PlusP's point is well taken, but I tink the problem does not come from the odd or off angle itself it comes from the fact that the weapon's frame is not properly supported in relation to the slide.
In other words, in a vise grip/ ransom rest, I doubt that there would be a significant increase in malfunctions at ANY Angle with a weapon like a Glock, but an operator may have serious problems properly supporting a weapon in an awkward shooting position.
I've worked with an LEO on an entry team who was having functioning problems when he first start shooting around a ballistic shield. He was shooting a Glock .40. I aksed around, and apparently this is a commmon problem for people new to shooting around a barricade, particularly with Glocks, but with practice, they develop the muscle control to support eh weapon properly in that position and the problem goes away....
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-Essayons