- why is there an insert limiting capacity to 2 rounds in the mag?
Hunting laws.
- is it OK to take insert out?
As long as you're not hunting without the insert, yes.
- would it be different w/ a post ban shotgun?
No.
Btw, does anyone know if I can shoot 3"magnum shells in my shotgun?
That depends on the gun itself. The manual will tell you if it will chamber 3" shells.
HTH.
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"If your determination is fixed, I do not counsel you to despair. Few things are impossible to diligence and skill. Great works are performed not by strength, but perseverance."
-- Samuel Johnson