Hello,
As you rightly say, the Mk2 Junior used the same .177 smoothbore barrel as the Mk1 when first introduced in 1973. This would have been a cheaper option for Webley especially as they believed that reusable darts would continue to appeal to younger shooters. I think the rifled barrelled pistols appeared around 1975 together with some other changes to the original design. I have in the past conducted some accuracy tests with both the MK1 and Mk2 Juniors equipped with both smooth and rifled barrels and have come to the conclusion that the difference in accuracy that can be achieved is negligible at the recommended 6-10 yard ranges. These pistols are after all intended for informal target shooting when having fun is probably more important than obtaining tight groups.
Brian