In lots of loading manuals they will omit the heavy bullets in the pistol section because the heavier bullets will not expand reliably at the volicities that can be obtained with a pistol barrel. That being said, I wish they would go ahead and test the heavier bullets because not everyone is loading with hunting in mind and a heavier bullet may be the answer if you are doing something like long range target shooting.
Another reason has to do with rifleing twist and volicity. Check the twist rate for the rifle and then the pistol. The rifle, in 223, may have a very fast twist, like a 1 in 7. Much more than a 1 in 12 may not stabalize the longer 60 gr bullet. The pistol barrel probably has a 1 in 14 or 16 twist. My 22 Hornet has a one in 14 twist and even at max volicity, I get oblong holes in paper shooting Winchester 55gr boattail bullets. Accuracy suffers but I still get about 2 inch groups at 50 meters. I still wish they would do the tests because not all barrels are the same and some may get that little bit of extra volicity and stabalize a longer bullet and the guy that has such a barrel could use the test info.
Sixgun