Went to the range yesterday - shot my XD40 for the first time since modding it a little.
I replaced the stock stainless guide rod with a tungsten one and replaced the stock 16lb spring with a 22lb. spring. The added weight and stiffer spring helps with muzzle flip.
Guide rod from Canyon Creek Custom, http://www.canyoncreekcustom.com/Home_Page.html
Purchased from Pistol Gear, http://www.pistolgear.com/index.php
I put about 100 rounds through and was consistently pulling to the left. No fault of the mods, probably just my grip and/or too much trigger finger. I found a cool video on grips through the XDtalk forum:
http://www.springerprecision.com/video.asp
Funny thing - there was one other older guy on the same range (there's 3 at this place) shooting a .45. I was about 1/2 way through my rounds and reloading a mag, and he came over to ask what I was shooting. I told him a .40 cal Springfield. He said "oh, I thought it might be a communist gun the way everything is pulling to the left." Man I walked right in to that one