Bore diameter

Kevin Stenberg

Well-Known Member
How do i measure the bore diameter when i have slugged a barrel. Usually the gruves are too narrow and shallow to get a measuring device in them?


Staff member
Yep, measure the outside of the slug. The grooves left in the slug give the diameter at top of lands.


California's Central Coast Amid The Insanity
I think Kevin is asking how to measure the barrel's "bore", not its "grooves".

Though not as accurate as measuring the "grooves" with a micrometer, I measure the "lands" with narrow end of a caliper.

Dusty Bannister

Active Member
quote: How do i measure the bore diameter when i have slugged a barrel. Usually the gruves are too narrow and shallow to get a measuring device in them?

The reason I would be concerned about the bore diameter is if I am using a two diameter bullet with the nose being a bore riding nose. Chambering a dummy round, will show as scuffs on the nose if the nose is too large. If I felt that it was necessary to know the bore diameter, I would probably set the base of the bullet with an undersized nose on a flat metal surface and tap the nose with a heavy hammer. That will slightly expand the nose. When it begins to drag in the bore, mic that area of the nose and you will be pretty close to correct.
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Well-Known Member
The only part of the bore that matters is the part in front of the throat, unless youre measuring cleaning rod wear at the muzzle. Check the throat and fit of a bore-rider by seating a bullet long and attempting to chamber it.


Well-Known Member
Groove diam is what you care about for bullet size. Bore diam is not important for most
Ian and fiver got it. Measure bore diam, if you care, with gauge pins. My only reason for doing that
is to look for thread choke in a revolver.


Kevin Stenberg

Well-Known Member
Dusty B was right on. I was interested for bore rider bullets and for pre-sizing for PP.
I have a 35 Rem and a 450 Bushmaster (I just ordered for a Savage rebarrel) . And would like to PP for both.
5er you gave me an idea. It wouldn't be as accurate as a precise made pin. But its free. I have short sections of steel rod. If i wrap the rod with duck tape to a dia. that just barely falls through the barrel. That should be very close to the bore dia,.


Well-Known Member
Wraping with duct tape will be pretty coarse. One layer is probably about .008 or so, so won't
tell you much.

Anyone serious about casting and shooting and accuracy, especially revolvers needs a set of pin gauges.

Costs less than one set of RCBS mold blocks.



New Member
Pin gages in .001" increments are too coarse to be accurate. You really need them in half thousandth increments. Z minus (-0002") are the ones to get. You don't need a whole set, you need a range typical for your caliber. Most 45 caliber barrels are somewhere between .4415" and .4435"

I use Meyer Gauge Co. for singles. USA made, great quality, good people to order from, priced cheaper than Enco or MSC.


Well-Known Member
Hmmm. OK, you may have purposes tighter than 0.,001", but I do not, in general,
so the minus set is fine for my purposes.



Redlands, Kalifornistan
I think I can muddle through with the .001" steps for my uses. Mine are Z minus 2 tenths throughout.


Well-Known Member
Kevin, I'm a big proponent of "the right tool for the job" but, if the bank acct. won't tolerate a C-note for pin gages right now and you don't have a lathe, I think I'd give a go at finding a piece of all-thread that was as close to, but plus on bore size. Chuck it in a drill and see if I could file and sand it down to the size you need.

By the way, if you can cite the bore dia. over the grooves, I have about 40 to 50 individual gage pins that are not part of sets, and are in no way high dollar. As I recall, most, if not all, are 7/16" (4375.) or smaller, but I'd be happy to do some prowling around the shop and see if I can find something that might be of help to you.

EDIT: Reason I said "bore dia. over the grooves" which you can measure from your slug, with that, dia. based on lands should end up in the neighborhood of .006 to .014 under that. For 35 Remington would we be talking around .342?
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