Mike;
I don't understand that 18 gr Bl on both the shot & rifle bbls. That means 18 grams lead. Now this would equal about 5/8 oz of shot which may be right for the shot bbl "BUT" a 277 grain bullet from a .22RF, I don't think so. Are you sure the rifle is not 1,8 gr Bl. This would be about a 28 grain bullet. Shorts & longs in the US are loaded with 29-30 grain bullets which is very close.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra