I believe now that the cause of the squibb was not tight enough of a crimp.
Possible I suppose, but seems unlikely it was lack of crimp.
IF, by any chance you saved out the case the squib fired from, you might check it for a neck crack or any thing else out of the ordinary.
A fast powder, light load .38 Special should not squib due to lack of crimp. It should fire normally with NO crimp at all. PROVIDED the case has the normal neck tension grip on the bullet.
A full charge of a difficult to ignite powder (such as W296) needs a solid crimp (along with proper neck tension) for good ignition, but a small charge of a fast powder simply doesn't.
The only other thing I can think of is that perhaps your scale was off. Did you test it with a check weight before you loaded those two rounds? DO you still have the second round? IF so, you could pull the bullet and check the powder on a different scale or on your scale after calibration.
OR if you're gutsy enough, you could fire it and see if it does the same thing? I wouldn't, but I'm not that gutsy...
Is there any possibility you misread your scale?? 0.5 read as 5.0 perhaps?? that would explain a squib easily.
We crimp even light .38 Special loads a bit, but its not for ignition, its to prevent bullets from being pulled by the recoil of revolvers.
Giving the round a bit of a crimp ensures it will work properly in all revolvers, particularly the light ones.
I've tested this, there are standard .38 loads that work fine uncrimped in heavy revolvers, but the bullets get pulled a bit in recoil in very light revolvers. So, I doubt it was the crimp on your load that created the squib. IF you were loading W296 I could buy that but not using HP-38.
I don't use the all copper pistol bullets, so I have no personal experience with them, other than the general awareness of how they behave differently than jacketed bullets. I wonder if perhaps the bullet was simply "too tight a fit" for that powder charge to push all the way through the barrel. The difficulty you had getting it out makes me wonder about that.
Might it be the combination of a very tight bullet, which doesn't compress the way lead does, a very light powder charge and the cylinder gap acting as a vent all together be the cause??
I don't know, just speculating.