After I pick an original (or several) to inspire my build, I make a blueprint. I will use a blowup of a gun in RCA to make several key measurements. I blow up a lock side view of the buttstock, for example, until the lock is the stated length. Then I know the magnification needed for some other blowups as well. From the barrel length I can easily guesstimate the length of the forearm, were the rear thimble is located etc.
But your parts will determine most of your key measurements. If the barrel, lock, buttplate and guard are not near-identical to the original you are using for inspiration, the plan must be changed to accomodate your parts. Here is a rough draft of the steps I take. No illustrations yet.
Drafting a blueprint To draft a blueprint, you will need your barrel, lock, trigger, guard, buttplate and a clear sense of the style of architecture you are trying to achieve. You can use plain brown wrapping paper or a roll of paper from the art store for your blueprints.
Step 1: Lay the barrel on the paper and trace it. With a swamped or tapered barrel this will be tricky. Be sure to use a square along the side flats to get a good tracing. Measure your barrel at the muzzle and breech, and double check those dimensions on your drawing. Measure the depth of your breechplug threaded portion from the muzzle using a ramrod blank, and mark this on the barrel tracing. Plan your touchhole location. If using a vent liner or an internal cone, locate the liner completely forward of the breechplug face, with at least 1/16” to spare. The touchhole can be exactly on center of the bore and side flat or slightly below center on a thick , large bore barrel if that helps the placement of the lock (more on that later).
Step 2: Establish the web between the barrel and ramrod on your blueprint. Plan the minimum thickness of the web between barrel and ramrod groove at the muzzle and between the barrel and ramrod hole at the front lock bolt, and mark that offset from the bottom of the barrel at both places on your blueprint. This web will normally be between 1/8” and 3/16” thick. Importantly, this dimension will affect the depth of your nosecap, though a ramrod with a swell at the muzzle end can also influence the depth of the nosecap. The ramrod is often (but not always) parallel to the bore, and could end up closer to the breech than to the muzzle on a barrel with a pronounced flare at the breech. Use a long straight-edge to draw the bottom of the web. Now, knowing the thickness of your ramrod, draw a line indicating the bottom of the ramrod as well. Note that after the ramrod hole is drilled, you may decide to re-shape the ramrod groove to follow the contours of your swamped barrel. This keeps the web thickness the same from end to end. But for now you are planning the web to establish the groove that will be used to get the hole drilled properly.
Step 3: Lay out the lugs, pins, forearm, thimbles and nosecap. This is a good time to determine the layout of your forestock. Considering your studies of originals or contemporary guns that are inspiring this build, decide on the length of the forearm, or where the ramrod will enter the forestock. Mark this spot. After considering barrel length and style considerations, decide on the number of thimbles and underlugs and locate these on the blueprint. Generally there will be an underlug to the rear of the ramrod entry hole or thimble, another close to the muzzle behind the nosecap, and either 1 or 2 more underlugs in between. The thimble(s) between the one at the muzzle and the entry thimble are normally evenly spaced, but it is not as critical that the underlugs are evenly spaced. Draw the thimbles and underlugs in place on your blueprint. Carefully plan where you will drill for the pins that will secure your ramrod thimbles and underlugs so that these holes will be located in any molding you will incorporate in the forestock.
Step 4: Locate the lock on the blueprint. Strip your lock of internals and lay the plate on your blueprint. There are three key points of alignment for placing the lock on the blueprint. First, the pan must be aligned with the touchhole, usually in the “sunset on the water” position, where the touchhole is right at, or slightly below, a line drawn across the top of the pan, and centered fore and aft. Next, the nose of the lock must be located to allow the front lock bolt to pass through the web between the barrel and ramrod hole, and be more or less centered in the area in front of the mainspring. Then, we hope that the tail of the lock will lie gracefully on the center of our planned wrist. Small adjustments at the nose and touchhole location can help the placement of the tail of the lock, but one may also find it necessary to change the flow of the wrist if a certain lock is absolutely required. If need be, the front lock bolt hole can slightly enter the barrel channel and a groove can be filed in the barrel at that point. Draw the outline of your lock on the blueprint and mark the screw holes, but keep a little wiggle room here and come back after you’ve located the buttplate and planned the wrist.
Step 5: Locate the trigger and guard. Whether you will be using a simple trigger or a set trigger, now that you know where the sear is, you can locate the trigger fore and aft on the blueprint. A double set trigger will be located so that it will fire whether set or unset. The pivot of a simple trigger will be located near the sear pivot bolt. Keep in mind the tang bolt must be able to strike the trigger bar front extension. Draw in the trigger(s) and locate the guard so there’s plenty of room for the trigger finger. Note whether you need to make some design changes to your triggers or guard so they work well together. Guards made of soft brass can be carefully reshaped after annealing to increase the depth of the bow, for example.
Step 6: Locate the buttplate. Next, measure from the trigger rearward and draw an arc using your length of pull dimension to locate the buttplate. Long lengths of pull based on old wives tales about your elbow to trigger finger measurement are not recommended. If you have a smoothbore or rifle that mounts well, use that for a general guide. Now place your buttplate on the arc you have drawn and notice that the placement of the buttplate will determine the drop of the buttstock at the heel , and its shape will determine the pitch of your gun.
Step 7: This is the creative part. Now you get to move things around just a little bit here and there to make the architecture evoke that of the original. You may need to modify the angle of a buttplate heel to top extension to get the pitch and comb line you want, for example.