a group within a group

[isobar]

this is what often happens to me when shooting a large amount of pellets at a target.i usually set up at about 20yds and use various rifles.i commonly get 2 distinct groups which are quite acceptable on their own but centre at about half an inch apart in the vertical.i am using open sights so there may be sight picture error,was wondering if anyone else has experienced the same?. atb

[tufty]

Are these groups with the same rifle?if not it's a simple zeroing error

[isobar]

Are these groups with the same rifle?if not it's a simple zeroing error

this can happen with any of my rifles and is only evident when i shoot a large number of pellets at the same target with the same rifle at one sitting.when i shoot groups with only a few pellets it does not show the two distinct patterns due to lack of holes in the paper.the rifle i used today was a webley mk3 and to correct my earlier statement;the two groups were about an inch apart;one on top of the other.some would say that a webley such as this won't group anyway,i think it is down to my hold or my old eyes playing up but appreciate your input. atb