Mr. Mortimer, that is a very well thought out and clearly worded rebuttal...
...which has NOTHING WHATSOEVER to do with the original poster's question.
He did not ask for a comparison of the .308 to Brenneke's latest super slug, nor the .50 BMG, nor a charging Buick.
He asked about Foster Slugs. Foster Slugs: Those hollow lead cups they tip upside-down in a shotshell and hurl at the vicinity of the target.
I would MUCH prefer a 12ga Foster Slug to properly selected .308 ammo if...
if...
if... You know what? No. I wouldn't. Pretty much ever, unless required by law.