"I-594 does not prevent you from letting another person shoot your gun at the range"
Correct. But that's not what I'm talking about. We have a new shooter (a police officer, FWIW). Pre 594: old competitor can loan a spare gun to new shooter by handing it to him, and getting it back six weeks later when the league is over. In the interim, new shooter can sight it in, clean it, practice with it, and compete at the weekly matches even if old shooter is out of town that week.
Saying 'no sweat, old shooter can just show up whenever new shooter wants to practice, and take the gun home for cleaning later' isn't quite the same. Now, it's fine to say 'screw the target shooters, if it will save one life...'. I just don't think that stopping bullseye shooters from sharing fussy target pistols is likely to save any lives.
Similarly, Everytown like to say 'but you can still loan guns for hunting'. And that's true; you can take your buddy duck hunting and loan him a gun as long as you go along. But when your buddy says 'Hey, I got invited on a duck hunt. Wanna come? And can I borrow a duck gun?', if you can't go that weekend, you can't loan him the gun. Hunting loans are only permissible if you are at the wildlife management area for the entire duration of the loan.
"If gun rights groups really believed that it does then they should attempt to amend the law..."
Ummm...initiatives in WA can't be changed by the legislature for 2 years; that clock is still running.
On the subject of the popularity of UBC - I've asked several non gun owner friends who voted for 594 whether they realized it outlawed the kinds of loans I'm talking about here. In general, they didn't realize that; they thought it only applied to permanent transfers (gift or sale). The initiative text was 18 pages, which was about 17 pages longer than it needed to be. My advice for people fighting these in other states is to get the actual provisions out there.