dvdcrr's post is a good one. I think too, there are just too many cartridges.
Yes, 6.5CM for gas guns and .260 for bolt guns is the way I prefer, but if you want both, the 6.5CM. I ran mostly 123 grain bullets in the gas .260 as they were more accurate due to length issues in the AR10 pattern magazines. (I sold my AR10 pattern .260, but kept the bolt guns ) That is where the 6.5CM has the advantage, fitting in AR10 pattern magazines. The .260 was in the lead across the varied groups, but with Ruger now pushing the 6.5CM, that might turn the tide towards the CM. In PRS (long range competition) most of the competitors have 6.5s and other cartridges, but we are seeing more and more go to 6mm (6CM, 6XC, 6x47 etc.) so the LR support has waned a tad for the 6.5s. The 6.5x55 Swede is a good caliber, but a long action, but I don't think it is in trouble yet.
I also have a 6.5PCC and I love it as a mid-range cartridge in the AR15 platform. The 6.5G is a slightly better cartridge, but a little more money, less AR15 parts commonality and there is the issue of feeding. Most guys around here do not fully load their magazines and have no issues. The problem comes when you load more than 10 in a larger magazine, which for most applications is not going to happen anyway. The 6.5G is more popular, but for my uses, the 6.5PCC was a better fit.
The 6.5 bullet selection is great now, and I think it always will be with so much competition in amongst the various cartridges. If you reload, pick one and move on. If you are trying to pick one (using factory ammo) to buy in a bolt gun, toss up between the 6.5CM and .260Rem. in a gas gun, get the 6.5CM.
Target shooting inside 300 yards, go with a .223 or some .223 case based wildcat. Most 6.5s shine from 300 on out to about 1000 depending on what you are doing.
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Good Shooting, MarkCO
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