?? 1240 tris - this is polycount measure which hasn't nothing in common with shadows (you maybe mean something about vertex shading
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If barrels will slow down computers too much, then i will simply create LOD for them. Barrels closer to camera will have more details and barrels further will have less deatail. Bump offset and normalmapping won't help, because main problem will be angularity of model, not amount of small details