What size mandrel when making a Hornady Modified case

[ronemus]

I use a gage pin 0.002" over bore diameter to expand the neck of the case - it's small enough that the case won't bind in the chamber and large enough that the bullet moves freely.

 

[338 dude]

Guys, Alex's method is ok with a bolt action. I'm assuming the OP's is an AR platform with the Grendel. Then what?

Another way to find touch is to color the bullet with a sharpie load Long chamber the round and there will be marks indicating it touched the rifling pull out the round seat the bullet .001 deeper re-coat with a sharpie re-chamber and look for marks repeat the process until you do not see the rifling marks on the bullet that +.001 will be touch this is also accurate and more repeatable using your fired brass in your chamber then a modified case from Hornady asked me how I know