Re: counterbore
The rifle will function fine without it, I've seen it, I've done it to try it out, and the test rifle is shooting just dandy. Now, I'm not sure WHY you would avoid it, but it can be done.
The amount of clearance around the nose is dependent on several things and not everyone runs a 0.005" clearance for a variety of reasons. Some gunsmiths do, some run a little more, some run a lot more.
Why are you worried about the counterbore in a switch barrel?