Mac,
We can argue this until the cows come home and still never get to the point of you being able to see that we do many things the factory says is unsafe.
We whittle on the triggers and fit barrels the factory says only they can fit. We shoot NATO 5.56 in a .223 that the factory says, DON'T DO IT! We shoot .357 Magnums that have 46,000 cup (according to Speer #10), that were used on H&R receivers pre-SB1 and SB2 and then we worry about a smaller case than the .357 with a little more pressure. The Models 157 and 148 H&R could be purchased as a shotgun or you could put .22 Jet, .22 Hornet, .30-30, .357 Mag, .44 Mag or .45/70 barrels on it.
The .35 Whelen barrels were put on guns prior to 1999 and they are 52,000 cup loads. Now you can't have a .30-06 at 50,000 cup put on an older receiver that had a 52,000 cup barrel on it. The factory doesn't know what they want, except for you to send your gun back to them for every need, real or imagined.
The bottom line is the .223 Rem has less base area to act on a receiver that is the same dimensions as one with a cartridge with much more base area and almost as much pressure.
However, if it will make you happy, I will say, I don't recommend you follow what I have done, but I have rifle barrels on shotgun receivers and I have used a .223 Rem on a shotgun receiver.