Every gun is a thing onto itself. Two of the very same model and year of manufacture can like different loads. Some times guns with the same case design have a tendency to like the same things. You just do not know until you try it. You have found that the 45 grain bullet can shoot well in your 223. I like a 34 grain MidSouth bullet for my Hornet guns, but they do not even list it for 223 guns, they are just too light. You would need to develop your own loads with them. Winchester 45 or 46 HP grain White box has a good reputation of shooting well in Handi 223's, so why not your 45 grain bullet?
My 22K Hornet seems to shoot Lil'Gun decently, but my Model 40 22 Hornet does not shoot Lil'Gun with out separating cases, wild swings with pressure, loose primer pockets, sticky cases, all signs of very high pressure with a powder that you can not get enough powder into to cause high pressure? I have a Handi 22K Hornet (heavy stubbed Douglas barrel) and a Savage Model 40 (22 Hornet). I have had excellent results with 34 grain HP Varmint Nightmare sold by MidSouth. Along with CCI 500 (small pistol primers) and light loads of W296. Not only are they accurate with this bullet they get high velocity and the cost is relatively low. My 22K hornet has a 20" barrel and gets 3150 fps with this load. Like I said it is a light load, so the cases do not stretch that much, even with the 22 Hornet.
Here is two groups shot with the 34 grain MidSouth bullets and my 22K Hornet at 100 yards.
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Good Luck and Good Shooting