What rifle are you using it in?  If it's a Peabody, many have a bore larger than .439" (often around .446")  I'd suggest slugging the bore on whatever rifle you're using first, especially if you want to shoot smokeless.
If you just want a mild load for accuracy, 10 gr Unique with a tuft of Dacron to hold the powder against the primer is safe  (assuming your rifle is in good condition!!), and with a good-fitting bullet, is capable of very nice accuracy.  Standard bullet is 370 grains. RCBS makes a .446" that's that weight if you need it.

I just checked back through my historical records and found 4 RBs in 43 Spanish with loads for 3 with 439186. 11 grains of Unique in one, 28 grains of 3031 in another, and 26 grains of 4759 in the third. The fourth one was an 1879 Argentine and required a bullet of larger diameter than the 439186.

Thank you for your help.

I experienced the same issue with similar loads of IMR 4198. The flame from the primer is just skimming over the top of the powder and failing to ignite it properly. Using a filler of some sort to hold the powder against the primer will help. I have used a 1" square of single ply toilet paper in the past, but now use Kynoch (Kynamco) foam wads. Kynoch uses the wads in their own NFB cartridges. Use of filler material is common with IMR 4198 when duplicating original black powder ballistics, but raises many pros and cons. Good luck!

Thanks again.  I'll try the magnum primers and a single sheet of toilet paper.

I use Reloader 7 with good effect.  Used load data that came with Lee dies.
Good ignition with just powder in case.