I think it's just the way the end has been upset to form the breech - unless the hammering process stopped at this point and what we see is the original diameter, more or less. Enfield made barrels for a number of actions at this point, sold through the National Rifle Association - there was closer co-operation between the NRA and the MoD at this point than there would be now. Most of its staff would be ex-military and had connections.
A washer has been used to bring the headspace to the right dimension - again normal by this point.