Rolf,
I am a chicken! I shape the fore end where the cap will go. Once I am staisfied with it I make a surrogate fore end from a scrap of maple and shape it exactly like the the fore end of my rifle. EXACTLY! I measure the surrogate for end with calipers to make sure it is exactlythe same sixe and shpe as the fore end of the gun I am building - but, there is no barrel - its solid wood. This allows me to hammer the annealed brass down onto the surrogate without the fear of damaging the rifle stock. It also allows me to have enough metal overlap into the barrel channel so when I remove the barrel from the rifle stock and install the muzzlecap I can file back the metal until it is flush with the barrel inlet.
That's how I do it. It has worked for me.
Best Regards,
John Cholin