How to slug the bore of a 5-groove revolver (SP101)

[Macchina]

As state above, I have an SP101 that I'm trying to iron out some leading issues on. I can easily slug the bore, but there are 5 grooves on this gun and I'm having a difficult time measuring the diameter. I am an engineer and have PLENTY of time spend behind a caliper an micrometer but without using an indicator and fairly elaborate setup I don't get how to measure the bore diameter on this one. I can measure from land to groove on the slugged bullet (.353") but without knowing my exact groove depth, I can't calculate bore diameter.

I'm sure there is a very simple method, but I can't think of it...

 

[Walkalong]

Don't worry about "bore" diameter. You want the groove diameter (.357? .358? whatever), so just measure the outside of the slugged bullet. You want your throats to be slightly over that. Say, .001 to .0015.

 

[Macchina]

Don't worry about "bore" diameter. You want the groove diameter (.357? .358? whatever), so just measure the outside of the slugged bullet. You want your throats to be slightly over that. Say, .001 to .0015.

I can't measure either bore or groove diameter with calipers. 5 lands and 5 grooves does not allow measurement across features.

 

[cheeze]

Measure the outside diameter with the bullet sideways in the caliper to get the largest diameter, then measure from in the groove to the top of the land on the bullet. Subtract that from the first measurement and it will tell you how deep the grooves are. Multiply that by 2 and subtract that from the first measurement and you will know the groove diameter.

 

[Drail]

There is a way to do it if you can get your calipers across a land on one side and a groove on the opposite side and then subtract the groove's depth measured on a slug. Five groove bores suck though. ( I can't believe that I am telling an Engineer how to do something.) In Kuhnhausen"s Ruger DA shop manual he explains how to do it, if you can access that.

 

[Driftwood Johnson]

Howdy

I know how difficult it is to try to do this with a caliper, trying to hold the end probe just right so that you get an accurate measurement or the depth of the groove. That is what you are up against and nobody has realized that.

There is a way to do it using a V block. I will have to ask an old friend of mine how to do it, he explained it to me once.

 

[Macchina]

I've attached a picture showing why I cannot measure 5R with calipers or a micrometer. For all of you telling me to basically "try harder" . I've used the nominal dimensions for a 357 bore here. A 16.584 degree rotation gets me closest to being able to measure land to land, but it still doesn't work...

 

[Macchina]

Howdy

I know how difficult it is to try to do this with a caliper, trying to hold the end probe just right so that you get an accurate measurement or the depth of the groove. That is what you are up against and nobody has realized that.

There is a way to do it using a V block. I will have to ask an old friend of mine how to do it, he explained it to me once.

Trying to measure the groove depth that way is a crap-shoot, especially when I'm looking for the diameter +/- .0005" (.357 vs. .356 makes all the difference). Since I don't have 3 points of contact on the caliper, a tiny angle change means a significant dimension change...


Here, we have 3 possible scenarios for measuring groove depth with the caliper's plunger. Note, these are still nominal numbers so the correct measurement is .0035"
This is why you cannot measure with a plunger if you don't have 3 points of contact (measuring the depth of a blind hole) or a very complex angle-restraining setup...

 

[Remllez]

Shoot jacketed slugs....I'm not being flip or a smart ***. Shoot a few hundred jackets outta the little blaster, if you think lead will shoot better afterwards. Switch back and give it a go.

You may be running those lead slugs too fast or with too low Brinell or both.

 

[Alland]

The only accurate way that I have found is to use a mic for 5 flute end mills. Now that I am retired I no longer have access to them.

 

[Walkalong]

To me, your pic shows a bore, not a fired bullet. You will be able to get that measurement.

Or are the lands that wide?

 

[Jim Watson]

There is a procedure to measure the diameter of a 5 groove slug but it calls for an odd angle V block not found in the ordinary hardware store or even machine tool supply.

There used to be a guy who had a lot of such measuring equipment, multi-point micrometers and V blocks, and would use it to measure slugs from however many groove barrels you had. Cost was cheap to free. Unfortunately, he is deceased and nobody I know of has taken up the service.

There is a procedure with three rods of known diameter lashed up into a fixture at:
http://castboolits.gunloads.com/showthread.php?119952-how-I-measured-a-5-groove-bore

Oh, by the way, you need to make up your mind as to whether you want the bore diameter or the more useful groove diameter.

What are the diameters of your cylinder chamber throats? That can be of more importance to a revolver than the barrel diameter.

 

[Macchina]

To me, your pic shows a bore, not a fired bullet. You will be able to get that measurement.

Or are the lands that wide?

The lands are that wide, the drawings represent a perfect slug from a perfect SP101.

A cross-section of a barrel and it's slug look the exact same (though material side is obviously reversed).

 

[murf]

get ya a couple drill bits (must be the same diameter) that will touch the groove indents on the bullet when held together (forms a triangle when the bullet is placed on top of the touching drill bits). measure from the bottom of the bits to the top of the bullet.

now, turn the bullet so the raised land is contacting the drill bits. measure again.

the difference in measurements is the difference in the diameters. take half that number and add or subtract from the straight-across measurement (groove diameter less one half land height) to get your bore and groove diameter for that barrel.

i admit, i haven't done this. it just sounds like it would work.

murf

 

[Geldarr]

This could be the ticket!

Check out Dardas Cast Bullets (http://www.dardascastbullets.com/). On the main page, there is a blurb that states they will measure your 3 and 5 grove slugs to determine what your proper diameter is. They do it for FREE, all you have to do is pay to send the slug to them. Follow the directions they give.

Also, think about buying some bullets from them while you are there! They are a small, family operation, and are very nice people to deal with. Their bullets look and shoot beautifully!