It's usually the Extractor.
But it can also be the Cartridge Guide alone or along with the Extractor.
The Cartridge Guide has 2 legs at the front of it that you can see when you open the bolt all the way and look down into the action.
Those legs are the Ejector. They are what the upper rim of the case strikes as the bolt is pushed to the rear upon firing or manually.
If those legs get worn or damaged, the spent round won't always pivot off of the bolt face in the direction and speed needed to clear the ejection port.
Add to that a worn Extractor that may be interfering as it's supposed to be the lower pivot for the case to kick off of the bolt face and you can get stove pipe jams.
Sometimes cases that don't clear can get crushed end to end along with a new round partially chambered as well.
Some aftermarket parts work just fine as drop in's. Some need some ( very little) touch up to smooth mfg'ring edges.
Most of the guns could use a good cleaning as well.
The guns are made for HS ammo but usually work just fine w/ Std Vel too.
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