In general, are auto loaders (semi-autos) less accurate at long ranges than say bolt action rifles? If so, why?

In particular, I'm thinking about getting a Winchester model 100 in .308.

Generally, yes, they are less accurate than bolt actions. The actions are tighter and have smaller tolerances (in general) in bolt actions. Bolt actions are easier on the cartridges being loaded. And a bunch more. We can only say "generally" because there are some darn good shooting semi-autos out there.

Well if I had the moola for a DPMS Panther . . . there would be no conversation.

But! not.
Iwould be equiping with quality optics and would want a tight pattern out to 300+ yards.

I'd always opt for the bolt. The 100s had a decent rep for accuracy, but they're old semi-autos. Bit harder to tell if you're going to get a good one or not. For hunting, there's not really any advantages of a semi-automatic rifle. If you want a classic, stylish semi-auto, then the win 100 is a good way to go. There's no guarantees you're going to get a tack driver. I would guess not, in comparison to a modern bolt gun.
My opinion: the safe choice for an accurate rifle would be a new bolt action. You can keep the price down via Stevens 200 or Savage 111.

My experience with autos is limited but as follows:
Win 100: shot one in .308 and it grouped roughly 2" but seemed to be fairly reliable

Rem 742: Shot one in .30-06 and one in .308. The .30-06 shot good groups and shot fairly reliable. The .308 was terrible!

Browning BAR: I shot a .30-06 and a .243 and both were reliable and accurate.

Not much of a record but my personsal battery today includes only one auto.....a .22 rimfire! There are quite a few Mauser, Remington, and Winchester rifles that have proved accurate and reliable.

Yes, in general they are.. A couple of reasons for this is because Bolt actions are 1 piece stocks instead of 2 pieces and have a more rigid platform for the rifle.. Also bolt action triggers are or I should say can be lightened up to around 2-3 pounds safely, while autos have to have a little stronger pull on them to keep them from fireing off after the first shot.

my BAR in .270 shoots great! sub MOA...

a good bolt should shoot great, but personally starting to think any gun that's mass produced is just stamped out, tolerances are so large, good luck having it shoot well.