I will second what brackshooter said.. I can add 100 grains to a 2216 by adding poly rope. it doesnt seem to affect spine in bareshaft testing either. But use the poly as a buddy had a bad experience with cotton rope, he shot a bear, it rolled breaking the shaft, the rope stayed attached to the insert, (i guess some glue off the insert stuck to it) anyway the rope was hanging out the entrance hole acting as a stopper/blood wick, we about never found that bear.. I dont tink poly would do that.