I had a little problem with the collapsing brace on my APC9K.
I found that when I shot the pistol with the brace fully collapsed it would jam every single shot.
Every single shot the brass would not eject.
I ended up realizing that the right side bar of the collapsing brace did not have the relief by the ejection port to allow for functioning.
So I got in touch with B&T. Talking to the rep, we figured out that my brace was assembled with the right side bar from the APC556/300 model. They send me a prepaid shipping label and under 2 weeks later I got my brace back with the proper right side bar.
This APC556/300 bar had a groove and two notches which are necessary for the rifle, but lacked the relief that the pistol caliber models need.
The correct right side bar has the relief cut out for operation while collapsed.
Unrelated, I like how the B&T collapsible rig has a notch cut out to interface with a grove in the Tailhook brace. This makes sure that everything is assembled straight and true.
I really appreciate that B&T sorted this out quickly and easily. Unfortunately, my gun still malfunctions about half the time when the brace is collapsed. The brass fails to eject and bounces back into the action causing all sorts of stove pipes, brass over bolt, or similar malfunctions. I have only tried this with cheap underpowered steel cases, maybe better ammo would do this. Still, I am disappointed with these malfunctions.