If anyone has a new HK416 curved trigger guard with the correct size front hole .12" I would be willing to trade a brand new HK416 Gen3 PM 30rd Mag for it. Just send me a PM with a close up picture of the sides of the trigger guard to make sure the hole is the correct size.
Go find an appropriately slim piece of aluminum or better yet a non conductive metal that is malleable and slip it into the trigger guard holes and use it to reduce the hole size.
I would try and thread it and find a small fastener that can screw into it.
So I had some time to work on my lower and installed my trigger guard today. I ran into a similar issue where the hole near the mag-well was slightly smaller than the one made for the pin. There was no way I was going to get the normal trigger guard roll pin through the small side of the trigger guard but the receiver hole was fine. I ended up just running a drill bit through the tg hole it to barely open it up. I cut the front roll pin the width of the trigger guard itself so I could remove it by hammering it all the way inside if for some reason I need to take it off.
This might just do the trick.