You might want to try one of the clones of the LBT style slugs. These give a shorter nose and a longer bearing surface. Might be what your barrel is asking for. If you enjoy hunting up old moulds, look around for Lyman 410426, a 260 gr. round nose designed for the 401 Winchester Self Loader. I've been using that slug in my 41 handguns for 20 some years with good success, and yes, I size it .411".
You may want to impact swage the throat of your chamber too. Measure what the throat diameter is and size you slugs to it, irregardless of what the barrel groove is. If the throat doesn't get filled well you get gas cutting on the base before the slug obturates in the bore. Marlins are famous for this problem with micro-groove rifling. If you cast to the throat you can generally get any of the big bore Marlin micros to shoot full power heavy slugs as well the jacketed stuff.
