I'm not sure I know of a case were a slippery slope argument has actually been realized for either firearms or hunting. Seriously, I don't think their is a country or place that had firearms and/or an established hunting culture that actually saw either disappear. Sure things became incredibly stringent to the point that it pushes most out of the sport, but even in my example of Japan where guns are incredibly difficult to get you can still get them and hunt.That will be the next one.
I am interested if there are cases I'm unaware of... but basically I think it's disingenuous to use this style of argument and in general that it's pretty flimsy. Also sure PETA wants to... but would PETA ever be able to do.... no so why are we even talking about it.