Why more powder for 223 than 556

[MistWolf]

Hornady 62gr website download data shows more powder in 223 for 2230 and xbr than for 556 and equal max loads for TAC. Emailed em and they simply said lab said that where pressure got high.

Would someone explain to this fudd how that works?

Because every barrel is a law unto itself. You can take two identical barrels that came off the assembly line next to each other and they can exhibit completely different behaviors when it comes to pressure, velocity, precision and ammo preference.

Every reloading book carries the warning to never start with max loads, but to start at least 10% under and work up watching carefully for pressure signs. They give the same warning anytime a change is made, such as switching primers from one brand to another. It does seem odd that a 5.56 chamber would reach peak pressure before a 223 barrel, but it's possible. One possibility is that the 223 wasn't cut to true SAAMI spec. I suspect that most makers of 223s deliberately cut the throats long to keep shooters from blowing something up shooting 5.56 ammo.