Howdy Again
That is not why the blanks used for Mounted shooting often have larger flash holes. They often have larger flash holes so the primers don't back out. It has nothing to do with the burning rate of the powder.
Whenever a cartridge fires the primer always backs out. This happens because when the primer ignites, sending hot gasses and flames through the flash hole to ignite the powder, enough pressure builds up in the primer pocket to back the primer out of the case. With any revolver there is enough free space between the case head and the recoil shield that the primer can, and will back out enough to bind against the recoil shield. This ALWAYS happens, every single time a cartridge fires. However a millisecond or two later, the backward thrust of the case created by recoil drives the case back against the recoil shield, reseating the primer flush with the back of the case head. Examine your spent cases sometime. If you normally seat your primers a couple of thusandths sub flush as is recommended, they will be seated flush with the case after firing.
In fact, sometimes even with a projectile, if enough recoil is not created, primers can remain backed out.
With a blank cartridge, there is no projectile to generate any significant recoil. So when the cartridge fires, there is no recoil to reseat the primer. So they will remain backed out. After one or two shots the gun becomes difficult to operate, because the primers have backed out and are binding against the recoil shield. Try it sometime with primers loaded in empty cases. The primers will back out and the gun will bind up. The solution with blanks is to open up the flash hole a little bit. This prevents enough pressure from building up in the primer pocket to back out the primers in the first place. They stay seated.
However, it is true that cases that have had their flash holes opened up should never be used for live ammunition. Cartridge manufacturers discovered a long time ago what the optimum size was for flash holes. The correct size is just large enough to allow the flame from the primer to shoot through and ignite the powder. But not so large that pressure from the main powder charge could come back through the flash hole and puncture the primer. Primer cups are not thick enough to withstand the pressure of the main powder charge igniting, the small flash hole keeps the pressure from backing out and possibly puncturing a primer.