...in your original post, did you mean 50 yds or 5 yards. A group of 4" at 5 yds is abysmal. From a rest, you ought to be getting ragged holes at that shorter distance, and most any bullet should do it and from most any type of gun...
For cast bullets in general, you need a bullet that's about .001" over groove diameter. If your barrel in .355, then a .356 bullet should work well in it, providing there are no other issues. I should note that, in loading for 9mm, I've never had the accuracy from cast bullets that I can expect from jacketed ones....and only in 9mm. In .40 S&W, .41 Mag., both of the .44's, and both of the .45's, cast bullets are usually more accurate than their jacketed brethren.
Another thought: I've had good luck eliminating leading in 9mm (and some other calibers as well) by re-lubing commercially cast 9mm bullets with Lee Liquid Alox using the butter tub, swirl method...I thin the LLA by 20% or so, then swirl the bullets...a little LLA goes a long way, then dump them out on wax paper to dry. They're a little messy to load with and handle, but the results have been terrific, cutting group size, at 25 yds, in half.
Good point, M&P
If it is a lube problem then it tends to show lead neat the muzzle. If it is fit it starts closer to the chamber, and tends to follow the rifling.
Best Regards, Rod