Lost Comparator insert

[Mech_eng]

I seem to have lost my .264 comparator insert for my bullet seating/chamber measurement tool. However, I do have a lathe.... Does anyone know what diameter I need to bore the hole in the comparator for a .264 bullet? Yes, I realize I will have to re-measure my chamber too. I'm being cheap plus it is a good little lathe project....

Any help would be appreciated

 

[spife7980]

.260

Doesnt really matter as its a relative measurement so anything within reason would work but I would stick with a spot between the groove (.264) and the full profile of the bore rifling (.256) for where the bullet would first be encountering the barrel material.

 

[Mech_eng]

Thanks!

 

[Mad_Charlie]

I asked Hornady awhile back, they said .257

edit: I just measured mine, it's .256

 

[Tchitcherine]

I'd like to understand this a little better myself...
Going by spife's diagram, Groove - Bore = 0.264 - 0.256 = 0.008", so
0.260" would be cutting 0.260 - 0.256 = 0.004" (half way), and
0.257" would be cutting 0.257 - 0.256 = 0.001" (1/8 way).
Are there pros & cons to use one vs. the other?
Or, just take into account that one is ogive kissing the lands, and the other is, half-jammed?

 

[spife7980]

I asked Hornady awhile back, they said .257

edit: I just measured mine, it's .256

That’s a good point. Plus they have that chamfer in theirs so take the angle into account. So if you’re going to chamfer the edge maybe slightly smaller so it has a buffer to open up. Cut the hole to the bore diameter and then cut the chamfer angle the same as your chambers lead angle for the closest fit.

I'd like to understand this a little better myself...
Going by spife's diagram, Groove - Bore = 0.264 - 0.256 = 0.008", so
0.260" would be cutting 0.260 - 0.256 = 0.004" (half way), and
0.257" would be cutting 0.257 - 0.256 = 0.001" (1/8 way).
Are there pros & cons to use one vs. the other?
Or, just take into account that one is ogive kissing the lands, and the other is, half-jammed?

If you measure too far outside then it can be hard to get a good purchase in the bullet. Slightly smaller gives you a more defined point. But honestly who could tell at that scale? Not me.