Who made the rifles and specifically the extensions. you have the "mil-spec" and "commercial" sized tubes. Commercial AR makers like bushmaster and RRA found a cheaper way to make the tubes, which produces a larger diameter. Colt and several other aftermarket makers (vltor, magpul, etc) produce mil-spec buffer tubes, but it would be easier to exchange the stock for one that fits your tube type.
Edit to add you might have to ditch the CAA stock for other options depending on who made the buffer extensions for your rifles.
Sorry another edit. It sounds like your putting a commercial unit on a mil-spec receiver extension so if I were you I would ditch the CAA unit and get a vltor or magpul for about the same price. I put vltor on my M4 carbine and havnt looked back. It locks up nice and tight, no wobble, and plenty of storage for batteries or other toys. Im actually thinking about getting a magpul CTR as it has a friction lock that makes it as sturdy feeling as an A2.