Please explain to me how the 220Swift is overbore.
"Overbore" is a condition of the expansion ratio of a cartridge being so large (low numbered) that an increase in capacity makes little or no effect upon velocity. It is a direct relationship between the expansion ratio and burn rate of the available powder.
A very good example of this is the 7mmRUM, it barely makes use of the slowest powders available to get an increase over the various other 7mm magnums. If the same powder that's efficient in the 7mmRM for example is used in the RUM case, there's barely an increase in velocity, even though the charge may be 10-15% larger in capacity
Throat erosion has nothing to do with "overbore capacity", it has everything to do with the type of powder you use and how fast you shoot. Even a .222 can be cooked by shooting it too fast and using a powder of the wrong burn rate! With normal use, 10,000 rounds is not uncommon for a .222 to withstand with normal accuracy!
The 300WinMag isn't overbore by any stretch of the imagination. There are cartridges out there that are quite faster and still have not reached an "overbore" situation. Even the huge 30-378 isn't at an "overbore" situation, but it's getting at the end of capacity usage before an "overbore" situation is reached. But with new powders being developed every other week I can predict that "overbore capacity" will never be reached again.
Some interesting facts.
To clarify, "overbore" was once true, but only in the aspect of the powders available at the time it was dreamed up. When the 25-06 was first experimented with there was no powder available that could be used to increase velocity over the 257Bob by any significance, so it was dubbed "overbore" at the time. This is no longer true of that cartridge, or in fact nearly all cartridges.
It was even once touted that the 270 was overbore, then came along 4831 and WOW! it became one of the most efficient cartridges ever devised!
Have a look at what Mr Allen has achieved with his "Allen Mags", if "overbore" capacity was true in this day and age then none would have gotten off the ground!
The 25-06 is a wonderfully accurate cartridge and will last a lifetime with proper care, don't shoot it until the barrel melts and it will give you a few thousand rounds down it before it gives up the ghost.
I have 2 and they both are a joy to shoot. The 25-06 really is a 1/4 bore "magnum", if the truth be known!