Go to the NAP website at www.newarchery.com and enter the faq section. The answer to your question lies in the kinetic energy produced by your arrow at a particular range. NAP recomends 50 foot pounds as a minimum for opening the spitfire and this is a function of arrow speed and total arrow weight so you are probably allright with the 2117s and a 125 gr head as long as your speed is not compromised by something. Check out the sight and do the math for yourself.