In the primer shortage of '94, magnum small pistol primers made a good substitute for small rifle primers, though softer cup (ie: caution about the potential for slam fires in semis). I used to run magnum small pistol primers in .38 Super and the belief at the time was that the magnum small pistol primers had the advantage of about 0.5 gr of powder over the regular small pistol primer - as compressed loads were the norm, it helped to make major in IPSC. Looking at the OP, the advice might be reasonable and as other have said here, start small and work up some test loads.