If it isn't leaking with the larger carrier, then you were definitely using one that was too tight. You always want to make sure you are using the largest
First thing to do when you aquire any new gun/mag is to clean the entire valve and gun. Replace any suspect orings.
When the marker is
Check the reg seat. Make sure it is clean. If it is mishapen or gummy, replace it.
The twistlock, check the barrel pin. Sounds like it was