Here's the link:
The "3 trips" thing, VarmintAl said "4 or more" in regards to the barrel so I was using 3 as an example if the receiver is included, it would almost certainly be more.
I had a FileMaker based calculator my uncle made running at one point using Chris Long's equation. As I recall, most normal rifle barrel length are in the 6-7 region but that's going off memory. I'll have to ask him to put that program where I can get to it again so I can run the numbers.
Got the program again, looks like my memory is faulty (again), here are the nodes around 25 milliseconds,