Designing the 2-diameter paper patch bullet.

[Distant Thunder]

I going to try to explain how to figure the length the base band, groove diameter section, of a 2-diameter paper patch bullet for your specific chamber.

First off all paper patch bullets are cast undersize and wrapped with paper to bring them up the desired diameter(s). With straight sided bore diameter ppb it's pretty simple, the shank of the bullet is cast to a diameter that when wrapped will be a snug fit in the bore (lands) of the barrel. These single diameter bullets are probably best shot from a special tight paper patch chambers. These chambers are cut tighter than a grease groove chambers for the same caliber would be cut and fired cases from these tight chambers will not accept a groove diameter bullet.

Many paper patch shooters shoot these bore diameter bullet in standard grease groove chambers and they can shoot very well. I've done it and it works! So in .45 caliber they are loading a .450" patched bullet in a case that would easily accept a .459 to .460 diameter bullet. When you add the very shallow seating depth of bore diameter ppb and the undersize bullet you have a rather loose fit in the case. The necks of the cases are often sized down to match the bore bullets or some form of crimp is used.


The shallow seating does two thing, it increases the powder capacity a bunch. When I was shooting my Shiloh .45-90 for long-range matches I had to work to get 85.0 grains of 1 1/2 Swiss under my grease groove bullet. With a bore diameter ppb in my .45-70 Hepburn I am using 83.0 grain of 1 1/2 Swiss and very little compression, but those bullets are only in the case .090". I just barely squeeze the case mouth using my Lee Factory Crimp die and the bullets are held in place enough to handle them for chambering and such.

The second thing that the shallow seating does, and I believe it's the most important, is that it gives the bullet absolutely the best possible alignment in the barrel that it can have. The result of this is incredible accuracy when everything else is right.

Now when these bore diameter ppb are fired in a grease groove chamber whatever portion of the bullet is inside the case and up in the throat/freebore bumps up to fill whatever the inside diameter would normally be, .459 or bigger. That bumped up portion then has to be squeezed back down to groove diameter and have the rifling engraved. There's a lot of lead moving back and forth in this process! That is not consider the best for accuracy in any bullet, certainly not with ppb.

The thought behind the 2-diameter design is rather than have the base of the bullet start out at .450" then be bumped up to .459"+ just make that portion of the bullet that is loaded in the case and sitting up in the throat/freebore of the chamber to patch to groove diameter and save it from all that bumping up in the first place. The added benefit is that the cases don't need to be size down for a .450" bullet.

Ideally your chamber is such that the inside diameter of your fired brass is .459-.460" and the throat/freebore will accept the same diameter. If your freebore is .459" and your brass is .460" or bigger just go with the .459" and a very light crimp will hold the bullets in place if handled with a bit of care.

Now to figure how long the base band (groove diameter) should be you just need to add two dimensions together. This length is labeled "B" in the attached drawing. "A" is the bore diameter section of the shank. "C" is the nose. "OAL" is the overall length of the bullet.

   

"B" will be the depth you want the bullet to seat in the case, shallow is good, so somewhere around .090-.150". My accuracy improved with shallower seating. This is with straight sided bore diameter ppb and the seating depth can be adjusted very easy with them. With the 2-diameter ppb the groove diameter part is going to hit up against the lead/rifling and there really isn't any room for adjustment. It's much like a grease groove bullet, you can push for all you're worth but they only go in so far.

The other length that will make up "B" is the length of the throat/freebore. For this you can measure how much of a groove diameter grease groove bullet sticks out of the brass before it contacts the rifling. In chambers without freebore this will likely be .100" to about .150", but could be a bit longer.

So you take those two, .100" in the case and .125" sticking out of the case, and add them together for .225". In my .40-65 I have .080" in the case, the freebore in .400" long (Yea, I know!) so the groove diameter base is .500". That's .080 + .400 + .020 that the breech block cams the bullet into the rifling when I raise it, which all equals .500".

Now in Matt's rifle, that's the bullet that started this thread, I figured .100" in the case + ,150" sticking out of the brass for a .250" base band length. By figuring .100" the case I have a little to play with + or -. If the bullet sits out a little more we have .080" in the case, if it seats a little deeper we have .120" in the case, either will work, but the shallower the better.

The only other thing I'll add for now is that the transition from groove diameter to bore diameter can be either just a simple step as in my .40-65 or an angle as in Beltfed's .40-65 design. I used an angle in Matt's mold. Both work and I see no difference.

I hope this helps. If you have any question about any of this please ask. We'll do our best to help. Or if you have anything to add or suggest I'd like hear that also.