It could be as simple as a dirty chamber that's sticking the case wall after firing, but not when it's cooled a bit, and you're getting the knife blade out.
The carbine headspaces off of the case length - I'd measure the Armscor case length and see what that tells you.
If the case length is in spec, the extractor is probably failing to grab the case rim either to wear, or and issue with the case.
Grab your closest reloading manual and calipers - spec the case heads and rims to see if they are in spec or not.
If the mystery ammo is extracting - the Armscor should as well unless it's a bit out of spec, and fighting a dirty chamber.
