Barnes MZ aligner tool
 

Any reason the 50 cal aligner tool wouldn't properly align a 45(.40) Barnes MZ in a 50 cal barrel? I have a bit of trouble imagining a fully seated saboted bullet not being aligned without the tool, but the previous owner of my Elite felt it important enough to recomend it. I plan to try both .40 and .45 bullets in this 50cal.

I don't know if it is recommended. I use a spin jag on my range rod to load various bullets. The shape of the spin jag is very similar to the MZ aligner.
But looking at the aligner, it is a nice looking tip to put on your ramrod for seating any number of conicals and round balls.

What has me confused is it is unclear if the tip size to order is associated with bullet size or barrel size. Clearly a 50 cal tip won't likely fit in a 45 barrel, but the tips vary by bullet style causing me to believe the interior contour of the 50 MZ tip will be different than the 45 MZ tip. I've gone ahead with ordering a 50 tip. Just wish I knew if it will align a .40 bullet.

A .50 tip is made for a .50 bore.

Like Muley says.As long as you have a .50 cal the .50 cal tool will keep the bullet in line with the bore. The contour should be OK for the .40 cal bullet tip.

We shall see. Thanks

Nothing to see. It's designed to fit the bore. How else could it keep the bullet aligned?

I suspect the 50 Barnes MZ specific jag was not designed with the thought of shoving a .40 bullet down the bore, where the 45 jag likely was. We'll see.

That alignment tool made specifically for Barnes MZ bullets is not like the spin jag. It's name is very misleading actually. It is solely designed to fit the olgive and nose contour of that specific bullet. It does NOT have a bore alignment ring that is 50 caliber specific like the spin jag does. It's simply a fitted end loader for a specific bullet. That in itself MAY allow for better alignment but not as well as a spin jag with a bore fitting ring to keep the jag straight would be.

Barnes Alignment head:

Spin Jag:

Notice the alignment ring on the Spinjag?

Yup, that is the reason for my concern. Suppose spin jag went one step further, offering interchangeable tips, with olgive and nose to match each most popular bullet. Would anybody buy it?