Neck bushing measurements?

[philip brousseau]

I have about a dozen neck sizing bushings and all of them measure out with my dial caliper as being. 003 to. 004 smaller than the number on the bushing? What am I missing here?

 

[rwj]

Respectfully; calipers will not provide accurate measurements when used on small diameter holes. I recommend using either gauge pins or an expanding ball gauge with an OD micrometer.

https://www.mscdirect.com/product/details/86425378

 

[coldboreshot]

Ball micrometer. Flat edges on an inside radius does not mix.

 

[Edd]

I have about a dozen neck sizing bushings and all of them measure out with my dial caliper as being. 003 to. 004 smaller than the number on the bushing? What am I missing here?

If your bushing ID is .300" and your caliper jaw width is 1/32" your measurement will be .0017" small. If the caliper jaws are wider or the bushing ID is smaller, the measurement will be off by a larger amount.

 

[philip brousseau]

Thank you all. I understand what you're saying.

 

[marchboom]

Pin gauges are the most accurate.

 

[marchboom]

The best way I’ve found to confirm a bushing ID is to size a piece of brass and measure the case mouth OD. It either reflects the number stamped on the bushing or it doesn’t.
Easy Peasy

Don't forget spring back.

 

[round]

Ball micrometer. Flat edges on an inside radius does not mix.

Do they make them small enough to fit inside a bushing ?

 

[coldboreshot]

Do they make them small enough to fit inside a bushing ?

.338 Redding bushing, .500 OD, wall thickness .081 x2, 500-162=338. I've had some Reddings that have been .001-.0015 variation but not .002-.003.

 

[Webster]

If your bushing ID is .300" and your caliper jaw width is 1/32" your measurement will be .0017" small. If the caliper jaws are wider or the bushing ID is smaller, the measurement will be off by a larger amount.

Good incite. The knife edge touches at two locations. I just measure the cases and use the bushing that gives me the neck diameter I want. Try them in order large to small. Keep it simple no pin gauges needed. No special tools.