Semi

Thinking out loud... Increasing the size of the flash hole would/will definitely allow more of the 'blast' from the HOT 209 primer to exit the flash channel through the 'opened' flash hole. The other benefit of this is that it will allow the more heat and more flash from the primer to enter the bore, which in-turn will allow ignition of the BH load. This will also or should reduce the amount of carbon build up in the 'flash channel' of the breech plug. Shooting primer after primer should not cause a problem or a build up in the BP.

But! I fear that opening the flash hole to much might end up causing another problem - with the increase of real blow back pressure against the primer and the what ever method your particular gun may have to hold the primer in place. This pressure should/will increase as the powder volume rises and or the weight of the bullet gets heavier and the pressure remains on the bottom of the primer longer.

Another adverse result of this pressure build up on the bottom of the primer - it may cause a bulge in the primer making it difficult to get the primer out of the BP. This can be reduced some buy using a copper clad battery cup instead of the aluminum cup that a lot of primers are now using. I use a Cheddite primer for shooting BH because of that reason, it has a copper cup, it acts more like a shell in a chamber in that it expands and contracts more than the aluminum cup.

It SEEMS to me the an opening of 33/35 was the suggested flash hole diameter for BH. And it SEEMS to me that the greater the opening the greater chance of real pressure blow back.

All of this is just THINKING no hard facts....

I can not remember if Utah actually shot regular BH loads and what weight projectile he might have been using....

For myself - when shooting BH it is pretty easy to twist drill the flash channel out after about 12 shots... and I have not had a problem with the flash hole plugging 'YET' now watch it happen but I shoot far more T7 than I do BH....