New Remington 870 Express Not Ejecting Slug Shells

[b_v]

Hey guys I have a 870 express that I just purchased brand new a few weeks ago. I have the standard 26in bird shot barrel that came with the gun and I also bought a cantilever fully rifled slug barrel that's made by Mossberg for the Remington 870. I went out to the range today to sight in the slug barrel and right away I had problem with the gun not ejecting the spent shells, which are Hornady SST 12 gauge sabot slugs. Out of the 5 shots only 1 of them was ejected properly. The weird thing that I cannot explain is, after the 5 shots from the Hornady sabot slug failed to eject, I put in 5 1oz slugs (Remington Slugger) and the gun shot fine with no hiccups. I've shot slugs along with steel and lead bird shots out of the bird barrel without any issue.

Now my question is, what is the problem here? Is it bad slugs/shells that are causing the ejection problems or is the problem caused by the barrel made by Mossberg or is it my gun's extractor that needs to be fixed? I'm at a lost here and would like some feedback from everyone here. Thanks in advance for your consideration.

[Synchronizor]

Exactly what is happening? Are the shells sticking in the chamber? Are they extracting, but not hitting the ejector? Is the ejector trying to flip them out, but they're not clearing the ejection port?

Also, what length are the Hornady shells? And do they cause the same problems if you shoot them in your bird barrel?