While an age old idea ( old as firearms anyways ) has been to knife split the head of a bullet to create expansion. That trick works for airguns too !!
But what about slugs that resist mushrooming even tho a hollow point exists ? HELP IT by creating a fracture path !!
Using a good quality "0" size jewelers Phillips screw driver, a Light 4oz tack hammer and a bit of practice in a light Tap tap you can lightly impress an X pattern within the HP hole without disrupting the Meplat.
The end result has been nothing short of amazing when applied to NSA .177 & .22 caliber slugs. THIS DONE FOR PESTING ONLY and may likely upset super long range accuracy pending how accurately you can do so ? Quality tool with ground tip controls the uniformity / shape.
Just a suggestion, but try chucking the Phillips bit into a drill press with the table set below the bit when the drill is pulled down. Adjust slowly until you get the right depth needed for the Phillips bit to impress the slug hollowpoint, and then set the stop.
Doing it this way, you can just set the slugs onto the drill table, lower the drill head until it bottoms into the hollow point and get uniform impressions without having to do the tap-tap trial and error method.
I did this years ago when I was casting bullets and the consistency worked out great then, and should work for you on these tiny hollow points as well.