Update
I used some pure lead and poured it into a spent .40 call casing that measured .403 I.D.
After it cooled I used a kinetic bullet puller to remove the slug. The finished slug measured .402
I then gave the barrel a good cleaning and after a light coat of oil I tapped the slug about halfway into the barrel then switched direction and taped it back out. The slug then measured .400 to .4005
My calipers show a small 5 after the .400 but not sure I trust them to be that close
Other changes
I purchased some new Starline 10mm brass from Wes
Purchased a Lee 4 die set that included the Factory Crimp Die
Loading the 220gr bullet in the new brass with the new dies on a lee 3 hole turret I did not have any of the wrinkles in the case I was getting at the bottom of the bullet and only 2 of the 150 I loaded did not pass the plunk test before the FCD. After I ran them through the FCD 100% passed.
I will see how they run this weekend. Also going to chronograph them to see if that changes at all
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Did you say you slugged the barrel from muzzle end but checked the squib from the chamber end? If so that implies the chamber end of the barrel is tighter than the muzzle. If so, nothing will fix that. Revolver barrels sometime are narrower where they screw into the frame but obviously an autoloader can't do that. So I want to be sure that's what you meant. As for the sizing die, there are sets from the same company that size while decapping and also those that size alone, with the decap die including belling. This got me just last week. I ordered a Lee sizing die so now I have two decap dies- one that sizes and one that bells.