I have the Rex rifles both in .357 and .45. The .357 worked fine right out of the gate with several different pellet sizes and weights. The .45 had some issues with a tight chamber and it did not like bullet diameters over .455" very much. I had to slug the barrel and found it needed slightly smaller dia bullets, so I picked up a smalle hand press and bullet sizing die in .454". Now the Rex works like a charm. The .357 I shortened a spare barrel to 13.57" and made a full moderated shroud for it. It's less powerful than the full sized rifle barrel, but more powerful than the pistol. The power/shot count drops pretty fast, but that's the price you pay for a small tank, and typically smaller compact gun. I really like both of mine a lot.
Here's the vid on my Rex .357 SBR (Shorter Barreled Rex). You can look for the Rex .45 rifle resized bullets, in my video listings if you want...
https://youtu.be/AEwLJlT6YNI