Interesting - an issue on WIX that I can actually assist with!
On modern US artillery pieces, regardless of caliber, all types of artillery rounds (HE, Illum, DPICM, etc...) have a hard plastic obturating band. When the round is rammed into the tube, the obturating band is malleable enough to engage the rifling in the barrel and form an airtight seal. Once the propellant is ignited, the round (and anything with it - including your .50 bullet) will be pushed out of the barrel.
The flight of the round may or may not be affected depending on the how the bullet is lodged in the barrel. The passing of the round and bullet through the muzzle brake should not be affected either.
There probably would be, however, some scoring to the barrel from the event that would affect the performance of the barrel in the future.
I am a former Field Artillery Officer; it's nice to talk gunnery every once in awhile!
Hope this helps!
Mike the Redleg