After finding some infos on a plausible pure-fusion bomb, I wondered if the device was not brute-forcing the problem of fusion ignition : the explosives has to accelerate the sabot against a stationary target, while smashing two sabots against one another would halve the amount of explosives needed for the same kinetic energy in output.
Then I found a blog article on the concept of two (rail)gun impact fusion for space propulsion and decided to combine the two systems. To obtain a significant yield, I transformed the device into a fusion-fission bomb even if the fallout would be notably higher.