Measuring the bore in a Trapdoor

[WildmanJack]

Hey guys, I just got an 1873 Springfield Trap Door in .45-70. It's not too bad outside but the bore is like a mirror. Now I slugged the barrel and realize that it has 3 groves. Now how in the heck do I measure the thing with three grooves.. I'm sure one of u know the answer to this.. Thanks..
Jack
:drinks:

[The Double D]

Try rolling the slug between the loose jaws of a dial caliper. Keep light thumb pressure on the loose jaw and watch the dial for the measurement that repeats as you roll the slug.

More scientific methods are to use sine blocks or ring guages.

[WildmanJack]

Sine Blocks or ring gauges???????? Oh man I never did Trig in Highschool!!!! I'm lucky I can use a slide rule to do multiplication!!!
Jack.
Dam, Slide Rule, man does that date me....

[twotrees]

firm, not hard plastic or shim stock and wrap that around the slug. Measure with calipers. Now measure the shim material and subtract 2X that thickness off the slug+shim reading. Plastic milk bottle plastic will work if you don't have proper plastic shim stock.

We do this all the time, in the lab, on polished surfaces, so we don't scratch them.

Good Shooting

[Jbar4Ranch]

A v-anvil micrometer.

[Larry Gibson]

firm, not hard plastic or shim stock and wrap that around the slug. Measure with calipers. Now measure the shim material and subtract 2X that thickness off the slug+shim reading. Plastic milk bottle plastic will work if you don't have proper plastic shim stock.

We do this all the time, in the lab, on polished surfaces, so we don't scratch them.

Good Shooting

This is the quickest, easiest and a most reliably accurate way to measure TD barrel slugs. Only exception is I use .003" shim stock, wrap around slug and pinch it togeter tight on one side of the slug with the fore finger and thumb of my left hand. I then mic the diameter with the flat of the micrometer jaws, not the blade part being carefull not to press the shim into a groove on the slug with the jaws (easy to do). Then simply subtract .006" (2X the thickness as twotrees mentions) from the diameter measured for the groove depth of the barrel.

Another alternative for the shim is a strip of beer can about 1/2 x 4". This material is usually around .004" (measure yours) and is easy to cut out with scissors and use.

Larry Gibson

[StrawHat]

Another alternative for the shim is a strip of beer can about 1/2 x 4". This material is usually around .004" (measure yours) and is easy to cut out with scissors and use.

Larry Gibson


Empty the can or cans first.

Depending on how many need to be emptied, you might want to wait until the next day (week) to do the measuring.

[WildmanJack]

Thanks guys, I think the shim idea is great. Never thought of that.. Here's to y'all.
Jack