In addition to the age part mentioned above, I tend to go by a wizards ability to hex (willing or not) technology being directly proportional to the effects of Murphy's Law on said tech. The more things/parts that any given piece of tech has that can break or go wrong the more likely it is that it will do so around a wizard. Face it, fancy phones like the Iphone break and go south with just us mortals playing with them! But, a good old Ma Bell wall mounted rotary phone... those things still work today if it has been kept out of the back yard. A wizard could hex it... but as there is less in one that can break, it's tougher.
Same holds true for weaponry. There is a lot more moving parts and complexity in a P90 SMG than there is in, say, a MP-40 issued to "Zee Germans" in WWII. Even less in a good, solid bolt action rifle. Or, even simpler, a breach loading shotgun.
Don't try to over think this issue to much guys. It's magic and does not have to work to any hard logical defining rules like, say, gravity. Then again... gravity gets a bit wonky if you start getting real particular and look at it close enough.