Cased telescoped ammunition firearm with headspace reduction

A weapon for firing cased telescoped (CT) ammunition rounds, comprising:

• a barrel member defining a barrel and a chamber cavity immediately rearward of the barrel;
• a chamber member that defines a chamber configured to hold a CT round for firing from the weapon, the chamber member moving between a firing position in which the chamber member is in the chamber cavity aligned with the barrel for firing the CT round and an ejection/loading position in which the chamber member is away from the chamber cavity for ejecting a spent CT round and receiving a next CT round;
• a breech member defining a breech closing a rear end of the chamber when the chamber member is in the firing position;
• and
• a carrier assembly coupled to and co-configured with the chamber member and the breech member to perform a counter-recoil operation in which (1) the chamber member is moved from the ejection/loading position to the firing position with the next CT round therein, (2) the breech member is urged into a closed position against the next CT round in the chamber to remove headspace before the next CT round is fired from the weapon,
• wherein the breech member is a cylindrical member screwed into a rear of a barrel extension at the rear of the barrel, the barrel extension housing the chamber cavity, and the urging of the breech member is rotation of the breech member into the closed position by forward movement of the carrier assembly during the counter-recoil.