I've asked Technical Support at Cimarron to weigh in on this arbor length issue. The wedge on mine pops in and out with only light finger pressure and it shoots great. Yes, the barrel is snug on the arbor but is easily removed using the rammer. I say, stop the worrying and enjoy the new gun.
I agree with you, one of my six-guns does the same thing. Go shoot and enjoy, its not rocket science.