[..] is that ur BM case cannot account for discrepancies in ur die OR ur chamber.
This has nothing to do with belted cases - No cases, chambers or FL sizing dies are even close to one size.
If you want to get cases that fit the chamber and don't stretch and split, you need to make the processes work for you - this is not idiots heaven - you need to know what you are doing and how to compensate for the differences.
...
Back in March of 2013, I bought a new Remington 40-XB Single Shot in .220 Swift.
Unfortunately, this was 3+ months after the Sandy Hook Elementary School mass shooting. Obama promised to all but eliminate guns and ammunition (or so it was thought).
Shooters went crazy and bought everything they could find - even if they didn't need it, they could use it to trade.
So, for my new .220 Swift, there was no brass available, ANYWHERE!
I friend heard of my plight and offered 100 brand new, beautiful Norma .220 Swift cases that had never been fired. Kewl!
But, the catch was that some idiot who didn't know what he was doing, tried (unsuccessfully) to make ,220 Wilson Rocket cases out of them, and failed.
He badly collapsed the shoulders making the cases unusable, or so he thought.
When a headspace comparitor was zeroed on a .220 Swift GO gauge...
... and measured a "new" case, the headspace (the actual "Space" in the chamber) was a whoppin' 0.1015" (~1/10th of an inch of air space in the loaded chamber).
Using standard dies (an FL, and a neck), the cases were salvaged without any stretching. The shoulders blew out and fully filled the chamber without any problems.
They have been reloaded 6 times without stretching.
So, it is not about belted or nor belted - it is ALL cases. Management of head space is necessary, and will pay back with accuracy and case life.