THE RPTR EXPERIENCED A VIBRATION IN HIS R ENG WHICH SUBSEQUENTLY; ABOUT 5 MI FROM THE ARPT; DETERIORATED TO A 5 INCH DROP IN MANIFOLD PRESSURE AND ENG BACKFIRING. THE RPTR ELECTED TO SHUT THE ENG DOWN AND LAND ON ONE ENG.

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APPROX 30 NM E OF MILWAUKEE; I EXPERIENCED A SUDDEN SMALL VIBRATION WHILE CRUISING AT 6000 FT ENRTE TO MWC. I TROUBLESHOOTED AND FINDING NO PROBS WITH THE ENG; SLOWED TO REDUCE THE VIBRATION. APCH WAS INFORMED THAT I WAS EXPERIENCING THIS PROB AND WE CONTINUED TO OUR DEST. WE WERE BEING VECTORED FOR THE VOR RWY 4 AT MWC WHEN APPROX 5 NM S OF THE ARPT; I LOST 5 INCHES OF MANIFOLD PRESSURE ON THE R ENG. THE ENG BEGAN TO VIBRATE AND BACKFIRE VIOLENTLY. I REQUESTED A VECTOR DIRECT THE FIELD AS I WAS VMC AND WAS CLRED FOR THE VISUAL AND HANDED OFF TO MWC TWR. I AGAIN TROUBLESHOOTED AND FOUND NO PROBS. I THEN SHUT DOWN AND SECURED THE R ENG; INFORMED TWR WE WERE ACTUAL SINGLE ENG AND LANDED UNEVENTFULLY. THE R ENG SEEMED TO BE CAPABLE OF PRODUCING PWR; BUT I DECIDED TO SHUT DOWN BECAUSE OF THE VIBRATION; BACKFIRING AND I DIDN'T WANT TO HAVE TO DEAL WITH ANY LARGER PROBS ON THE VISUAL APCH NEAR THE GND. ONCE THE ENG WAS FEATHERED; ALL VIBRATION STOPPED. I HAD RECEIVED 1 HR OF DUAL ON SINGLE ENG PROCS 5 DAYS EARLIER. THIS GREATLY HELPED THINGS GO SMOOTHLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE WAS FLYING A BEECH DUCHESS BE66. HE SAID HE WOULD NOT DO ANYTHING DIFFERENTLY OTHER THAN HAVE DECLARED AN EMER SO THAT EVERYONE WOULD BE AWARE HE WAS ON A SINGLE ENG AND AT SOME POINT ON FINAL COMMITTED TO LAND. HE STATED HE HAS PRACTICED SINGLE ENG GARS BUT THERE IS NOT MUCH CLB PERFORMANCE WITH AN ENG OUT.