After researching and reading about its innovative breech
design, I learned that the Firestorm has some advantages
as well as potential drawbacks.
The breech plug was designed so that a sidelock rifle that
could use pellets as well as loose powder. The rifle was made
in both #209 percussion and flintlock models with a 1 in 48"
twist barrel.
THE “FIRE STORM” POSITIVE IGNITION
The Fire Storm’s removable breech plug with the
“Pyrodex Pyramid” directs the ignition fire 360° around
the base of the pellet. As the pellets move forward,
the flame is drawn up through the center of
the pellets, creating a “fire storm” in the breech
for reliable ignition!
http://www.tcarms.com/firearms/mzModernFlint.php
One drawback is that when pellets are used, they are not fully
seated in the breech, and it results in the potential for
poorly developed ignition which can hurt the flintlock's accuracy
and rapid ignition.
One way to combat that is to first load a base charge of loose
black powder fffg underneath the pellets, or to just entirely use
loose powder.
One person reportedly removed his Firestorm's breechplug and
turned the face of it flat on a lathe. He stated that the gun
went off better and also shot better groups afterward.
Another said that he owned a TC Firestorm for 5 years, and it
shot the 370 grain TC Maxiballs well but not the TC Maxihunters.
He also said that the 295 grain Powerbelt Aerotips shot well.
So it's not that the rifle shoots badly, it's just that for some,
it's capable of shooting better probably because of the breech design,
or its plastic stock [bedding], or both.
When another person mated his Firestorm lock with a GM
aftermarket barrel and a wood stock, he said that his "new"
rifle shot extremely accurately.
There's not many sidelocks or flintlocks that will shoot pellets,
but using pellets alone could very well hurt ignition and
accuracy. Otherwise the flintlock version can shoot reliably
and accurately with roundballs and Maxi's/conicals. The
removable breechplug and stainless steel barrel are other
pluses in its favor.
Here's a close up of how it's vent is set up from the factory: