Longarms don't have an autolock like other weapons.  They are completely manual aim.  Can you elaborate more as I am really sure what you are asking.

I think he means that the red box for target selection switches from one player  (he wants to kill) to another (that he doesn't want to focus on at the moment).  I have this frustration too, especially when it switches back and forth really fast and can't make up its mind to stay on one target (at different floor/height levels on the map, etc.).  I wish there was a way to force lock-on on one mech.

agreed the lock on/ aim system can be highly frsutrating