Two possibilities, if it kicks the snot out of you.
It doesn't fit you, or you use heavy loads.
I shoot 7/8 ounce target loads at 1150 roughly, and 1 ounce at 1200 for hunting.
My 20 gauge AyA and my Beretta 687 are 5 pounds 15 ounces. I have no problem with 100 rounds of target loads at Sporting Clays. I do have a pad for Clays.
Larry, you are most likely correct. But ....... I have a 20 ga Superposed (RKLT) that weighs about two ounces more than the Merkel and I can shoot it all day without a recoil issue, mostly skeet. The stock dimensions are the same except that the Browning has a tad more pitch. I made and installed a wedge under the butt plate on the Merkel to make them the same. I've also loaded some 3/4 oz shells (14.5 gr Green Dot for about 1,200 fps) but have not had a chance to shoot it after these changes because of a combination of crazy club hours (Covid), bad weather, home projects, etc.
The one fundamental difference between the guns is that the Browning has a POW grip and field style forend whereas the Merkel has a straight had stock and splinter forend. There's not much wood to grab on the Merkel. We shall see.........