I have recently acquired a set os "S" bushing style dies for 6PPC and found that they don't tighten the case neck enough to hold a bullet.

Why might this be?

You bought the wrong size. Too large. Somewhere in your measurements you miscalculated. Also bushing sizes must correspond with the tension you want on the bullet and variances in neck wall thickness. You obviously need bushings that are "tighter".

Measure the neck of the brass you are using with a seated bullet. Subtract at least .002" from that, .003" is better. The result is the loosest bushing you'll need as likely that will only give you a very light grip on the bullet. There's usually about .001" springback in the brass. You likely need at least .002" grip and you have to take into account the springback of the brass as it's sized. I found I ultimately had to buy minimum 3 bushings "around" the measurements I wanted and I've usually ended up with several more than that. If you get a batch of brass whose neck walls vary from what you have now, you either have to turn all the necks to the same neck wall thickness or you have to adjust the bushings. The positives of several bushings is that you get to experiment with yet another variable, neck tension. It DOES affect accuracy. In my experience the tighter the grip the less the accuracy. In a hunting gun you have to balance that with the bullet setback as a result of recoil. In medium recoilers, like a light 338, you need about .002" grip. Any more than that accuracy suffers. Less and the bullets in the magazine are set back on recoil. Good luck.

Good advice above.

Loaded round minus .002- .003 is what Redding recommends.
.001 tighter for moly coated bullets.

I am currently using minus .0005 which lets the bullet be seated by the rifling when chambered. Single loaded ONLY !

Travis F.