Powelly gauge for measuring 3-5-7 land rifling

[mdeland]

I would add that the width and length of the V groove along with the helix angle of the land on the slug, aid the contact points in registering the cylinder diameter.

[TexasMac]

Mike,

I never implied that the Powley gauge must contact the center of the of the grooves to be accurate. And I agree with you that as long as portion of two grooves (and not the edges) are in contact with the sides of the "V" the Powley gauge will give an accurate reading. The illustrations confirm that and as shown will correctly measure 3 & 7-groove slugs. There definitely is a range of groove to land width that will work, but if the grooves are appreciably narrower than the lands the grooves will not be sufficiently wide for a portion of two grooves to contact the sides of the "V".

Look at the illustrations. As the grooves are narrowed, the angle of the lines drawn between the center of two grooves does not change. The angle between the center of the 3-groove illustration is 120 degrees (360 degrees divided by 3) and is constant regardless of the groove width. Now envision the 3-groove slug with 1/2 the width as in the illustration. At best only the edges of the groove will touch the "V" and edges of grooves are typically rounded and not a good surface to make measurements. Let's take it a step further and reduce the grooves to 1/3 of the lands. I guarantee that no two grooves will contact the "V" even with the help of the helix angle.

Wayne

[Trigger1212]

Gents

Interesting discussion!

Mac, do you have all three versions of the gage? Or are your points from a theoretical stand point (and that’s fine, just wondering)?

Mike, have you ever measured a bullet that you felt the measurement was a bit suspect due to the land placement while using your gage?

Can someone provide instructions/drawing of the gage to have one made? Mike, did you make yours or was it procured by other means?

Thanks!

Wade

[Clarence]

Wade,

There is only one gauge under consideration. It is really designed for measuring slugs from a 5-groove barrel, but it can, with limitations, measure 3-groove and 7-groove slugs. The limitation is that the contact points for those slugs must be tangent to the angle of the gauge for an accurate measurement. Wayne pointed out those limitations: that is, if the 3-groove or 7-groove lands are too wide, the contact points will not be tangent and therefore, the measurement will not be accurate. The diagrams he provided graphically make his point, and also indicate the angles that are required for more flexibility in barrel configuration for 3-groove and 7-groove barrels.

Clarence

[mdeland]

Wade , I machined mine of mild steel with a Jet mill ,from a drawing by Powley. Yes ,I have had my doubts about it until I got a plug gauge set I could prove the gauge and formula with. I could make the Powley gauge measurement with the plug gauge, run the calculation and immediately verify the accuracy if it matched the plug gauge diameter. I new as long as the V in the Powley picked up some portion of the slug land cylinder, without interference from the grooves that it would be accurate.
It doesn't matter if the V picks up the top,middle or bottom side of the slug lands , it will be accurate as long as the diameter is not diminished or added to. It also doesn't matter wither or not the land contact with the two sides of the V, is in the same place on the opposing side of the V. This is where the helix comes into play , to in effect, widen the land cylinder to be read .
Say for instance the helix angle on the left side of the slug contacts the left side of the V in a slightly different location than it does on the opposite side. The two sides of the V in the gauge still is measuring the cylinder profile of the slug lands from a different opposing position. This in my mind is an advantage of the gauge over a tri-mic, that has very small contact points that cannot use the helix angle because the contact point have no appreciable length to them as does the V trough of the Powley.
I have measured quite a few 3 and 5 groove barrels and have not yet done a 7 groove.
Now can one tell if the gauge is only contacting the slug lands and is not being interfered with a wide groove bottom. Well yes , a sharp LED lamp ( I use an auto pin light) can be focus on the back end of the V directed into the V. Now the slug and gauge are viewed from the front of the trough with Optivisors which will allow any contact areas to be viewed. Just went out to double check the preceding statement and it works perfectly.

[TexasMac]

Wade,

There is only one gauge under consideration. It is really designed for measuring slugs from a 5-groove barrel, but it can, with limitations, measure 3-groove and 7-groove slugs. The limitation is that the contact points for those slugs must be tangent to the angle of the gauge for an accurate measurement. Wayne pointed out those limitations: that is, if the 3-groove or 7-groove lands are too wide, the contact points will not be tangent and therefore, the measurement will not be accurate. The diagrams he provided graphically make his point, and also indicate the angles that are required for more flexibility in barrel configuration for 3-groove and 7-groove barrels.
Clarence

Clarence,
You said it much better than I could. Thanks.

Wade,
I have a 3-flute (60 degree) V-angle micrometer I used for measuring 3-groove Trapdoor slugs. I tried using it with a 5-groove slug and it would not work and is my one experience of the potential problem I attempted to highlight. I have not used a Powley gauge so the previous illustrations are my method to confirm it will or will not work on 3 & 5-groove bore slugs. It works but has limitations as Clarence better described.

Wayne

[Trigger1212]

Gents, as stated, a good discussion!

Clarence, I understood how the gage works (now!) and the fact that there is only one Powley gage, I was referring to having ones made with different v angles to hopefully work better with the 3-7 groove bullets as pointed out by Wayne.

Since Mike had the gage and was using it wanted to see if he was comfortable using it for 3-7 groove bullets.

Mike, hate to be a pest but at the beginning of the post you said you had the dimensions/diagram to build one. Can you provide that info?

PS, what are optivisors? Magnifying glasses?

Thanks

Wade

[TexasMac]

I see I made a typo in my last post. The last couple of sentences should read "I have not used a Powley gauge so the previous illustrations are my method to confirm it will or will not work on 3 & 7-groove bore slugs. It works but has limitations as Clarence better described.

Wayne

[TexasMac]

Gents, as stated, a good discussion!
Clarence, I understood how the gage works (now!) and the fact that there is only one Powley gage, I was referring to having ones made with different v angles to hopefully work better with the 3-7 groove bullets as pointed out by Wayne.
Wade

Wade,

Making separate Powley-type gauges for 3, 5 & 7-groove bores would seem like a good opportunity for an enterprising machinist. At a reasonable price I would consider picking up a set assuming good clear instructions were provided. List them on eBay, GunBroker, etc. If I ever get a good mill I may have to make a set. Concerning making your own, as long as the V-angle is accurate & the sides of the "V" are sufficiently long to measure a .50 caliber slug I expect that it or they will work for 99.9% of the BPC rifles made. For old flintlock and caplock rifles greater than .50 cal, many of which have odd-groove bores, a taller gauge would be needed.

Wayne

[mdeland]

Here you go Wade.

[mdeland]

2

[mdeland]

3

[mdeland]

4

[mdeland]

I forgot to mention that this gauge will just as easily measure the bore of a barrel as well by turning the slug in the V to contact only the groove bottoms as long as the lands don't interfere with that reading by again changing the diameter.
Pretty useful little tool Homer Powley invented here!

[mdeland]

Hope you can read the print in the last picture as it is a little blurry but contains the information, along with the drawing, to make your own gauge. It was all I used to do it.
As long as the gauge can pick up the true diameter at the three equal distant points, it doesn't care if the rifling is 3-5 or 7 groove.