Sil,
Look at it this way-
It's not just anybody borrowing a gun, you're way over-simplifying it while at the same time making it more complicated than it is.
ATF's presumption is that, WITHIN CERTAIN PARAMETERS, there's an actual "transfer" being made, whether or not that's the reality.
Those parameters are limited in scope, and clearly defined (mostly, anyway).
This does not apply to shipping companies, since ATF can see quite clearly there is NO "transfer" by their definition, and the shipping company is not the "end destination".
It does not (as yet) apply to you letting your buddy shoot your gun at the range (except possibly in Colorado, under their new STATE law).
It is not mere possession under any & all circumstances that determines a "transfer".
ATF has decided that transactions between FFLs must be logged, and made a presumption that if a gun passes through the hands of an FFL-holder IN A NON-REPAIR CONTEXT, it's more likely to involve a "transaction" than not.
That's further affected by crossing state lines.
Gunsmiths have FFLs, too. They have to log guns in & out.
If a gun comes to them from another FFL, it has to be logged out & go back to that FFL.
If it comes in direct from an owner without FFL involvement, it still has to be logged in & out, but ATF regs allow the gunsmith to return to the OWNER direct, without any 4473 or FFL participation at the owner's end.
Rather than requiring each shipper to provide certification in triplicate that there IS no actual transfer of ownership involved, on EVERY shipping event, ATF just establishes a broad policy to cover the big picture, as opposed to scrutinizing each & every shipping event individually.
Trust me- you don't want that.
The current system has its flaws, but it could be far worse.
Idaho,
You don't need an FFL to ship a gun to a gunsmith directly, or to return a gun to its maker. UNLESS local law requires an FFL to handle everything both ways, which I believe is the case in California. Could be wrong on California.
Denis