I am currently making a ''classic'' 3D shooter game, where instead of 3D models there are simply 2D sprites with masking. The problem is, when I use the z-buffer, a bullet ricochet will be partially obscured by the wall it hits. Any suggestions?

polygon offset.
or,
an easier way (depending on the situation)
is to fudge the projection matrix, and push out the far and near clip planes. This ''offsets'' depth values semi-consistently... where as poly offset does some random stuff here and there.

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I really do not recommend to play with the projection matrix. It can be correct on some particular cases but it is really hard to play with.

Polygon offset is supported since OpenGL1.1, so that should not be a problem.