We put test the XL blocks with 640 grain bullets.   Sure, they fit, but will the blocks overheat?   By following best-known practices for casting big bore bullets, we were able to get acceptable results.   Bullets dropped when the mold was hot were about .0015" smaller than when the mold was lukewarm.   This small amount of shrinkage is acceptable except on the bore ride section, and even there, it's not a serious problem (the customer's bore measures 0.487" - .488", so our range of 0.4866" - 0.4882" is not too shabby, even though we would prefer to have perfect 0.488" dimensions).   In conclusion, 640 grains is close to the edge for the XL blocks, but decent bullets can be produced with a little care.