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The 5 grains per pound rule and how it prevents a dry-fire
Crossbow makers set a minimum bolt weight, commonly 5 grains per pound of draw weight, because an underweight bolt behaves like a dry-fire.
What the rule means
The guideline is that total arrow weight should be at least 5 grains for every pound of the bow's draw weight. Total weight means the finished bolt: shaft, nock, insert, vanes and the point or broadhead all counted together.
Some makers express it differently, giving a flat minimum such as 400 grains or more for a given model rather than a per-pound figure. Either way the point is the same: there is a floor below which you must not go.
Why an underweight bolt is dangerous
When you fire a crossbow, the limbs release their stored energy into the bolt. A bolt that is too light does not absorb enough of that energy, so the surplus goes back into the bow itself, which is effectively a partial dry-fire.
That surplus energy can crack limbs, damage the cams and string, and it commonly voids the warranty. A correctly weighted bolt soaks up the energy and turns it into forward speed and penetration instead.
Working it out
Take the bow's draw weight and multiply by 5 to get the minimum total grains. The finished bolt, with its point fitted, must be at or above that number. Remember the point weight is part of the total, so a heavier broadhead helps you clear the minimum.
If a bolt listing quotes only the bare shaft weight, add the insert, nock, vanes and point to judge the finished total against the minimum for your bow.
Match weight to your bow and use
Meeting the minimum keeps the bow safe, but many hunters run heavier than the floor for more momentum and penetration. Confirm the minimum for your specific model first, then decide how far above it to sit.
Micro-diameter arrows from Ravin and TenPoint are built to their own weight specs, so on those bows check the maker's finished-weight figure for the arrow rather than applying a generic number to a standard bolt.
Check the minimum weight for your bow
Open the bolt finder toolSelect your crossbow to see minimum bolt weight and matches.Find 400 grain crossbow bolts on eBayaffiliateRelated
Sources: TenPoint · Ravin · Archery Trade Association