The attached image shows the range of some common TNT cannons compared in the latest release and the latest snapshot. The cannon in which the TNT projectiles rest on a ladder has not changed. The other two cannons have the TNT resting on a slab and a fence, and their range has been reduced dramatically in the snapshot. These cannons each used three charges (TNT used to launch the projectiles) and the water in them was not moving, so no TNT was pushed by it in the snapshot.
It seems that the way blocks obstruct explosion energy has changed in such a way that slabs and fences obstruct much more of the blast than they are supposed to.
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Just confirmed that this started happening in 14w31a, was still fine in 14w30c

Confirmed in 14w34b

This is the cannon I'm using to test this. Since people's first response is usually "this has something to do with the water flowing pushing TNT changes doesn't it?" NO, IT DOESN'T. TNT EXPLOSION PHYSICS HAVE COMPLETELY CHANGED TOO. This cannon is setup to have water source blocks, and is simple to build:
http://i.imgur.com/VUO45Jt.png
Loaded: http://i.imgur.com/28R0smo.png
You can try this in 1.7.10, or in 14w30c and earlier and it shoots like normal, like it always has. This starts happening in 14w31a.

Almost definitely probably related to MC-53215 getting fixed in 14w31a.
That would be awesome, thanks Searge !
Tested in 1.8.2 pre1 but the bug is still exist. Strange...

Can anyone else confirm that this has not been Fixed?
This is Fixed for me. Explosions physics are back to how they were in 1.7.x.
not fixed at all . 99.99% of all tnt cannons broken. they work in 1.7 they break in 1.8.... the system before made seriously cool and complex cannon possible this pretty much throws cannon tech back into the middle age 1.12 by now btw
Confirmed. UTTER OUTRAGE. OUTRAGE!