Sure sounds like excessive loads. This is exactly what a 99 does when the load is too hot. Compare a fired cased with a freshly sized one. If the shoulder has pushed forward on the fired case, there's your answer. It will be hard to close the bolt on that fired case also.
If it's NOT hot loads, it has me stumped. I assuming you can open the action and extract an unfired cartridge easily?
The bolt being slightly low before firing indicates to me that the fit between the cam face on the lever and the mating surface on bolt isn't so good. Closing the lever should make the bolt rise all the way. Very hard to fix that. You'd have to add metal to either the cam face or the bolt, and neither should be welded upon. In my previous incarnation in the machine tool rebuilding business, we'd have had it hard-chromed and ground back to size, but that is a very spendy process for an old rifle.
By how much is it too low? Two thou or ten thou? If the bolt isn't being lifted all the way into battery by the lever, it may be that the bolt is trying drop/slide down off the locking shoulder on firing. That might give the binding symptom you describe. Bolt slips down and back, the case shoulder gets pushed forward by the pressure. When everything relaxes again, the bolt is wedged shut by a case that is now too long for the chamber. But I'm speculating. Not claiming to be a know-it-all on 99s - I only have a couple. However, if I'm anywhere near close to being right, that rifle needs attention. That sort of movement of the bolt on firing will damage both the bolt and the lever, and could eventually do something dramatic.
I've never heard of a "forward action screw" on a 99 either. . . .
Primary extraction it ain't. If it were that, the back end of the bolt would move freely for a few thou before resistance is felt, when trying extract your fired case.
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