The best pellet seating tool is a brass bolt or machine screw, with two nuts on. The diameter of the threaded part of the bolt needs to be around 4mm in diameter. You need to file the end of the threaded part into a truncated cone, then put on the two nuts and knit them up against each other to set the depth needed to seat the pellet.

It's adjustable, cheap, made of the right material and does the job perfectly.