The Speer loading manuals used to address how to use their plastic bullets in normal brass cases vs. their special plastic cases you would normally use. If I remember correctly it said you could drill out the flash holes of brass cases and using primers only, fire the plastic bullets for practice.
There was also a warning about NOT using those cases with drilled out flash holes for normal loads because the larger the flash hole, the more pressure permitted into the primer. This may be what you’re running into with the blown primers.
Here you go:
Speer maanual #13, pg 596…
“The Target-45 system is slightly different from the 38 and 44 versions. T-45 bullets are used in standard brass 45 Auto cases. Bullets are pressed into a primed, resized case with finger pressure until they stop. If primer setback occurs, open the flash hole with a 7/64” drill. Do not use these modified cases for conventional loading. Mark them with a bright, permanent color or notch the rims with a file to clearly identify them as practice cases.”
