I hope to try the Sinclair mandrels, perhaps for the next cartridge I buy a set of dies for.
The Lee collet die is also a mandrel die. It does not use bushings and it works by sizing the ID not the OD. The spring collet squeezes the brass around the mandrel, and so it is indifferent to case neck thickness or variations in it.
One difference with a mandrel-only die like the Sinclair is that the neck has to be sized-down first by some other means like a standard sizing die without the expander ball. Another difference is that there are a variety of mandrel diameters available rather than cartridge-specific mandrels. The other difference I can think of is the separate mandrels can be used for neck-turning, whereas the Lee collet's mandrel is not intended for that.
I understand how the collet die works. The beauty of the Sinclair mandrel is that you just buy the body, and the mandrels are like $20 per caliber. Yes, I double pass, FL sizer (no ball) and then mandrel. Its an extra step, but I feel that one extra step, in combination with FL sizing produces superior ammo. I also get alot less cases stretching because you arent dragging the expander ball back thru the brass, which stretches the brass length slightly, and work hardens it even more. How do I know it stretches less? I can see it when I trim. The pieces sized with a mandrel I have to trim less than those sized with a ball.
I guess Im not a fan of the Lee die because its only for sizing the necks. Neck sizing doesnt really provide any real tangible benefits in the long run other than ease. It doesnt extended brass life, because brass still stretches every firing. When that brass stretches, it becomes worked hardened, regardless if you are only sizing the neck which is the most fragile part of a case. And you still have to FL body size the brass every few firings depending on your rifle. The minimal work we do when sizing is minor compared to the abuse it takes when its fired, so the "neck sizing increases brass life" mantra is a crock. When you have to full size a neck sized case every 4 firing because the case is blown out and you can barely close the bolt, do you think that sizing that brass imparts more or less stress into the brass than the minimal pass required doing a FL sizing every firing? Its much akin to resizing brass that came out of MG vs brass that came out of a bolt gun.
My other complaint about neck sizing is that while your brass may be fired formed perfectly to your chamber, which is a good thing, you still have a tolerance stack up in the action, bolt and barrel which does exacerbate concentricity issues. A thou here, a thou there, thats tolerance stackup, and its something that every rifle has at some level. Custom rifles made by a skilled smith with quality actions and barrels have less issues, but factory guns can be awful. The chamber might be just slightly out of round, or off center, or canted just slightly. Let me explain how that affects the entire thing. I look at neck sizing like a clock with how brass is indexed coming out of the chamber. The brass is fired, and now its the same as your chamber, and we will call that 12 o'clock. So this brass, that is perfectly fire formed to your chamber, was clocked at 12 o'clock when fire formed. You process, neck only, and now that brass gets loaded. How can you guarantee that you get the reloaded round back into the chamber at 12 o'clock? Its almost impossible and that is where the tolerance stack comes back to bite you. If it comes back into the chamber at 6 o'clock its going to be way off where it should ideally because the brass is not perfectly concentric to the bullet in relation to the bore. Now you take a round that is FL sized with a 2 thou shoulder bump. That brass is consistently the same size, shot after shot after shot. It doesnt care where it is clocked because the brass is always concentric and can be indexed in any position in the chamber. The brass being concentric to the bullet, puts the bullet in the same location every firing in relation to the bore. I would rather have a result that is absolutely consistent and consistency with high levels of detail always averages out better than something that can be inconsistent even with high levels of detail.
You look at every "pro" shooter, and I use that term loosely as there are very few people out there that are 100% full ride shooters, but they win and sometimes they even win money. You look at what they are doing. Are they neck sizing? No. There might be a few out there that are doing it, but they are few and far between.