Well, lets see 2 6.5 SAUMs built buy a regional smith that has a a few 1k BR winners under his belt and has built me some hummers in the past. Yet these wouldn't shoot in a bucket, 2 different actions, one a Defiant the other a trued 700, McMillian stocks, NF scope, heh the first one ended up getting tossed off the bridge into the river after having a conversation with a hackberry stump, second one was rebarrelled to a 300 Ackley that shoots it's tail off.
Had 2 6.5x284s that refused to act right, both built by reputable smiths, as with the SAUMs, just would not back good groups up, something that showed promise one day would then shoot an inch the next.
The one that should have shot and was the first one I messed with was a 6.5x55 George Wally Improved, a fellow that won the IBS 1k shooter of the year liked the cartridge, said it was the easiest thing to tune he had ever worked with. So I had one built by Clay Spencer, Kelbly Atlas action, jewell trigger, Spencer barrel, McMillian stock and a March scope. The thing shot some good groups fire forming with the 100g sierras but wouldn't shoot in a bucket with anything else, I checked the twist, was a 1-8, tried berger 130/140, sierra 142/140, hornady 140/123, every powder combination, primer, Lapua/Norma brass, different neck thicknesses, .100 jump to jam. Hell I put a lot of rounds through that thing to try and get it right, the other ones I gave up on after 200-300, I had around 600 through this one, it became an obsession, had my falling out with Spencer over it too, that was the biggest loss on the gun, ended up screwing the barrel off and depositing it, dies, brass, bullets, powder in a friends truck at the range one day. Funny thing is, I bought a used 308 Palma barrel threaded for a Panda off ebay for $50, I used a spacer and screwed it on to that rifle, the thing shoots great, I have some groups in the zeros from it and it's never acted up.
So yeah, I've done my time fighting a 6.5, they were all built by legit smiths and one in pretty high regards.
In screwing with those rifles I got to second and triple guessing everything I did in loading and shooting, I bought new presses, bushings, dies, rests, bags, reevaluated myself at the bench, even got others to shoot loads for me. Then messing with other rifles in that time that shot really well, some 20s, 22s a few 6mms and a couple 30s it drove it home that the 6.5s were just going to be an @$$ for me.
the latest one I've inflicted on myself is a 6.5x47 supposedly one of the easiest cartridges out there to get to shoot, showed some promise so far but hasn't backed it up too well, if this one fails I probably need to swear the things off and shoot quarter bores.