There originally was a small hole for ignition from the percussion cap.

The Allin Conversion took a closed breech and milled a flat three-inch 
section out of it.

They soldered and screwed a hinge reinforcement in the front, installed 
a breech block with a cam to hold it closed in the back and a firing pin 
that angled from the hammer to the center of the chamber, and they 
reamed out the barrel and installed a sleeve, reducing the caliber from 
.577 to .50 .  The chamber of a cartridge rifle is always larger than 
the barrel, and the sleeve was used to engineering this.

  http://galootcentral.com/components/cpgalbums/userpics/10072/Allin_06.jpg

Hope this helps!  Gary Katsanis
Albion New York, USA