I was fortunate to know an excellent tool grinder who fabricated my cutters out of carbide end mill blanks. He retired but I have seen a few companies on the innerweb that advertise such services. Things to consider before having a cutter made:
First and foremost, know your barrel. Pellet rifle barrels that I am familiar with (limited knowledge) are choked. That is to say the last couple inches of the bore of the barrel are reduced. My barrels are all Lothar Walther. I don't have much knowledge past LW. Slug your barrel to around the half way point to understand your exact dimensions. Then slug the barrel 100% to understand what if any choking might be in your barrel.
Secondly, decide on a pellet design. I chose the Diablo design because of the accuracy I was enjoying as my benchmark. I figured that if the homemade pellet did not fly as good as the factory, I was missing something. Kinda lazy but modifying the tool didn't seem realistic so this was my only shot at instant gratification. Now I am looking for ways to modify the pellets design for profiles such as wad cutters and hollow points but unfortunately all I have come up with is a secondary op to make hollow points (accuracy degrades slightly in my efforts till now).
I would gladly share my drawing for my .25 caliber cavity end mill, but if I were you I would not use my dimensions. You are better off getting your dimensions from your barrel slugging efforts.
You will likely find that your pellet weights will be close to factory pellet weights. You can fine tune your pellet weights by changing the profile of the ram die used to set the cavity in the back of the pellet. The shallower the cavity, the more lead. The more lead the heavier. This is also achieved by increasing the wall thickness of the pellet skirt which is also a result of the ram die profile.
It was a super fun project, but when it comes to accuracy, there are no shortcuts. The more work you put into it, the more satisfaction you will get out of it.
One final thought. As John Newman (
@pontiac428) smartly pointed out, your handling lead. Lead is not only lethal at high velocity, it can be very bad for you just sitting on your work bench. Always practice safe handling procedures, store it properly, wear gloves and follow proper hygiene protocol.