Don, I don't think you are generating enough kinetic
energy to safely be shooting NAP Spitfires from what
you have told us. Not only do the Spitfires have a large
cutting diameter (1 1/2"

, but they also are the only
mechanical that uses blade retention clips instead of
'o' rings...a lot of energy is used opening the blades
and with the 1 1/2" cut, you probably need 55-60 lbs. of KE
to use these. If you're set on using a mechanical, I'd
recommend Rocky Mountain Snypers or Rocket Steelheads(100 gr.)
...both produce excellent penetration.
Here is a link to figure your kinetic energy:
http://www.bowjackson.com/
Take a look at some of these broadhead test results:
http://www.american-hunter.com/broad...dhead_test.htm
I was shooting a similar setup a few years ago and I also
have a 27" draw and was using Spitfires...I did not get
favorable results. This is a excellent broadhead, but
IMO needs to be shot from a setup generating some high KE.
Good Luck
TB