Why small flash holes? Philosophic, habit, or engineering ?

[preventec47]

Having been a lifelong reloader or modern cartridges, the only thing I
see about muzzleloading that is sacred is that the propellent and bullet
be put in from the muzzle end.  Since we have over a hundred years
of knowledge accumulated in modern firearms, why not apply some
of that to the muzzle loaders?  No doubt that is what in-line muzzle
loaders have done.  That leaves me to wonder why or how the
209 primers became popular.  All shotguns except 12 ga magnums
have SAAMI max pressure of 12,500 psi so I am wondering why
use them in black powder weapons that according to lyman black
powder handbook produce up to 33,500 psi with black powder
loads up to 120 grains.  Back to the specific question of why
the small flash holes in front of the primers when shotguns and
other modern rifles dont use the same small and long flash holes?
I've seen several attempts to use various brass case heads
with the primers pressed in and this seems like the right
direction to prevent blowback as the brass will expand
and seal when fired if the breech plug is sized properly.
Buy why restrict the fire getting to the powder via
a small flash hole if the firearm is designed to handle
the rear thrust of firing on the case head in normal calibers ?

If loading a primed rifle is a safety concern, then by all means load
on a dummy primer or case head and replace the primer/case
head after loading.   As summary, why cant we simply duplicate
as much as possible the techniques used in modern rifles for
ignition so long as we load from the muzzle ?

[Varmint Hunter]

I'm not too sure as to your "why" question. But if you can achieve good ignition through a tiny flash hole that greatly restricts back pressure then "why not"?

Standard 209 primers, fired through the tiny flash hole can even ignite the BlackHorn 209 powder, which is harder to ignite than other BP substitutes. There is also the option of using magnum 209 primers which makes an even smaller hole seem practical & maybe even safer.

[preventec47]

The reason I ask is apparently in some models there is a lot of
blowback into the action and onto the shooter with a faulty
ignition of the powder.  It seems obvious if the flash hole were
bigger then more flash would be available for ignition ( just
like in a shotgun shell ) and maybe less blowback as the
primer flash is going into the gun instead of being restricted
by the funnel.  So why not a larger flash hole?

[DavidKansas]

Because with the larger flash hole you are going to increase the amount of blow back,unless you use a bullet case for the primer holder. The smaller flash hole works for me and I see no reason to change.

[Varmint Hunter]

QUOTE: It seems obvious if the flash hole were
bigger then more flash would be available for ignition ( just
like in a shotgun shell ) and maybe less blowback as the
primer flash is going into the gun instead of being restricted
by the funnel.

This sounds counter-intuitive to me as it is widely acknowledged that the tiny flash hole is more than adequately large enough. There is a very valid reason that the hole is kept small.

Just my 2 cents

[alsaqr]

Because with the larger flash hole you are going to increase the amount of blow back,unless you use a bullet case for the primer holder.

Exactly.  One of my guns has been fired a couple thousand times and the flash hole was enlarged.  It allowed a lot of blow by.  Got a new breech plug and have very little blow by.   

[preventec47]

This sounds counter-intuitive to me as it is widely acknowledged that the tiny flash hole is more than adequately large enough. There is a very valid reason that the hole is kept small.

For the sake of discussion as I dont actually know,  I'm not
sure we know where blow back comes from in all cases.
I think it is possible in some situations that the primer explosion
is so great that it cannot all be funneled through a small hole
and a lot of the excess pressure spills out in what we call blowback.
Now there is the other situation where if the hole is too big
the the blowback could be comming from the main gunpowder
charge through the flash hole.  How can we determine which?
In any case, I must point out that there are several models
where ignition is inadequate or weak and even weaker with
the use of 25 acp primers.  That is the only reason I am
inquiring about possible improvements or different techniques.

[Varmint Hunter]

Primers produce a negligible amount of pressure and it's channeled in one direction.. A full charge of BP or BP substitute will probably produce 30,000 lbs of pressure in all directions.

Opening up the flash hole would not seem to be  a very good idea. Ideally, the breech should be sealed.

[Empty Quiver]

It is probably so the primer doesn't blow off and get lodged in your cheek. I have an older sidelock that will ocasionally blow the cap off and leave the hammer at half cock. I think most of the fouling on my inline is coming from the primer itself, as the breach area is filthy after snapping a couple drying primers. A larger hole may also allow the primer to knock the bullet away from the charge before powder ignites, and give you accuracy problems.