Again, because OAL is very similar for both cartridges, then assuming both bullets are loaded close to max OAL for the cartridge, then both bullets will be in almost exactly the same position in the chamber. That means both are positioned pretty much the same distance into the freebore before firing. If there's blowby for one, then there will be blowby for the other.The ~.135" length of .380" diameter unused .357 Mag chamber is the concern. A .357" bullet will have blow-by and instability in that situation. A 125 gr XTP is the same whether loaded in a .38 Special case or a .357 Mag case, and it will have to navigate the .357 Mag chamber if fired from a .38.
Again, the temptation is to assume that the bullets start out in different positions because of the difference in case lengths, but in reality, assuming that the loads are close to max OAL, they start out in nearly identical positions. So the idea of the .38Spl bullet rattling down this long length of .380" diameter chamber in the .357Mag chamber just isn't reality unless the .38Spl load is made to be considerably shorter than max OAL.
Set the two loads next to each other on the table. If they are both about the same length (and they will be assuming both are loaded to about max OAL) then both bullets will start in the same position in the chamber.A 125 gr XTP is the same whether loaded in a .38 Special case or a .357 Mag case, and it will have to navigate the .357 Mag chamber if fired from a .38.
Of course, if the .38Spl load is made to be considerably under max OAL, then things will be a bit different.