Sounds like you need to go back and start over. I'd start with checking whatever I'm measuring with.
Your "Brass at mouth is .386 to .387" is over max saami. You shouldn't be over .385" max.
http://www.saami.org/PubResources/CC_Drawings/Pistol/9mm Luger - 9mm Luger +P.pdf
Your "After sizing it is .372 to .373" is on the small size. I've always gotten .374"/.375" over the decades from several different reloading dies. I'm currently getting .374" from a set of lee dies.
It's the inside diameter of the case after belling that is important. The outside dimension of the case means nothing.
Crimping of the 9mm is 1 of the most mis-understood area's of reloading. It is also 1 of the most important!!!. Typical taper crimp dies are adjusted (depends of mfg) 1/4 turn to 1/2 turn down/deeper into the press after the taper crimp die touches the test case mouth. What 1/4 turn means:
the dies are threaded with 7/8-14 threads. That means that the die is nothing more than a 7/8" in diameter bolt that takes 14 full turns to travel 1".
14 x 4 ='s 56 or it will take 14 full turns or 56 1/4 (quarter) turns to travel 1".
1" - 56 1/4 turns ='s .018" per turn.
That means that the crimp die is only affectin .018" or 18/1000th's of an inch of the top edge of the 9mm case.
1/4 turn ='s .018" (18/1000th's)
1/2 turn ='s .036" (36/1000th's).
The idea is to turn the taper crimp die down enough to get a desired crimp. Myself, I typically use a 3/1000th's crimp on all the semi-auto ammo I reload (9mm/45acp) How many times has someone printed " I use a h&g #68 bullet with xxx powder with a 1.250" oal and a .369/.370 crimp". Countless billions of rounds have been put down range by millions of target shooters over the decades using a 3/1000th's crimp for the 45acp. The 9mm is no different, try measuring 6 or 7 different factory loads. They all end up .377"/.378" for the taper crimp on a 9mm.
Pictures are worth a 1000 words. Here's a picture of 2 of my favorite bullets to use/shoot in the 9mm's. The green bullet was designed for the 9mm. The red bullet was designed in the 1800's for the 38lc. The green bullet is a 125gr hp (hollow point) & the red bullet is a 150gr hb (hollow based). They both are sized to .358". What to look for in this picture:
Look at the walls of the 9mm case. They are smooth and tapered as they should be. You don't see any hourglasses, watermelon stuffed in the case bulge from under expanded brass. You will also see a faint shinny ring at the top of both reloads. That shinny ring is a 30/1000th's long taper crimp that crimped the top of the case mouth down to .377"/.378". When a taper crimp die is setup correctly only the top 20/1000th's to 36/1000th's of the case is affected.
Something else to keep in mind. A 3/1000th's crimp affects only .0015" of the bullet. A bullet has 2 sides, .003" divided by 2 ='s .0015". A typical piece of #20 pound printer paper is typically .0032". Onion skin tracing paper is typically .0018". So if you hold up a piece of onion skin tracing paper and look at the edge of it. That edge is thicker than the depth (crimp ring) left in a bullet from a properly adjusted taper crimp die.
The magic of that .0015" deep taper crimp is that is around 30/1000th's of a inch long. The back edge of a utility knife blade is around 30/1000th's.
Something to think about:
Great you got that coke bottle, watermelon stuffed in the case look thinking WOW "look at all that case neck tension". Don't need no taper crimp here, I'll save on brass wear and tear, better chamber fit and make sure I don't swage the bullet down. So I will only use enough crimp to remove the flare from the bell in the case.
Well as others have already posted different brass has different thickness and couple that with things like brass springback from different types/blends of the grades of brass. Then add the work hardeneing of the different brass from 1x, 5x, ???x times reloaded. The end result is huge differences in brass.
Consistency ='s accuracy
You take that watermelon round and shoot it. It will take different amounts of pressure from the load to expand/seal the chamber of the bbl. The closer you reload to the dimensions of the chamber, the more consistent you ammo will be.
A crimp, not only is it there as an extra measure for bullet setback it is there to aid in consistent ignition. Namely the short start pressure of a reload. You hit the loud bullet, and BANG, everything is off to the races. The fp hits the primer, it in turn ignites and burns the powder. In that millisecond the powder starts to burn the pressure from the primer and the expanding gasses of the burning powder is already starting to push the bullet forward. The crimp gives the bullet that extra hold that is critical to not only get a consistent burn from the powder, it will make for a consistent short start pressure. That in turn allows for a consistent bullet launch.
To much neck tension ='s huge swings in pressure to seal the chambers.
No crimp ='s throwing gas on those huge swings from the differences in neck tension from the different hardness of the brass, thickness of the brass, how much work hardening of the brass/springback. The only thing that is consistent is that your taper crimp will allow your powder to have a more even short start pressure.
Back in the day (decades ago) sihlouette shooters used full house loads in 296 in their 44mags. They found that their loads were more accurate with standard primers compared to the factory recommended (gotta use mag primers to get a good powder burn). The higher pressure from mag primers was starting/moving the bullet quicker than their standard primer counterparts. Which led to inconsistent short start pressure/powder burn which showed up down range with lower scores.
Those same reloads/bullets pictured above. A 10shot test group @50yds with that 150gr hb bullet. Note the fliers. I used mixed range brass for that test. As others have stated, some brass is thicker than others. And rp brass is thinner than most. The base of that long bullet was being swaged down when being seated so deep in the thicker cases. I swithed to using nothing but rp brass with this loadd and it cut down on the fliers.
A 10-shot group @50ft with those 125gr hp's.
The op really should start over and take some time to go through every aspect of reloading the 9mm. Start with sizing the brass and go from there. Ask others what their measurements/#'s are for all the different areas of reloading the 9mm. As it stands right now.
Your used brass # is too big.
Your sized brass # is too small.
You # for belling means nothing.
Just something to think about.
