doublegunshop.com - home Forums General Discussion Classic & Custom Single Barreled Sporting Rifles @ doublegunshop.com A pressure problem or something else?

If the lands and grooves both span 60 degrees, almost all measurements should be between the bottom of a groove and a land. These measurements should be 0.354, not 0.349.

You would have to measure almost exactly from the extreme edges of two opposite lands to get 0.349 with a bore of 0.349.

Moving slowly around the circumference, you should almost always get 0.354

This is the point I have been trying to make, I should get .354, I am getting .349.

The way to measure a "slug" from a barrel with an uneven number of grooves, is with a special "V anvil" micrometer or with a special "V block"( special for the number of grooves) and common micrometer. You can get a pretty good approximation by measuring with a dial or digital caliper and carefully "rolling" the "slug" until it slips past the jaw of the caliper; or wrap the "slug" with a piece of thin shim stock or a thin blade of a "feeler" gauge, measure it, then subtract twice the thickness of the shim stock.
Mike

Mike, I can see where that might be necessary if there is a considerable difference in the width of lands and grooves but on my slug they are they same size, there's no trouble using a dial caliper. The 60 deg. arc is plenty to insure you can touch the lands on the slug.

Recoil Rob,
I think you should add twice the .005 to the .349". This will give you .359" It is hard to "see", but .005 taken from the radius, would be .010" from the diameter. It is the same as "turning" something on a lathe- a .005" cut reduces the diameter .010"; in other words .005" off each side. A simple test would be to insert a bullet into the neck of a fired case. Whichever diameter bullet that will easily enter, can be used, even if a little over groove diameter. As long as the neck has room to expand/release the bullet( normal cup and core-not solid)a slightly oversized bullet doesn't excessively drive the pressure up. Once the parallel part of the bullet has traveled it's own length in the barrel, it has been sized to the groove diameter. This occurs while the pressure is rising.
Mike

Mike,

Here's some better diagrams below of the slug I made of my barrel (the drawing's from the other guy that has the same problem) I have measured from across one land to the depth of one groove and arrived at .349.

Add another groove depth and we're at .354. You wouldn't want to add a third groove depth.

If the groove depth is .005 that means the bore is actually .344 and the groove diameter is .359.

The other thread shows a burst a fair bit down the bore, is the fellow sure nothing was in there.

I still think, it may not be quite right to assume a .005" groove depth. I was probably seeing it wrong, but a fellow on the thread was demonstrating how to mic a bore slug. It looked like he had a picture of a 60* anvil. I think the right one for a three groove slug would be a 120* anvil on a calibrated micrometer.

It could be that a three groove barrel with wide lands like your drawing shows, could displace quite a bit more bullet metal and push up pressures some for the same groove diameter, maybe. I don't know though, that burst has the look like something was in there.

Recoil Rob,
You are measuring groove to land, measure groove to groove.
Mike

Mike,

With 3 groove, grooves opposite lands, you cannot measure groove to groove. We measure groove to land and then added in the one groove depth to get groove diameter, or, subtract one groove depth to get bore diameter.

Rob