I would also suspect a bit of excessive firing pin protrusion from the standing breech face on the lower bbl.
I had Josef Jurjevic [one of the bast double-gun smith's of all time, IMHO] tell me back in the early 70's that one millimetre or .040" was the minimum you wanted for consistent ignition and that 1.5 mm or .060" was the outer limit & any more would cause firing pin drag, making most double guns hard to open after firing and for some, that outer limit might be too much.
Its easily checked dropping the hammers w/a block of very hard wood or the classic bit of horn held against the breech face and then using a feeler gauge held flat against the breech face next to the firing pin being measured.
I'd also give the nose of that pin a close inspection for any chips or irregularities. If any are noted and they are not deep, the pin should be removed and they should be stoned out smooth. It is also how one goes about reducing too much protrusion, should it be found the culprit.
I'm assuming the hard opening is only experienced with fired cartridges; is that correct?
Those are fun guns! I've a 20 of the same vintage bought new w/28" bbl.'s choked MOD & FULL and it has shot many limits of dove & large bags of feral pigeons w/ 7/8's oz. of #6's in the Federal Field loads w/the two-piece 'Pushin' Cushion' plastic wads and thousands of clay targets using #8 7/8's ounce reloads mostly, but also new CF WW AA's, and older Remington & Federal target loads in the years I've had it and I've never had the first issue w/it.
I've never fired any of the Winchester Universal white box ammunition in it that you are experiencing the opening issue with.
I did quit using the Remington 'Gun Club's' for reloading because of extraction issues experienced w/certain 20 ga. reloads in some std. weight 1100's. Those washed steel heads do not have the same type of memory as brass and they take more effort when resizing/de-priming when using single-stage loaders like a MEC. I'm only reloading the WW CF and AA HS hulls these days.
