Would it be possible that the pressure cause the bullet to expand and fill the bore instead of say bouncing down the barrel like a basketball?

Yes, that's a bad mistake to make. Please pay more attention!!!

But, I've had customers do this already. They didn't put 6 rounds down range, but 1 or 2. The 260 is a .264" bullet, so it's .044" smaller than a .308", so it will bounce around in the barrel some. But, most of your powder gases will blow past the bullet reducing the pressures, while pushing the bullet out the barrel. The damage would be from the bullet hitting the rifling, not an over pressure issue.

Like stated, I would clean it heavily with a pointed jag with a cotton patch soaked in your preferred cleaning solution. As you run the tight patch through the barrel, I'd feel for any resistance or bumps in the rifling. I'd still shoot it to see if it shoots still. I'm betting it would be fine.