I have been using a B mag for 2 seasons now. I shot a sporter barrel last season and the accuracy was not impressive. I switched to a heavy barrel B mag for this season and it shoots amazingly well with 5/8" groups at 100.
For me, the jury is still out on performance. Very little, if any fur damage which is great but I've had trouble anchoring foxes with it even with good shot placement. Many have run from the spot of the shot and fallen over dead, others have required a boot to the chest or a follow up shot. I have shot at over 45 red foxes and only brought home 25 (I believe, give or take) foxes this year, and I believe there are more foxes that are dead out there somewhere that I couldn't find. I have also brought home 4 coons with it and 2 of those required follow-up shots...