Lee 9mm Undersize Resizing Die

The full-length Lee resizing die was recommended to me for loading 9mm bullseye rounds.  However, I have come across a couple of articles and videos recommending the undersize sizing die.  Do you recommend the undersize die, or is it up to experimentation and trial-and-error using both?  What are your experiences with either?
Thanks in advance!

So is undersize a shell that is too small in diameter, or too large?

I use a Lee FActory crimp as a decap resizer to get a larger shell

so Bruce, you back the crimper out, resize and then use something other than the calibre of the casing to de-prime?

David

I would highly recommend using the EGW undersizing die for resizing 9mm brass. This is also a Lee die that is sold by EGW, but it undersizes the brass more than the normal Lee one and lengthens it. It really helps with accuracy when resizing used brass. I wouldn't use it on new brass though.
Also recommend using a wet tumbler, having the primer pockets and inside makes a difference with reloads also.

Gentlemen,
This is the first time I will be doing any highly specialized loads, so I greatly appreciate sharing your techniques.
So far, a resizing die seems to be the way to go.  It was also recommended to me o use a Lee Factory Crimp Die, which already made sense to me.
Are there any considerations for using the Lee/EGW dies on a Dillon XL650?  I've hand-loaded ammo in years past and this is my first progressive loader.
Cheers

I use both 9mm Lee U-die and the LFCD.  Reduced group size at 50 yds by 1" and better case neck tension for my revolver crimp jump.  The LFCD is used to remove the case mouth bell with no crimp.  FWIW I'm not a competitive shooter but just a benchrest shooter looking for the perfect load yet to be found.  Going to change from 115 JHP's to subsonic 125 and 147 JHP's.   Spending money and having fun.  After 3 years I should be further along the path to my goal but I always get side-tracked when I get ahead of myself and think I know what I'm doing.

I found the Lee FC die was too small for most brass wall thicknesses so it effectively swaged the bullet in the case, moreso for lead bullets. Ymmv. I had that die for 45&9

Excellent info, Gents!  If I may, what's the best seating die for match 9mm?

I use the Redding precision seating die.  Not cheap but worth the expense to maintain concentricity.

I second the Redding comp seating die, expensive but cry once.  Was having some issue with rounds fitting in tight chambers even with the Lee FCD.  Some suggested my issue was seating bullets a little sideways which created a minor bulge.  It makes sure the bullet is straight before seating.  Also swapped out the factory Dillon powder funnel for the PTX made by a member here.  It actually expands the mouth instead of belling it.  Bullet naturally goes straight when placing then the die makes sure it's square before seating.  My problem round percentage has gone down to almost zero now.  Hornady also makes a similar die and is cheaper.  I just went Redding because others recommended and didn't want to buy twice and maybe cry twice.