Found It

My 1957 vintage Ruger Standard had an oddity when I got it.

The magazine would forcefully eject when you rotated the heel-catch.

It's not supposed to.

I found the reason for this to be the sear spring being backwards and intruding into the magazine well.

So I turned the spring around and put the long leg under the pin it's supposed to bear against behind the wall of the magazine well.

That created a braaaaaap noise next time I went to the range.

Uh oh.

Took it apart and put the spring's long leg back inside the magazine well, but with the spring oriented correctly so it was out of the path of the magazine (which put enough force on the sear to actually stop the hammer); and made note that I needed a new spring.

Numrich has listed them as out of stock for a long time and Ruger denied they still have or make parts for it, while also offering to let me send it in for repair at some unspecified and unguaranteed price.

Well, just the other day, Numrich got the springs back in.

New spring on top.  The short leg pushes against the sear counter-clockwise, which pushes the long leg clockwise against a pin set in the grip behind the magazine well.

Can you see the difference?  The old spring would put enough pressure on the sear, when installed correctly, to appear to function while hand cycling and dry firing.  The more dynamic environment of live firing though, it didn't have enough to keep the sear from bouncing from under the hammer and letting the pistol run away.

What a bother!