I stopped by the local "gun" department at Walmart and bought some lead sinkers yesterday for this project.
I thoroughly cleaned and lubed the bore before starting. I ran the lead from the muzzle end due to the larger size of the sinker and I wasn't sure how it would work from the breech end. Getting it started was not difficult, but took some effort as it was stripping the extra lead away.
I measured the slug with my dial calipers and it shows that my bore is right at .400" or maybe just a touch over around .4003". The .4003" is an estimate of where the needle was on the dial of my calipers. My calipers are the type that have one complete revolution per .100" so the space between each .001" is fairly large.
My calipers measure the Berry's 180gr RS bullets right at .400" so my calipers are reading pretty darn close at this range.
I have attached an image of the lead slug setting next to the Berry's 180gr RS bullet that I pushed through the barrel.
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That is a nice tight bore for a Glock. I and jealous
My stock G40 barrel is .401", sorta goes along with what some tests have found.
Some Glock barrels are extremely good, others are not so much.
It is not possible to see, but on the bullet in the image it has very light marks on it from the grooves in the barrel.