I purchased a Remington RB in 20 gauge, in pretty decent condition except the barrel has been shortened. My reason for acquiring this was to some day build a 20 bore rifle out of it, shooting black powder (of course) and a slug. I would am thinking a 1 1/8 X 30 inch oct barrel in the style of an English game rifle. I have a 20 bore brass case and determined that it will take up to 5 drams or 136 grains of powder loaded to 1/2 inch (12.7 mm) from the mouth. That should leave me enough room to seat a 1 inch (25.4 mm) long projectile (half in and half out). The inside of the case measures .654 (16.6 mm). Sooo... I can get a .620 barrel and have the twist and rifling depth made to my order, what would be best? If I drop to a .640 slug and .010 deep rifling that would be about perfect,,, I think? I would really like to use a full size grease grooved slug. What would a .640 X 1 inch slug weigh??? ANNNND,,,,Is this action up to the task??? Need ideas here from you bore gun guys.....SOOO many questions.....THANKS,,,Ed