Sounds like your fit on the hinge is better with the 12g forend than the 10g forend, but it is pulling everything together so snug that it is creating some resistance elsewhere. Likely on the back side of the locking lug.
I will use Parker Reproductions as an example. Interchangability of barrel sets from gun to gun is very good. But sometimes a set of barrels fitted to a frame can be very firm to open and close. And it usually ends up being the back of the lug rubbing on the bridge in the frame too much.
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B. Dudley
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