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Brass expands to chamber dimensions. The sizing die brings it back to unfired condition however it will stretch lengthwise. You're wrong to toss your brass after only a couple of reloads. Check length every two reloads and trim if necessary. It sounds like an overload. if it had not fully chambered it shouldn't have fired at all.
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Yes and no.
In an unsupported chamber, the brass will expand at the base, outwards. Heck, even in a fully supported revolver chamber, I've seen a 44 case that has had too many instances being loaded with 296 begin to loose its shape at the base.
My friend has a Ruger .40 caliber handgun and matching PC carbine, and I wonder about case support on that particular setup because his ammo has horrid bulges on the base. His dillon sizing die does not fix this problem.
And frankly, my Lee/RCBS/Forster dies don't size COMPLETELY to the bottom of the case either. In any caliber I own. I'll admit I don't own a small base die, however. Never tried one.
I just threw out 50 pieces of PMC 44mag brass that just didn't look right even after re-sizing. They had 5 loads through them, probably 2-3 of those with H110 or Win296.
To the OP:
they get longer (especially rifles, but all cartridges in general) when fired. You can trim them back and keep using them, but then they get thinner. At some point, they get thin enough that they will bulge at the base, or suffer from a case head separation.
Or you can avoid that problem and get rid of them before they fail you.
Some calibers will last years and years. I have .45acp cases that must have 10 loads through them (never trimmed), and they will be chucked because the extractor beats them to heck, not because of bad lengths or thin-ness or bulging.
My .308 from my M1A is a great example. It has to be trimmed every 2-3 times it is fired. I trim twice (once with new brass, once around load #3 or 4). On the fifth loading, they get tossed. Were it a bolt action .308 the brass would last longer, but the rifle is murder on the brass. Mostly because the brass is extracted while still burning hot and still expanding from the pressure of the charge.
I'd be comfortable reloading 9mm cases at least 6 times as long as I wasn't using max loads and I was not seating my bullets deeply or using excessively heavy bullets (147's for example). with reloads, I try to get the minimum amount of power to reliably cycle my action, unless it is a 44mag hunting or critter defense round. As a result, my handgun brass lasts a long time.