Powelly gauge for measuring 3-5-7 land rifling

[mdeland]

Here is the simple Powelly gauge used with a mic or caliper to exactly measure and calculate barrel groove diameter from a bore slug.

[Trigger1212]

Mike

Ok i’ll bite, how do you use that thing?

Saw the word Powley and the first thing I thought of was the ballistic slide ruleoad calculator. Did he come up with your gage too or someone else with same last name?

I’ve heard of wrapping a bullet with off number of grooves with shim stock then measuring the dia then subtracting the thickness of the shim to get groove dia. Have never tried it but sounds like it should be simple to do. Have you ever tried that method? Results?

Wade

[mdeland]

It's quite easy really although looks rather technical. You of course drive a slug or cast one with cerosafe then put it in the gauge. With one of the slug lands top center you mic the thickness of the gauge and the slug together which give you value "A". You then plug your measurement into the formula stamped on top. It will give you a reading to ten thousands it is so accurate. I've checked it with one of my ground plug gauges and it's dead nuts accurate. Same guy that makes the slide rule powder gauge.
Shim stock would not be as accurate but would give you a pretty good idea as long as it was snug against bullet diameter.

[mdeland]

This gauge works on precisely the same principle as an expensive tri-Mic and is just as accurate. It is in fact a poor mans tri-mic.

[Trigger1212]

Mike

I was scratching my head after reading your last post, how the heck was that supposed to work? Looking at your picture of the gage it just didn't make sense and I am a mechanical guy.

Finally figured it out! I was looking at your post using my phone and it looked like where the bullet was setting in the gage was just an open milled spot between the two "legs" of the gage. I googled powelly gage and came across another reference to the gage on the forum and in THAT one you described the V notch in the gage. I then looked at your picture on my lap top and then could see the v notch, makes sense. I could have used that when I had my trapdoor!

1. Can you explain how to work the formula with some actual numbers? Show an example of the calculation. I see it stamped on the gage but do not know what the A, B or C dimensions are supposed to be.

2. Where can a fella get one of these gages? Anyone making them?

Thanks, always enjoy your interesting posts!

Wade
Semper Fi!

[mdeland]

I'll dig the Powelly gauge out in the next few days and show you how to run the numbers with a plug gauge that we know to four places what the size is to verify that the calculation will work as described with odd land rifling measurements.
I made mine here in the shop and have a schematic around here somewhere that shows how to machine one.

[Kurt]

In my Lyman #6 manual there is a article using a 36 degree V block drawing that explains how to use the Powelly way measuring odd number grooves and also the bore.
I will see if I can dig it out.

[mdeland]

The .419 plug gauge is put in the Powelly and the width of the block plus the gauge is recorded and then subtracted from value C which is .2885. This will give you value B.Value B is multiplied by.8944 which will yield your bullet diameter.

[mdeland]

The bullet must have some part of one of the lands on the bullet at top center for the mic reading to feed into the formula. If your mic readings are off so will be your finished calculated diameter. My calculated diameter came out to .4187. I was off by .0003. The plug gauge is ground to .419. The Powelly gauge is accurate!

[TexasMac]

This is a timely thread as I've been working on an article on the various methods to measure the groove diameter of odd groove bores.

Although the V-block as pictured & described in this thread is designed to ideally work with 5-groove slugs (included angle of 108 degrees between the sides of the “V”), it will also work on 3 & 7 groove slugs as noted, but only as long as the lands and grooves are equal width and, as Mike noted, one of the grooves must be centered at the top. It will not work on 3 & 7-groove slugs if the grooves are narrower than the lands.

For 3 or 7-groove slugs with different width lands & grooves a similar V-block can be made with included angles between the sides of the "V" as follows:
3-groove: 60 degrees
7-groove: 128 degrees, 34 minutes, 17 seconds
BTW, the above are the same included angles found in the jaws of expensive to very expensive V-angle micrometers used for the same purpose.

Wayne

[mdeland]

Well, actually Wayne , I see where your coming from in that the degree of angles change with the land count for an equal distant tri- mic measurement (mid land) but as long as the V in the gauge angle can pic up some portion of the slug land (without interference from the groove width bottom), this would usually mean rather deep, flat bottom grooving, it will measure 3 and 7 land accurately as well. The reason is because it is registering (reading) a cylinder, just as it does the plug gauge.
The pitch (helix angle)in the rifling on the slug in effect adds to the land width that is being measured as well.

[TexasMac]

Well, actually Wayne , I see where your coming from in that the degree of angles change with the land count for an equal distant tri- mic measurement (mid land) but as long as the V in the gauge angle can pic up some portion of the slug land (without interference from the groove width bottom), this would usually mean rather deep, flat bottom grooving, it will measure 3 and 7 land accurately as well. The reason is because it is registering (reading) a cylinder, just as it does the plug gauge.
The pitch (helix angle)in the rifling on the slug in effect adds to the land width that is being measured as well.

Mike,

As I noted the Powley (not Powelly BTW) gauge DOES WORK with 3 & 7-groove slugs but only under the condition that the lands and grooves are of equal width. In that case the sides of the "V" does catch a small portion of the grooves as long as one groove is centered at the top. But I stand by my comment that it will not work on 3 & 7-groove slugs if the grooves are narrower than the lands. I agree with you that the pitch of the rifling does help a bit but not much.

Wayne

[mdeland]

Wayne , I'm still not getting why the lands and grooves have to be of equal width for the gauge to work on 3-7 groove rifling.
It seems to me that if the V is picking up any portion of the opposing slug land and is not contacting any portion of land bottom, that it would still be measuring a cylinder, No? Oh, by the way thanks for correcting my spelling of the name. Don't know where I picked that up.

[TexasMac]

check out the illustrations below to get an ideal of what would happen if the bore grooves were narrower than the lands. It would not affect the measurement of the 5-groove slug but could result in being unable to measure the 3-groove and 7-groove slugs. Picture the 3-groove & 7-groove slugs with grooves 1/2 the width of the lands. In that case two grooves of the slug would not contact the gauge correctly. This is why V-anvil micrometers are made with the angles as noted earlier. I.e., with the proper "V" angle it does not matter if the grooves are narrower than the lands.

Wayne

[mdeland]

I have always understood the illustration you have shown and how the V would contact the various land counts in different positions. I think you are saying that the V must always contact the center of the lands to accurately measure a cylinder. I think all it has to do is contact some portion of the land surface from edge to edge to accurately measure the cylinder (as long as the cylinder diameter is not diminished or added to) buy the contact point wither it be groove bottom orbit or over center of land edge corner . Both 3 and 7 groove lands make contact with the V away from the edge corner.
I just measured a Trap Door Springfield 3 groove this week end and it came out with .461 groove diameter that I'm pretty sure is an accurate read.
Good discussion, thanks for the chance to bounce this stuff around with different ideas and points of view to test our thoughts on the matter.