I have used the TOW stamps at least 100 times in different variations, and they work fine, WITH PRACTICE.
They key is not to expect or try to make the impression with one strike. For example, if you are going to use the crossed septre viewing mark stamp on the breech and tang
of your Bess hold it in position, and angled to the side so only about 25% of the stamp is in contact with the barrel. Give it a modest tap with a hammer. Neither a heavy blow or a sledge is required. Once the first strike is made, you can "feel" the stamp click back into the first impression when you position it back on the barrel. Once the stamp is back in it's original position slowly angle it more to the center so about half of the stamp is in contact. Strike again. Repeat this process until the entire process is complete across the radius of the barrel.
If you try to make the entire impression at once, you will undoubtedly make a double impression.
It is not difficult, but it does take practice, especially on a round barrel. The stamps work, but like everything else it takes time to learn how to use them properly.
I've also used them with success many times on Pedersoli Bess barrels, which are much harder than the barrels made for the TRS kits.