How do you obtain a "perfect square" on a tapered/swamped barrel?

[BrushCountryAg03]

Howdy everyone,

I'm asking this question for the purpose of being able to a perfect vertical line perpendicular to the bore for measuring & marking pins, keys, tennons, etc on the SIDE OF THE BARREL either in or out of the stock.  So far, I've been "lucky" and gotten "close enough" but after having worked on my 4 ga Blunderbuss kit, I've come to the conclusion that I must be missing something in regards to measuring down and marking locations.

If anyone has any advice on working with extreme flares/tapers and measuring, I would love to hear and would be extremely grateful.

Will

[Rolf]

Inlet the barrel, drill the holes for pins before cutting the forestock to follow the barrel swamp.

Best regards

Rolf

[BrushCountryAg03]

Unfortunately, the kit came with a precarved stock (I had no choice in the matter) and deprived me of some very useful 90 degree angles.

[Ky-Flinter]

Will,

Would it work to carefully measure the width of the side flat of the barrel and mark the center of the flat every 2 or 3 inches with an extra fine felt tip marker, then connect the dots.  That line should indicate the centerline of the bore.

Then again I may be totally misreading the problem.

-Ron

[Dave Dolliver]

Install the breech plug.  The installed breech plug should give you a reference flat that will allow you to clamp in a drill press vise and let you rub a file on a side of the barrel.  Rotate the vise and barrel 90 degrees and scribe another line on another side of the barrel.  Repeat for the other 2 sides.  You now have lines on the true top and bottom and the true middle sides of the barrel. 

If the breech plug you have should have a draft angle then shims will be needed on each side to compensate for the draft angle.

Dave Dolliver

[Dave Dolliver]

Sorry, I mis-read the question. Thought the barrel was round.

Dave Dolliver
 

[Dave Dolliver]

For an octagon barrel:  Use a machinist's square with the 45 degree head that's used to mark the center on the end of  a round piece of stock.  Place the 2 legs on the octagon flats and adjust the scale to just touch the flat between.  The end of the scale will indicate the center of the flat.

Dave Dolliver

[jerrywh]

 Clamp the barrel in a vise and draw the line straight across the ends of the jaws where ever you want the lines.