First, angling the surface doesn't do much because the force isn't generated with air shear... or most of it anyway.
The idea on pneumatic locking is pretty simple really if you understand the issue. The bolt is being forced back (compressing the spring) by the breach pressure generating enough force via the bolt face to compress the spring for a bit. The carrier that houses the spring doesn't "see" it's location so it doesn't know if the force is because of a ball or the firing pressure. Adding spring force doesn't make sense because then you will have a higher apply force on the ball and will be less effective there. So, quite simply you create a piston/ air spring to increase the holding force with a rise in breach pressure. You could also make a literal lock like a pneumatic sear, but I am not sure how necessary it would be. The easiest example I could use... well I haven't released that concept to the public yet. The second easiest easiest example of this would be the etha air-sprung spool (though they call it a poppet).
If you do this, then when firing you will get holding force from the magnet, the spring AND the air spring. If you send me an email address, I can do a quick 2d sketch and shoot it to you.