Almost all off the shelf shotgun slugs (including brenneke) are perfectly safe to fire in even a full choke. Now accuracy is another matter, you may get the best accuracy with a C, or IC. You also might find that the results are close enough out of a M choke to not matter / bother with switching.
Maybe ask this question on a SPAS forum or some guys over on shotgunworld might be able to help you out. There are some pretty good guys on that forum that might be able to point you in the right direction.