From Streamlight FAQ
Q: Are there Issues Using Tactical Lights on Glock® Pistols?
A: Some Glock® .40 caliber pistols, models 22 and 23, exhibit feeding malfunctions, either nose down or nose up (stovepipe), when used with tactical lights. The problems tend to occur with individual guns, with some pistols becoming totally unreliable while other identical, even close in serial number sequence, guns have no problems. Most models 22 and 23 are reliable.
A sensitive gun may malfunction with any tactical light - the TLRs, the older M models, and even Glock®’s own brand. There is evidence that the problem sometimes develops with use, and may progress until the pistol is unreliable even with no light attached.
On the basis of testing by Streamlight, we believe the problem is magazine related. It appears that the rounds are unable to rise fast enough for proper cycling. We have observed proper feeding for the first few rounds, consistent failures at mid-magazine capacity, and a return to proper feeding of the last few cartridges in the magazine.
We have tried both stronger and weaker recoil springs, and compound-action recoil buffers, all without success. Sometimes new magazine springs, either new Glock® or Wolff, will cure the problem. In one case of a pistol which was totally reliable when new but progressed to malfunctioning on every magazine, even with no light installed, we found two solutions which restored reliability, but which might not be acceptable to some users. The first was using 10 round capacity Glock® magazines. The gun will not cycle reliably with 15 round mags with their steeply stacked columns but works flawlessly with 10 round mags. The second solution was a new magazine follower from Brownells®, their part number 069-000-006. When used in a 15 round magazine with a new spring, reliability was restored. However, the follower would not lock the slide open after the last round.
Ammunition is also a factor with any weapon. Some brands and weights may be totally reliable while others jam repeatedly. Make sure your gun is thoroughly tested with your duty ammo.
