Start with a piece of drill rod or O-1 steel round stock. Don't screw around hunting up exotic stainless alloys that someone thinks is a good candidate, use common materials that can be heat treated if necessary. Don't heat treat unless necessary, and if so, don't heat treat to the highest strength possible ending with poor ductility.
Discard your notion that stainless steel might be something or other. There are dozens of alloys and heat treated conditions for different purposes.
The correct way to approach this is to measure the length of the rod you have, then the minimum diameter, make an estimate of the port pressure, calculate the load applied to the end of the piston, apply a safety factor, then determine whether the rod is stable. The down stream end that contacts the carrier is the only place that needs "strength" against battering so it won't mushroom.
Make a nice rod out of common drill rod. H13 is high speed steel; I don't know that I would go that way, but it is available as round bar stock, and machining will be no issue. O-1 is available as drill rod and it will be straight with a dead nuts diameter. Then test it in your rifle. Be amazed that such a simple solution works without drama.