I rapid fired three rounds of 3.5" magnum 00 buck through an autoloader once.
And I mean once.
When i bought my pardner pump I bought some 3" 00 plated buck with it, and kept them loaded for HD. I bought 10, fired 5, decided that was more than enough practice and kept the other 5 in the gun for a while.
I eventually got bored and fired off the last 5, and bought remngton 2 3/4" 00 buck after that. The magnum shells arent unbearable, but the 2 3/4"s dont feel inadequate, and they certainly do kick less.