifle shot few PD with it etc just fun rifle to have around. I've got 3rd season deer tag here (Co) and I'm taking 284 and 270 so we'll see.
I know nothing about the 260 have never owned or shot one so no help there.
I like my 260 and used it for tactical matches and its shoots very well with just about anything you feed it. Zac Smith has a great article on the 260. Google his sites9 Demi God and Thunderbeast Arms) and read the info he has posted. It's very informative!