Lee Factory Crimp Die

[RealGun]

An O-ring doesn't limit the travel (slop) in a thread "locking" situation. The O-ring is a cushion applying pressure to the threads and will keep the male threads snug up against the female threads. If there is any movement of the stem, "locked" with an O-ring it will be the same every time pressure is applied...

The reason an O-ring works is because there is slop. If you loosen the nut and wiggle the die back and forth, you will have experienced the "slop".

 

[SSN Vet]

FWIW, I stopped using the Lee FCD for pistol rounds for fear of swaging down fat bullets. I started buying Lee taper crimp dies which are similar to the seating dies without the stem. I used them in the 5th station of my LnL AP as the final step.
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I've done the exact same thing, except my problem wasn't with fat bullets but with 45 acp cases with thick walls. I've found that Federal cases are ~40% thicker than Winchester and can add and extra ~0.0015" to the finished cartridge diameter. Before I started segregating my brass, I could always tell when I had a Federal case, as the post sizing ring in the LFCD would bump. But since all of the federal cases passed the plunk test in my 45s, I bought the Lee taper crimp die and haven't looked back.

 

[ArchAngelCD]

I have been using Lee FC dies for a very long time and I have yet to have them post size any round I have made. I like them and will use them as long as they serve my needs. I feel they only post size mistakes and I try not to make mistakes. I have heard the stories of problems and I don't doubt anyone, I just never had those problems.

You can always remove the Carbide post sizing ring and use the dies without fear.