From my own personal experience some stocks just do not fit the contour of one's face, causing what you call cheek slap. Over the years, I've had this same issue occur with two different stocks. I was never able to solve this problem even thought I could line up the sights as the two stocks I had just slapped the craped out of me causing me to eventually lift my head off the stock. There are usually two things that result with cheek slap, missed targets and pain in the facial area.
My suggestion, either purchase a different stock or a different gun as you will most likely save yourself a lot of frustration, time and money, and pain to your face. The shooting sports should be fun and when a stock does not fit properly it will not take long before you feel beat up after a few rounds, and as you have experienced that does not relate to having too much fun.
In my opinion, an adjustable recoil pad will not help your situation at all.
Good luck to you,
Unfortunately, as others have said, your shotgun doesn't fit. The best option is to have one made to your dimensions. If this isn't feasible or cost effective right now, a gel cheekpad may do the trick.
Neil Winston has proven, clearly without a doubt, that the gun barrel moves upward with recoil when fired - so we'll keep that without question. Further, he concluded that the butt stock end "pitch" doesn't make a damn as he changed one significantly and it had no major affect on cheek slap. These have driven me crazy ever since he noted them as my cheek slap is gone and it was brutal.
Now - if you're a "heads-up" kind of shooter it's not uncommon that you're getting "cheek slap" if you have a good portion of the butt stock recoil pad hanging up above your shoulder and the recoil pad is "matched" to the outline or profile of your butt stock (that is it's not something like a Morgan or KICK-EEZ pad which can be mounted considerably below the physical outline or profile of your butt stock end to make better shoulder contact). In this condition the gun recoil kicks upward quickly but the recoil does NOT move your body and head back, thru your shoulder, in "exact unison". Rather, the recoil kicks up into your cheek first (maybe in microseconds) before any actual backward body shoulder and head movement to the rear. With your head connected to your body the recoil smacks that sensitive part of the head (cheek) contacting the gun. Your body eventually moves backward but NOT in exact unison with the recoil.
Many shooters claim that a KICK-EEZ pad solves their cheek slap problem. Mounting the KICK-EEZ pad - further below the butt stock profile- down where it makes much more direct contact with the shoulder - results in the recoil now bringing the shoulder and head (cheek) immediately rearward with the recoil of the gun, eliminating cheek slap.
I believe one's shoulder, head and cheek must react in direct unison with the gun recoil in order to prevent cheek slap. Although I changed both my gun's "pitch" and added a KICK-EEZ pad, in hindsight I believe it was the KICK-EEZ pad and the fact it was mounted lower, and much more into my shoulder, that really stopped my cheek agony. Recoil then brought all of us (the gun and me) back in unison when firing the gun.