3400 fps with a 125 gr bullet out of a 14" 309 JDJ??????? Unless this is a typo I think you need to have your chrno checked.
The unit repeated those numbers several times on different days and was verified by my shooting buddy. I was using 125 gr Nosler Ballistic Tip with a near max load.
Been thinking about adding a 4th barrel, 25 Souper 9 twist (25/308). It would be fun.