Hello Andrew
S&W had Offered the 8-3/8" Barrels for a very Long time on many different models including the model 29. I Can't be certain if they dropped it in the more modern ones, but Know it was available many years on the model 29. if you give us a serial number of your gun or if it is a dash series which will be stamped in the crane if it is, This will help us zero in on a date of it. An Older model 29 will take many magnum Powered loads without ever getting loose. That is an Old wive's Tail and most that shoot and own them for awhile do NOT Enjoy shooting full house loads unless you are hunting with it. Regards, Hammerdown