I am reloading some of my 40 once fired brass and noticed something "unusual".

Dillon 550B, RCBS 40cal carbide dies.  I screw the sizing/decapping die down till it touches the plate.  Back it off 1/2 turn as per the 550 videos.  When I size the 40, the bottom fraction is not getting sized 1/16-1/8 of an inch (i did the sharpie test).  I know this as it will not drop smoothly into my gage.  The brass drops perfectly into my Glock and Kimber 40 barrels.  The test rounds go off without a hitch.

Move over to my old RCSB.  same set up but only back off the die 1/4 turn.  Same results on the sharpie test, barrel test and firing loaded rounds.

New factory ammo drops into the gauge with no issues.

ANY SUGGESTIONS short of investing in a roll sizer?  I don't have this last 1/16" issue with my 308 or 223.

Yes you will likely have to invest in a bulge buster to get that last little bit.
See if your individual pistol will chamber that brass even if your gauge doesn't.
I have yet to run into this issue with 40 but it is famous for it.

As Meistermash said if you want to get the last little bulge out of the case.  .40 S&W is loaded to the point of insanity pressure wise and the brass bulges.  Further, Dillon dies probably size the case more than needed to ensure the bullet has sufficient tension, which accentuate the appearance of the bulge.  If the cartridge fits your pistol(s), don't worry about it.  Those chamber gauges are a gimmick to lighten your pocket.  Pull the barrel from your gun and check against that as that is where the cartridge will be used.

Thanks guys.  Thought I lost that last marble.

Dillon dies do not size all the way down.

They are designed for speed and ease of feeding the cases into them.

The Lee carbide 40 S&W die does a decent job and gets down more than other sizing dies.

Then there are special, "bulge buster" dies for 40 S&W too.

Another trick with the Lee carbine dies is to size 40 S&W twice through them, if you don't have a bulge buster die.

Same number of steps with either case.

If they run in the gun I would ignore the fact that they haven’t been fully resized.

Running in the gun is what matters

I run all of my .40 S&W brass through a Redding G-rx die. Another trick is I don't load them hot, you simply don't need to. 180 grain bullets at 900 to 925 fps is plenty. I load 200 grain bullets at 850 fps.

I find the 135's and 155's recoil a lot more than I think they should. IMO there is no reason to run the 135's faster than 1100 fps and the 155's no faster than 1025 fps. Lighter loads are lot easier on the brass and the firearm.

I found recently that the S&B brass in my pile was bulging whereas the rest were not.  Im inclined to believe it's just softer brass but I had to cull it out nonetheless.

Hey! We gots a reloading forum!

The rounds pass the chamber test you have no issue. The less you work the brass the better.

Redding has a full length die part number 91272. Better than my Dillon but not quite all the way on the Glock fired 40 cal