Sometimes a regular expander die doesn't do a good job for cast bullets. Just a fact. They might work fine for you, your sizing dies, your brass, your gun, and your bullets. So try and see.
My Lee 357 and 45ACP dies do a good job. But the 10mm expander is mediocre. And the 9mm is completely unacceptable (and this is mostly due to the sizing die, as I'll explain.)
My Lee 357 expander works fine because it expands most all the way through where the bullet seats, not just the mouth. (This is similar to how the M die works, only the M die is even better.) But none of my Lee autoloading handgun expander plugs come close. They only flare the mouth.
You might think a flare just on the mouth is ideal. More neck tension is better, right? Well, you don't get more neck tension with cast bullets beyond a point. After that, the brass just swages the bullet down.
My Lee 45 ACP flaring die works fine with cast bullets, partially because my Lee 45ACP sizer die barely sizes the cases to begin with. With the thinner cases, the neck tension is actually noticeably loose with jacketed bullets. Perfect for cast bullets, though, with all my brass.
My Lee 9mm sizing die really cranks down those thick 9mm cases. And then the flaring die will only touch the mouth. Seating cast bullets squished the bases down to as little as 0.353". This caused poor accuracy and lead fouling. Instead of an M die, I bought the 38S&W expander plug, and I use that for my cast 9mm. It was only a few bucks, and a couple days in transit. Lee Precision ships fast, IME. Note, the 38SW/9mm expander plug swap is a unique situation where there is a cheap solution. In has been posted in many forums. You can't order a stock plug to "fix" most other Lee flaring dies.
A good rule of thumb for your cast pistol reloads: if you are getting leading and bad accuracy, take a look at your reloads. If your expander only flares the mouth AND your reloads look like a snake that swallowed a cow, you might wanna investigate a little further by pulling a bullet and putting calipers on it. A good flare die will make your empty cases look like the "coke bottle" before you seat the bullet.
So as you can see, folks could load cast pistol bullets their entire lives and see no need for anything but a stock expander. But that may be little more than dumb luck, and it could have as much to do with their sizing dies and brass as it does the shape of their flaring die. Their dies just happen to work well for what they're shooting.
I use an M die for all rifle reloading, cast or jacketed. Wouldn't do without it.
Because bottleneck rifle expander plugs expand the entire neck, you can more than likely shoot cast rifle bullets just fine with your regular dies (with a couple exceptions). You may just need to put a little flare on the mouth to avoid shaving, and to prevent the bullet from tipping while seating, is all. Also, the gas check which is used on most cast rifle bullets would effectively protect the bullet from case-swaging, anyway. But the M dies are awesome on a stick for rifle cartridges for a couple of other reasons - in addition to the fact they flare the mouth perfectly for holding the bullet straight with no shaving.