I recently purchased a 16 lb Storm X Factor.  After practicing with it, I told my ball driller it felt light.  He weighed it and sure enough it was 15 lb.  He ordered a 16 lb and the box said 16 lb.  He ordered a new one and has drilled it, but I haven't picked it up yet.  

He is going to sell the 15 lb ball for $70.  I was considering buying it and experimenting with a drop in weight since all of my other equipment is 16 lb.  

Any opinions?

I had a similar situation. Bought a ball from a friend who threw 15's. I assumed it was 15 also.

 After plugging and drilling, it felt light. I asked mt friend and he said that it was a 14lb he had experimented with and thought I knew.

 I decided to keep it, just to try out the ball and if I liked it< I would buy a 15. I liked it so much I decided to start using all 14's because my wrist and fingers didn't hurt after using it.

 3 years later, I am still in 14's and not sorry at all, but it took a while to re-build my arsenal in a new weight.

 If the 16's aren't causing you a problem, I might keep it to throw when the lanes are a little dry because you can throw the lighter ball harder to help with drier conditions. And besides, you can always re-sell it later yourself if you decide you don't like it. You might not get $70 like the shop can, but you will have experience with a lighter weight ball to see if it helps.
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Just call me Darreyl, it's much easier to say!