Same reference - WWG - 4th Edn.
Class 3 = single barreled ML - smooth bore -
219grains/8drams and bullet for proof of 316grains.... of a specified shape/form and lead density, but for Shot barrels it shall be No.6 soft lead.
Irrespective of any additional special condition of proof that might be requested by the gunmaker on a particular gun.
This may be the answer to your query but in "general terms" only.