I noticed that you said the gun you were looking at was a .54 caliber, but even if it is a .50 you might want to think about this.
These plastic stocked guns usually are pretty light weight and a stout powder load, even when used with a patched roundball can produce a pretty good recoil.
Before you do any of the following suggestion give your new gun a good workout by firing a number of shots to get a feel for it. Then consider the following:
To help reduce the "felt recoil" some folks remove the butt plate and fill the hollow cavities with loose lead shot.
Now, I can understand why someone wouldn't want to go out and buy a 25 pound bag of lead shot just to add a little weight to their plastic stocked gun but if you can find some of last seasons "dove loads" for a cheap price you might want to buy a box of them.
Adding 25 ounces of shot to the butt will add about 1 9/16 pounds of weight to the gun and that addition can make a difference.
Because the stock might hold more shot than that, and because you really don't want the shot rattling around like a kids toy, cramming some cloth in on top of it will reduce the rattling.
Note: Adding shot to the stock, which is below the centerline of the barrel adds the weight at a low location. That is going to make the muzzle rise more when the gun is fired than the gun did without the shot in place. Because of this, it is recommended that you take the gun out and fire a number of shots to see if the stock slaps your face or does any other things you dislike.
After shooting the gun, if you like what the added shot did for it you can spend another couple of bucks and buy a tube of siliconized acrylic window/door sealer at your local hardware store. Pumping in a bit of this on top of the loose shot will semi-permanently hold it in place.
I say semi-permanently because you can always scrape the rubber out if you don't like the shot or want to add more.
After you have coated the loose shot with the sealer you can use the rest to patch up those holes around your windows and doors to keep the summer bugs out. That will make your wife happy and remove one more thing from your wives "TO DO" list. :rotf: