The slab works as a full block rather than a half block, causing the blast to be (almost) 0. Can be demonstrated by blocking yourself off with slabs.

In 1.7(.10), blocking yourself with slab would make you get blown away (https://youtu.be/Ayj9F3_oZnM)

In 1.10(.2), blocking yourself with slab would make you only move about 0.5 blocks (https://youtu.be/oNwpruChhr4)

An evidence for this bug is when Mojang nerfed TNT in 1.8.0 and 1.8.1 compared to 1.7.10, Last Username made a bug report regarding this issue MC-66089 (The provided cannons had a ladder, fence, and slabs for mounting blocks, yet only the cannons with slabs and fences were affected). When Mojang "resolved the bug" in 1.8.2, only cannons with the fence as a mounting block were fixed (reverted back to 1.7.10 range). Cannons with slabs on the other hand were "un-nerfed", but were still weaker (1.8.2) compared to 1.7.10. TNT on 1.8.2 was supposed to act like 1.7.10 as TNT on slabs were supposed to act similarly between 1.7.10 and 1.8.2, but sadly it did not. An observation I have found is that TNT cannons using slabs fire shorter distances because TNT launched from slabs acts similarly to TNT launched from a full block.

Additional information:

Slab spread cannons work on 1.8.0 and 1.8.1 but are just weaker than 1.7.10. Whiskeys on the other hand are broken as described above. To reproduce the issue:

1. Create a standard slab spread cannon with 7x3 cannonball segment and 7x2 charge segment. In 1.8 and above, make sure that water an infinite source.

http://i.imgur.com/RtHGuJc.png

http://i.imgur.com/FPVbTkf.png

2. Fire cannon in 1.7.10 and 1.8.2.

3. Compare distances.

To confirm issue:

1. Create a standard whiskey cannon in 1.7.10 and 1.8.2 with slabs as the mounting block. http://i.imgur.com/gzxir0K.png

2. Fire cannon and compare distances.

To confirm that slab spread cannons work on 1.8.0/1.8.1:

1. Create a standard slab spread cannon with 7x3 cannonball segment and 7x2 charge segment. In 1.8 and above, make sure that water an infinite source.

http://i.imgur.com/RtHGuJc.png

http://i.imgur.com/FPVbTkf.png

2. Fire cannon in 1.7.10 and 1.8.0/1.8.1.

3. Compare distances.

This issue has nothing to do with "Explosion impulse on non-living entities is not attenuated by power, distance, or obstructions": MC-100260

Last Username said:
The problem with slabs is not caused by that patch as provided by him (https://github.com/OvercastNetwork/SportBukkit/blob/9de4c6e27928a02f534fae75b585c0b4c9d08824/CraftBukkit/0142-Fix-explosion-distance-and-occlusion-being-ignored-f.patch)(MC-100260). Removing the patch happens to make TNT go farther, but it also completely breaks the occlusion mechanic that was working up until 1.8.

UPDATE 10/19/2016
It is now possible to test and confirm this bug in future development snapshots after 16w41a now that MC-100260, another bug that this bug is dependent on, is now resolved. You may now test this bug from versions 1.8.2 to 1.8.9 and future 1.11+ versions/snapshots. You may use 1.7.10 as a reference for it is the newest versions without MC-107982 present.

https://github.com/OvercastNetwork/Issues/issues/2504
https://github.com/OvercastNetwork/SportBukkit/issues/206