Compression fittings and systems, kits, and methods incorporating the same

A compression fitting comprising:

• a nipple comprising:
• an annular groove;
• a longitudinal passage;
• and
• an engagement feature;
• a ferrule sized to fit over the nipple and compress a tubing end between the ferrule and the nipple, such that the nipple is configured and adapted for insertion within the ferrule, the ferrule having a complementary inner diameter;
• a first clamshell member;
• a second clamshell member;
• and
• a retaining ring seated within the annular groove of the nipple and engaged with complementary geometry of the inner diameter of the ferrule to distally retain the ferrule such that the first clamshell member and the second clamshell member sit radially over, but not within, the annular groove of the nipple;
• wherein the first clamshell member and the second clamshell member are adapted and configured to:
• engage with complementary geometry of the engagement feature of the nipple to resist at least distal movement;
• and
• radially compress the ferrule to cause the ferrule to compress the tubing end against the nipple and form a gas-tight seal with the tubing end.