This is the Biomial Distribution of 20 bolter shots on canrifexes does. This is just basic stuff and doesn't include twin linking, Misfortune , rending or anything else. If your whole army has to focus on shooting down those guys. They are going to fall.

Binomial Distribution
20 Bolters
0 47.010%
1 36.162%
2 13.213%
3 3.049%
4 0.498%
5 0.061%
6 0.006%

now 15 Str 6 shots.
0 47.010% 9.483%
1 36.162% 23.708%
2 13.213% 28.153%
3 3.049% 21.115%
4 0.498% 11.217%
5 0.061% 4.487%
6 0.006% 1.402%
7 0.000% 0.351%
8 0.000% 0.071%
9 0.000% 0.012%
10 0.000% 0.002%