A friend of mine has a young BC coming up for 18 months old male un-castrated.  He is obsessed with his dad (who she also owns) and has a strong herding instinct but his dad will never tell him off for anything so he will shoulder barge him and not lightly but full on.  I felt the full force of him at the weekend as he wasn't looking where he was going and barged into me.  My friend's concern is that he could do some real damage but we are at a loss as to what will work to stop him doing it. 

She has tried telling him to down and then putting him back on the lead but we are concerned that she might lose a recall - I have 3 BCs myself but have never had a dog with this behaviour other than during play.  With him it is all the time and as soon as she lets him off the lead.

Does anyone have an experience of this and how did you sort it out?

Hi dvnbiker, I would like to know the answer too. I have a two year old WSS that does this to my puppy of 9 months, not only charging him but scragging him about. I hit on a diversionary tactic because the older dog is stick-happy :)--he wants to carry anything and everything but is much less reliable in bringing it back, so it can't be anything of value. I give him something to carry to keep his mouth and the rest of him occupied and he hasn't really done it since. I let the puppy off the lead a few minutes before, so he gets a chance to run, and then give the troublemaker his luggage for the day...both of them seem very happy with the arrangement and the older boy ignores them both.

I've got 5 BCs and a couple of them will barge each other during play, given the opportunity. I think your friend's right to be concerned about possible damage.  It's not so much damage that might be obviously occuring now, though that is a concern 'cause as you've felt it can be quite hard, it's how it may show up later in life.  One option is to not let him run around with his dad whilst off lead!  I've gone down the route of shouting a loud "oi" which usually grabs the offending girl's attention, praised highly when I've got the attention and they're not barging (or hanging off scruff of neck!), if I've felt the need to "oi" a further time, then I've gone to the dog, got hold of her collar and made sure she walks besides me for a while, usually going on lead.  In my case if barging took place at the start or end of a free run, so I made these times very controlled, one dog off at a time, to cut down the excitement level.
The above has worked for us.

I've got a similar problem with my young kelpie. I've been making a concerted effort to distract her with toys when out on a walk which is working brilliantly at the moment.