dmc235,
I can only give empirical data, as I have not, nor will not try this particular substitution. Let's look at this from another angle for a moment. You are proposing to put a 10mm cartridge in a 9mm magazine. Does this sound like a good idea to you? Also, you are talking about buying a new barrel, custom made and then altering it with an o-ring to tighten the fit. That will cost some major money. Why not trade the full size in on a tactical and just add the extra money to the purchase?
Regarding the extractor. I feel confident that the larger .40 case (.380 - 9mm vs .423 - .40) will cause problems. The base of the forty is both larger and thicker than the nine is. If it works at all, it is sure to place undue stress on the extractor. You could probably get a .40 extractor that would fit your 9mm, if you can get HK to send you one (iffy proposition at best). And will the firing pin hit the primer 'on-center'? Probably not, but 'close enough' to cause the gun to go bang. But you are going to an awful lot of trouble to modify a firearm that doesn't need to be altered, as what you are trying to create, basically exists in a slightly large caliber. I would personally recommend against this. And if you do, the first time you fire it should be with a really long rope tied to the trigger, in case it blows up. This is not criticism, as I personally have modified a few pistols in my day for personal reasons and usage. In each instance, it turned out to be a lot of work that generally didn't accomplish the goal I was trying to reach. There's not enough time for us to repeat all the same mistakes. Trying to help. Jim
