This is a Silicon cap
There are three types:
• Without plug
• With small plug
• And with long plug
24.10.2015 Up date
Ver 1 has wider base but it can't be close tightly on the caulking gun
The new Ver 2 has 17.5 mm base and it can be close tightly on the caulking gun
Ver 1 plug where too wide ,This was corrected in Ver 1.5 and 2
This is work in process and I don't know which is the best option.
I will update this Thing .
Please let me know how ti work for you.
I have print it from ABS with support but I don’t know it it’s a must