I had the action of an older 2 3/4" SxS tightened and put on face. However after firing, its extremely difficult to break the gun open. If I pull the triggers on snap caps there is no problem opening the gun. Only thing I can think of is that the pins are too long (might be rebounding off the snap caps but not off live ammo?). I have run into this before. What else might it be. Its an extractor gun.

Let me relate an experience.
I have two 16 gauge sxs shotguns. Both German manufacture.
I've shot thousands of rounds of skeet, sporting clays when 16 gauge ammo was still carried by Cabelas at a low price. This was Herters ammo.
I've shot RST, Fiocchi, assorted paper Peters and Remington hunting.
All of the above without any problems.
Federal Game Loads and the gun is difficult to break open. My belief is that the hammers are not retracting. There are no separate firing pins
Never was able to come up with a solution other than not shooting Federal Game loads

There is most likely sopme sort of a bolt that mates into the lumps. If the sunsmilth didn't get that angle right, recoil could cause it to act as a wedge.

Of course, rule out other causes before doing any work on the angle issue.

I've found Parkers are REALLY techy!

Hey, the firing pins could just be sticking in the primers (not retracting). Since they had to "reach" longer before putting the gun back on face, they may be long now, or the firing pin springs got weak, but that helped ensure they'd fire before, but now they need to be replaced to retract back after firing.

I think Duane is likely onto something. I bought a new single barrel 20g shotgun for my 9 year old grandson last Xmas and a box of 28gm cartridges. The gun was quite hard to open after firing each time with these shells however when firing just primed cases, some 3/4oz reloads and a box of factory 24gm loads the gun opened easily after firing.
No sign of firing pin drag. The pressure of the loads should all be about the same, except of course the primed cases, so like Duane I suspect it is a recoil/locking bolt related issue.
In the light 20g single the 28gm loads definitely recoil hard.

As mentioned above, try several different shell brands/types. Steel headed shells can bind when opening on a tight fitting gun - were brass heads might not. You can't go by color, the steel is plated, you have to go by the magnet or price....

Had the same thing happen on a recently rebuilt Krieghoff on the cheaper steel headed Winchesters. So I passed up a pallet of them a year ago at a steal of a price... only to find out later they were a different batch built to better tolerances and were fine in my guns. I'm kicking myself now!

The cocking linkage is likely worn and has some take up slack in it before the hammer(s) start to retract.

Since the hammer retraction movement is tied to the opening motion of the bbls thru that linkage, that slack allows the bbls to open a bit before the firing pin(s) are pulled away from the fired shells.

Depending on the amt of wear and slack in the sytem, and the particular primer used, the firing pin will be driven into the primer deeper than others.
The bbls start to rotate open but the firing pin is still imbedded in the primer and the tip of the pin is then dragged out of the primer by the opening action of the bbls.

'Harder' primers can alleviate the problem in some instances as the firing pin doesn't drive in as deep,,it doesn't drag as much coming back out. It makes it easier to open, sometimes seems to 'fix' the whole situation. But it mearly covers up the problem.
Remington STS and Cheddite primers are a couple of the 'harder' ones that generally do better for the over the knee opener guns.

A quick check to see if and how much slack is in the linkage betw the bbls and the hammers is to :
First make sure the gun is UNLOADED
Then close the SxS up,,take the safety off.
Dry fire both bbls.
Now hold the gun horizontally and unlatch the top lever while still supporting the bbls.
Holding the unlatched bbls,,let them slowly drop open of their own weight.

They will open as a rule, slightly,,sometimes quite a bit untill the cocking linkage takes up the wear and slack we are talking about and the hammers are then engaged to be retracted.

Stop and hold the bbls right there from opening any further.

Notice how far the bbls have rotated open.
How far are the bbls off the action flats and above the frame at the top.
The center of the bore,,the center of the primer,,, is the same distance displaced from the firing pin.
That is how far the firing pin has been dragged in the full down position under full main spring pressure buried in the fired primer.
That's what drags and keeps the gun from opening.

Fix the worn linkage betw the bbl movement and the hammer retraction and the hammer w/firing pin starts to back off the primer with the first motion of the bbls. That makes for easy opening.

Snap caps will generally not show the same issue as the centers of them are not the same hard material (brass or steel) that dents and holds the pin in one position.
They are either soft rubber or spring loaded plastic and still allow the firing pin tip to drag along a bit, at least the dia of the snap cap 'primer' till the hammer starts to retract.
Some are a hard nylon type matrl and don't imprint at all.

The firing pin holes are likely worn oversize, allowing the primer to extrude into the hole, somewhat, crimping the pin(s). Some of those old doubles are pretty soft!

quote:

I've found Parkers are REALLY techy!

Especially when the gun is new and the shells are cheap. The regional Parker service person "unstuck" my new 20 ga Repro and advised me to use high quality ammo with brass butts until the gun was well broken in.

I did and that worked.

In my post above, all the cartridges I was shooting in the new 20G were steel headed, you are hard put to find brass heads today. The lighter loadings with less recoil did not lock up the gun, the heavier loading did.

All about inertia...the blte angle is pretty ctitical, but again, check out all the other obvious things. A ptetty sure sighn would be that the lever locks up before any opening movement at all

Does the gun have disk set strickers. In which case new stricker springs (if it has them) might help. But as others have said, keep loads light and try several brands.

Rejointing will have affected the timing of the cocking bars. They will have worn with age. They may well need a spot of weld and refitting so as to get timing correct. Ditto for the ejectors.