The LOP adjustment on the Ballista is actually a cylinder lock, not the spring detent. It is either engaged or not into one of six holes in the buttstock body. Perhaps the LOP button/cylinder mechanism is stuck in the retracted position due to dirt or other debris (button stuck depressed and locking cylinder retracted). If it is and the LOP is held only by the ball detent used to align to one of the six positions until the button can be released and the cylinder engage, then yes, it will collapse readily upon firing. The LOP button should move freely if it is functioning.
Removing the butt pad support and LOP mechanism for cleaning and inspection is very easy. There are two 2mm hex screws located near the rear of the buttstock body, one on each side. These act as the stop screws to ensure the butt pad support does not pull off when adjusting the LOP. Simply loosen these two screws until the butt pad/LOP assembly pulls completely off the rear. With it off, you can easily see the lock mechanism, the ball detent and the cylinder recesses. Easily cleaned and lubed before reassembly. Reinstall the stop screws until the butt pad / LOP assembly moves freely and does not come off. Usually all the way in with the screws and the back off one full turn does it. Use non-permanent thread locker on these screws to securely lock them in place as they are not torqued.
If you wanted to lock the LOP, these two stop screws will also accomplish that if there is something preventing the button/cylinder mechanism from locking the LOP.
Regarding the barrel extension interface being the same as the UA TPG3-A4, they are identical and interchangeable. UA might use a different barrel profile, length or twist rate on some barrels, but otherwise they are identical.
Hope this helps.
AG
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