No, the Contenders & G2 Contenders - and the Encores / Prohunters are two separate, non-interchangeable worlds.
If you want a .300 BLK, go Contender, not Encore - it only makes sense that it be the smaller, lighter, handier platform.
Although a lot of people do, the reason I don't do multi-calibers is Contenders (and when I had them, Encores) -- i.e. the reason I had a lower for each is, because the pins are in there pretty tight, and the tighter the better for maximum accuracy (you can buy oversized pins for super tightness), which makes them a bit tricky and possibly frustrating / time-consuming to swap them out. But it's just a balance of "how much $$ you have to buy a frame for each barrel?" and "how many different configurations do you really want/need?", balanced against your time it takes in swapping and whether your shop / tool bench is ready to go for easy swapping.
Again, AR15 require no tools to swap, AND stayed zeroed because the sights stay with the upper - that's the best multi-caliber platform I can see. However, ARs are not light hunting guns, nor are they "PC" hunting guns. One extra advantage of AR15s and AR10 types in a multi-caliber setup, if that's your thing, is that some people drop a fair amount of money into a lower, what with expensive fancy triggers, fancy / custom grips, mag releases, bolt releases, oversized trigger guard bottoms, "front grip" doodads on the front of the magwell, selector levers, etc. If you get a lower you really really like, and have dumped a lot of money into it, AND really happen to like / need multiple chamberings for whatever reason, that platform makes sense (unless you're hunting, which for ME, AR10 types are too heavy for hunting, and AR15 DI types, though not "too heavy", are typically "not ideal" and "heavier than they need to be" - "not ideal" encompassing less-then-top-choice of chambering and Un-PC.
Also, each AR upper is typically $300-$600 minimum, not $150-$350 like Contenders or $200-$500 like Encores.
