I had the hk45 briefly. Yup, the o rings, think its just one, at the end of the barrel really does need to be there, but they last a very long time, and are cheap to replace if you need to. You could shoot it without it, but accuracy will suffer. Pros for the HK45...well, they're built like a tank. Decent capacity for a 45, full featured, full size duty handgun. I found the paddle mag release took some getting used to, but not altogether disagreeable. What I personally was not a fan of was the relatively high bore axis, makes recoil a bit more noticeable than most 1911's I've shot, more than the Ruger SR45. Other than that, a very good 45.
As far as the lock, I bought mine used, never knew it was there, and I sold it within a year, so it never knew anything about it