Small to large firing pin bolt head

[thirty06]

OK bought new bolt head to change 112 .223 cal. to .243.
Surprise bolt has large firing pin. No problem have a #25 drill bit.
Can anybody tell me about how far to drill? How easy is this to screw up?

[Dirk]

Drill only the rear portion of the shank up to the cross pin hole. I've done this before and that's how far I went, IIRC

[BillPa]

You can open it to the vent in the bolthead. Past that point forward the firing pins are the same diameter.

Bill