Making Flat Base Bullets by Art Coover of Modesto, CA was originally published in Fouling Shot #162 - Mar/Apr 2003.
In the quest for a more accurate bullet, one of the most dramatic methods of improving accuracy is to make flat, uniform bases. The best and most cost effective route is the spin jig cutter design. The spin jig can be made with easy to find parts and a few hours by someone with a moderate amount of mechanical ability.
Using the spin jig cutter, one can get perpendicular bases, consistent bullet lengths and weights and gas check seatings that are perfectly flat. In fact, in .30 caliber applications, bullet weights tested were within tenths of a grain of each other.
Currently, there are several common methods of getting flat-based bullets. One can nose pour them. This method does make flat-based bullets. Nose pour molds are expensive and sometimes take six months to get. One can bump or swage bullets in a die. This is a good method except that one can never take material off a bullet to make it lighter. One can hand tighten bullets into a drill chuck and run bullet bases against a file on the drill table. One can even turn the bullet bases flat on a lathe. These methods are good, but time consuming and somewhat expensive. There are commercially available tools, but they are prohibitively expensive. One commercial device consists of a yoke that pivots over a sanding surface. The bullet is placed in the yoke. The bullet base is surfaced using a sawing motion. This method is good but the precision of the spin jig is more reliable.
The spin jig works by placing a lead bullet into a collet that has a stop that contacts the nose. The bullet is spun one direction while a case-trimming cutter is pressed against the bullet base. The cutter is spun in the opposite direction. The trimming is done until the cutter can no longer trim lead from the base. The method is simple and one can trim bullets at assembly line speed.
If the trimming process were limited to just shaving lead as above, it would improve accuracy. Now picture the cutter blade and the base of the bullet slightly offset on the horizontal plane. Now the cutter makes arc shaped cuts until less and less lead is removed. After cutting bases, assembly can be readjusted to make spire pointed bullets into flat nosed bullets.
A person can easily make the spin jig cutter system with moderate mechanical ability, using common hand tools and a modestly equipped shop. The following can be purchased at most tool shops:
Spin Jig: A spin jig is an indexing tool used on the mill table.
5C collet: The spin jig should be fitted with a 5C col-let that fits the bullet one intends to trim. For example, a 5\16 universal collet is .3125” in diameter. This is perfect for .308 caliber. The 5C collet fits inside the spin jig to hold the bullet. Be sure to select a 5C collet that fits your spin jig.
5C universal collet stop: A universal collet stop has a threaded rod that adjusts how deep a bullet can be put in the collet.
Forster-Applet case trimmer: The reader should own two of these: One for its intended purpose: case trimming. For this, the Forster Applet trimmer is one of the best. Get the second trimmer for bullet trimming. For bullet trimming, the Forster Applet case trimmer is cut in half. Two anchoring holes are drilled in the base of the half with the crank. These holes are for screwing it into the wooden base.
Steel strapping: This can be some scrap strapping 1/2 to 1 inch wide that is straight. The strapping is a guide strip.
C clamps: The spin jig trimmer requires two of them. Do not attempt to permanently affix the spin jig to the block. That is what the C clamps are for. They allow the spin jig to be pulled away from the assembly to change the collet.4x4x8’’.
Block of wood: The block of wood has a simple job: Prevent the knuckles from banging on the worktable.
Shims: These are for making the axis height of the trimmer and cutter even.
Mounting block: This can be a scrap piece of plywood or Particleboard one can mount the case trimmer on. It should be a 1/2’’ or 1’’ thick plywood approximately 4.25”x 10’’. Both the case trimmer and the spin jig are mounted on it.
In the quest for a more accurate bullet, one of the most dramatic methods of improving accuracy is to make flat, uniform bases. The best and most cost effective route is the spin jig cutter design. The spin jig can be made with easy to find parts and a few hours by someone with a moderate amount of mechanical ability.
Using the spin jig cutter, one can get perpendicular bases, consistent bullet lengths and weights and gas check seatings that are perfectly flat. In fact, in .30 caliber applications, bullet weights tested were within tenths of a grain of each other.
Currently, there are several common methods of getting flat-based bullets. One can nose pour them. This method does make flat-based bullets. Nose pour molds are expensive and sometimes take six months to get. One can bump or swage bullets in a die. This is a good method except that one can never take material off a bullet to make it lighter. One can hand tighten bullets into a drill chuck and run bullet bases against a file on the drill table. One can even turn the bullet bases flat on a lathe. These methods are good, but time consuming and somewhat expensive. There are commercially available tools, but they are prohibitively expensive. One commercial device consists of a yoke that pivots over a sanding surface. The bullet is placed in the yoke. The bullet base is surfaced using a sawing motion. This method is good but the precision of the spin jig is more reliable.
The spin jig works by placing a lead bullet into a collet that has a stop that contacts the nose. The bullet is spun one direction while a case-trimming cutter is pressed against the bullet base. The cutter is spun in the opposite direction. The trimming is done until the cutter can no longer trim lead from the base. The method is simple and one can trim bullets at assembly line speed.
If the trimming process were limited to just shaving lead as above, it would improve accuracy. Now picture the cutter blade and the base of the bullet slightly offset on the horizontal plane. Now the cutter makes arc shaped cuts until less and less lead is removed. After cutting bases, assembly can be readjusted to make spire pointed bullets into flat nosed bullets.
A person can easily make the spin jig cutter system with moderate mechanical ability, using common hand tools and a modestly equipped shop. The following can be purchased at most tool shops:
Spin Jig: A spin jig is an indexing tool used on the mill table.
5C collet: The spin jig should be fitted with a 5C col-let that fits the bullet one intends to trim. For example, a 5\16 universal collet is .3125” in diameter. This is perfect for .308 caliber. The 5C collet fits inside the spin jig to hold the bullet. Be sure to select a 5C collet that fits your spin jig.
5C universal collet stop: A universal collet stop has a threaded rod that adjusts how deep a bullet can be put in the collet.
Forster-Applet case trimmer: The reader should own two of these: One for its intended purpose: case trimming. For this, the Forster Applet trimmer is one of the best. Get the second trimmer for bullet trimming. For bullet trimming, the Forster Applet case trimmer is cut in half. Two anchoring holes are drilled in the base of the half with the crank. These holes are for screwing it into the wooden base.
Steel strapping: This can be some scrap strapping 1/2 to 1 inch wide that is straight. The strapping is a guide strip.
C clamps: The spin jig trimmer requires two of them. Do not attempt to permanently affix the spin jig to the block. That is what the C clamps are for. They allow the spin jig to be pulled away from the assembly to change the collet.4x4x8’’.
Block of wood: The block of wood has a simple job: Prevent the knuckles from banging on the worktable.
Shims: These are for making the axis height of the trimmer and cutter even.
Mounting block: This can be a scrap piece of plywood or Particleboard one can mount the case trimmer on. It should be a 1/2’’ or 1’’ thick plywood approximately 4.25”x 10’’. Both the case trimmer and the spin jig are mounted on it.
(or nose flattening) spin jig.
First, cut the Forester Applet case trimmer in half. Drill two holes in the base near your cut to screw the trimmer into the plywood base.
Mount the crank end on the plywood base so that the crank hangs over the edge and the trimmer is centered in the width of the mounting board.
The shims are for raising the height of the trimmer, if necessary, to the axis height of the cutter. The spin jig collet will eventually face the Forester Applet cutter. The steel strapping is mounted perpendicular to the axis of the trimmer. Check and recheck to insure the strapping is straight and perpendicular. The front edge of the spin jig indexes on the strapping and the spin jig is clamped to the plywood. Do not be tempted to permanently mount the spin jig, the C clamp allows the collet to quickly change calibers and gives easier access to adjusting the 5C Universal collet stop.
Align the spin jig and the trimmer blade so they line up perfectly when viewed from the side. Viewed from the top, they should be offset no more than the radius of the bullet base. For a .308 bullet, they should offset about .140”. It would be beneficial to construct an alignment plug the diameter of the collet and the length of a bullet to check the concentricity of the tool. It is simply a matter of loosening the C clamps and moving the spin jig along the steel strapping index for adjustment.
Using the spin jig trimmer system begins with com-mon cast bullet practices. Start with sized bullets. The 5C collet stop fits inside the spin jig collet. It gives a consistent index to the bullet length. Adjust the 5C uni-versal collet stop to your desired bullet length. It may require adjusting, trimming and measuring (and possibly remelting) a bullet. Put the bullet to be trimmed in nose first. With one hand on the trimmer crank and the other on the spin jig crank, turn the cranks in opposite directions. The trimmer will shave arcs of lead from the bullet base. Eventually the set bullet length is reached. Trim the next bullet.
Everyone’s bullet trimming needs will be different. One can adjust the amount of lead shaved off the bullet base to an almost indiscernible amount Eventually the user establishes a cadence of trimming and changing bullets.
We encourage you to experiment with different ways to improve bullet performance. Try using this system to truncate an ogive on a round nose or making more accu-rate handgun wadcutters. Besides an occasional drop of oil, the spin jig requires very little maintenance. Cutting material as soft as lead with the hardened cutter will give it an indefinite life. Although it is not the only factor, having a flat-based bullet is a significant part of the accuracy equation.
Mount the crank end on the plywood base so that the crank hangs over the edge and the trimmer is centered in the width of the mounting board.
The shims are for raising the height of the trimmer, if necessary, to the axis height of the cutter. The spin jig collet will eventually face the Forester Applet cutter. The steel strapping is mounted perpendicular to the axis of the trimmer. Check and recheck to insure the strapping is straight and perpendicular. The front edge of the spin jig indexes on the strapping and the spin jig is clamped to the plywood. Do not be tempted to permanently mount the spin jig, the C clamp allows the collet to quickly change calibers and gives easier access to adjusting the 5C Universal collet stop.
Align the spin jig and the trimmer blade so they line up perfectly when viewed from the side. Viewed from the top, they should be offset no more than the radius of the bullet base. For a .308 bullet, they should offset about .140”. It would be beneficial to construct an alignment plug the diameter of the collet and the length of a bullet to check the concentricity of the tool. It is simply a matter of loosening the C clamps and moving the spin jig along the steel strapping index for adjustment.
Using the spin jig trimmer system begins with com-mon cast bullet practices. Start with sized bullets. The 5C collet stop fits inside the spin jig collet. It gives a consistent index to the bullet length. Adjust the 5C uni-versal collet stop to your desired bullet length. It may require adjusting, trimming and measuring (and possibly remelting) a bullet. Put the bullet to be trimmed in nose first. With one hand on the trimmer crank and the other on the spin jig crank, turn the cranks in opposite directions. The trimmer will shave arcs of lead from the bullet base. Eventually the set bullet length is reached. Trim the next bullet.
Everyone’s bullet trimming needs will be different. One can adjust the amount of lead shaved off the bullet base to an almost indiscernible amount Eventually the user establishes a cadence of trimming and changing bullets.
We encourage you to experiment with different ways to improve bullet performance. Try using this system to truncate an ogive on a round nose or making more accu-rate handgun wadcutters. Besides an occasional drop of oil, the spin jig requires very little maintenance. Cutting material as soft as lead with the hardened cutter will give it an indefinite life. Although it is not the only factor, having a flat-based bullet is a significant part of the accuracy equation.
