... but doesn't have the aftermarket bits & bobs available that the Marauder does. Noise level goes to the M-Rod, as does the trigger.
Pretty sure that is my B51 you are talking about and I just answered the post above this one with similar details you just did in this thread 8)
If the tool marks on the M-Rod pressure tube aren't gouged enough to become stress risers the M-Rod isn't a bad deal. All my guns are hunters and get used in the field so none are pristine. Guess I am saying use doesn't bother me as long as there are no signs of abuse.
Before you trade the Career you might have a look at Chambers UK. They had all the pieces in .177 to convert your .20 this time last week. I am sure you could sell the .20 parts and get most, if not all of your money back. I just had my 707-II carbine in .20 out hunting this morning, love that little rifle. I treat it like a heavy .177 - imagine shooting 13 grains from a small bore. Shoots like a laser and hits like a brick on a bottle!