9mm P cast bullets that tumble when loaded with S121 or similar (H TiteGroup, IMR Target, Lovex S022, VV N320 etc.) fast-burning powder is not uncommon at all. I have experienced it as well.
The reason for the tumbling is pressure. To make minor PF the load with almost any bullet weight runs close to max pressure (35000PSI). This pressure exceeds the tensile strength of the cast lead bullet, thus deforming it and causing it to strip in the rifling.
Frontier CMJ's have a relatively thick and hard copper jacket, which can handle these high pressures much better.
When shooting cast lead, whether powder coated or wax lubed, the slower powders (MP200, S221, Lovex D036, D037.1, VV 3N37, 3N38, Alliant Unique etc) work much better, since they operate around 25000PSI with minor PF loads. This is acceptable with +/-25BHN bullets.
I also suspect that the lower operating pressures help to reduce pistol wear, which is especially important on pistols with a weaker action like the Beretta 92 and derivatives. (I have broken the locking block on my Taurus PT92 already. This is pretty common.)
I also think that the faster powders and resulting high pressures cause cases to bulge and primer pockets to stretch, resulting in cases to be scrapped much earlier than with the slower powders.
The reason for the popularity of the fast powders is the lower powder cost per shot, compared to the slower powders. If you add the cost of case wear, the saving vanishes.
In low pressure cartridges like 9mm Short, 38 Spl and 45 Auto, S121 and similar fast powders seem to work very well. Their max pressures are around 18000 - 21000 PSI which is well below what cast bullets can tolerate. (Edit: Most common loads in these calibers develop pressures well below max, even when loaded with S121.)
Cast bullet hardness also needs to be addressed. Not all bullets and alloys are equally hard. To get to 25BHN (Brinell Hardness Number) you need to have enough Antimony in the mix and quench harden the bullets from the mould (and again from the PC oven). Since Antimony is somewhat costly, many casters cast bullets that are just not hard enough for high pressures. Than may well be true for commercial casters too. To measure is to know.
Powder coating (PC) is normally harder than lead, so a generous layer of thermoplastic coating can help to strengthen the bullet surface and prevent stripping to some degree.
Another important parameter to consider is bullet to bore fit. Ideally, a cast bullet should be at least 0.001" (0.025mm) bigger than the groove diameter of the barrel. Since PC is somewhat "springy", most casters use bullets that are 0.002" (0.05mm) larger than barrel groove diameter. Do get your barrel measured (slugged) to determine the actual size. I have seen several 9mmP pistols with groove diameters as large as 0.359", in fact, I own one of them. Standard is 0.355" to 0.356".
If the bullet does not fit snug in the barrel, some hot gasses will escape (blow by) past the bullet and cause flame cutting of the bullet's bearing surface. This causes leading, stripping and tumbling.
Do it right and you will enjoy many years of accurate shooting at a moderate cost.
