Building a benchrest rifle

[Lee Martin]

Step 48 – Setting the Trigger
___________________________________

Trigger attachment doesn’t occur randomly or even on appearance. The layout has to account for these variables:

• Sear point on the cocking piece
• Trigger sear bearing surface
• Stock inlet
• Pillar locations – on ours there will be three
• Trigger guard bolt spacing

Before any of that happens, the Jewell trigger and the hanger are fit. My dad had some spare brass blocks from past projects. No photos of the production steps, but they’re basically milled on a Bridgeport and cross drilled.

Martin trigger hanger:



Jewell trigger is test fit:



The cross pins are made from 1/8” round stock:



The action is laid on the stock to spot mark the trigger guard and pillar settings. This’ll give us an idea on where to attach the trigger. From there the sear is determined.





Since we’ve done over 15 actions, we have plenty of patterns to work from:





Noting the trigger and cocking piece arrangement on a finished receiver, we tentatively marked mine:



-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

The Seating Die and Bump Gauge
______________________________________

I was eager to see how the reamer cuts and what the neck mics at. We’re a ways from barrel chambering, but the bump gauge and seating die can be done in advance.

A bump gauge is nothing more than a collar which contains the neck and shoulder portion of the chamber. Used with a fired shell and a caliper, it’s valuable in setting the sizing die. I’ll thoroughly cover the process in the reloading portion of this thread. For now, here’s how it is made.

We start with a 3/4” piece of round stock. The exposed end is faced and lathed to one-inch long:





Center drilled to remove most of the steel. This is followed by the finishing reamer:







As JGS advertised, the neck came out exactly 0.264”. The completed bump gauge:



I went a different route with the seating die. It’s a given you’ll use an inline seater for precision reloading. Additionally, most cut them with their chambering reamer for an exact match. No real decision point on that front. The fine adjustment top is however. For years, guys used the Wilson set-screw depth attachment. Photo below:



These work but lack indicating marks. Eventually, Wilson released their micrometer that dials in one-thousandth increments. Sinclair International offers a similar product.

My dad always makes his own inline seaters, complete with a micro-feed stem. One can be seen here:



The Sinclair version is $40 and considering the amount of time it takes to do ours, I went retail.

First, we had to cut the blank to the correct length. The stem has limited travel, so it’s important to be close on the body height:



Trimming to length:



Chambering:



Sinclair micro-meter top:



Full seater die assembly:



-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

Step 49 – Milling the Trigger Slot
______________________________________________

We’ll now mill the bottom of the receiver for the trigger assembly. The top of the Jewell protrudes through the action and into the cocking slot. Then the sear points are marked on the cocking tab and notched. A recess is also made for the brass hanger.

Great care is taken to ensure the action is absolutely level in the vice. The underside and runway should be dead on 12:00. Several leveling tools are used to accomplish this:



The marking paint is inscribed for the trigger edge cutouts:



A spot drill lays out a series of holes, exactly 0.300” apart:





These are finished with a drill bit:





Like our side port, an endmill connects them. The individual cuts are ~0.030” deep:







Top of the trigger is test fit:



We checked the hanger recess depth on another action. It was 0.175”:



A larger endmill makes the inlet, again around 0.030” a pass:





The corners are radiused with a small endmill:



Assembly tested. As you can tell, we have to go deeper.



The roughed trigger pad is shown below. Every edge will be lightly filed and the flats smoothed with a blending dremel (illustrated in the next post):

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-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[squawberryman]

As much as the parts and processes, I find myself staring at the tools. The stories they'd tell....

[Lee Martin]

Step 50 – Affixing the Trigger Assembly
__________________________________________________

Flats are milled on the bottom of the receiver for the hanger tabs:





The hanger is positioned and a spot drill marks the screw locations:



Drilling the holes:





Tapping the holes:



Enlarging the screw holes on the brass hanger:



Attached:

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-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

Step 51 – Setting the Pillar Locations
___________________________________________

The underside of the action is coated with layout paint:



We’ll be using three pillars, one behind the trigger and two out front. Those holes are drilled and tapped:







The last pillar intersects the cocking recess. Anytime you drill through an edge, burrs are raised. With clearance less than 0.001”, these block the bolt. Fine honing removes these edges without altering the inside diameter:



The trigger screw sticks through as seen here. It was quickly ground to length and reset:



Top view of the flushed screw and installed trigger.



-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

A well produced video of a guy shooting benchrest in Iceland:



He's using a practice target in which different loads (likely adjusted by charge, seating depth, and neck tension) are shot across the bulls. At the end he zeros his caliper on the bullet diameter and measures the group edge-to-edge.

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

Step 52 – Setting the Sear
________________________________

The cocking sear is marked off the trigger position. To align the two, we first have to relieve the back of the hanger. Shown in black Sharpie, that portion of brass is removed so the tab can slide into the trigger channel.







The spec sheet for our cocking tab:



Tentative marking of the sear cutout:



The assembly is inserted and a witness mark is made on the edge of the receiver. This appears as a scribe line just behind the bolt notch.



More marking paint is slathered on the bolt, cocking carriage, and receiver. By now you can appreciate how important layout paint is to the machining process. It makes the action look like hell, but is an invaluable tool.

The height adjusted scribe first marks the outside of the action relative to the trigger. It is locked with a set screw and the bolt is inserted. A scribe line is made, again at the pre-fixed height, on the bottom of the cocking tab:



Milling the sear with a cross section cut:



As the above schematic shows, our cocking piece is 0.250” wide. However, the Jewell trigger channel is only 0.246”. Using our Bridgeport, we narrowed the tab to 0.242” for proper clearance (but only the length that rides into the trigger plates):





Measuring the angle of the Jewell sear. The little square device on the right does this electronically.



A drawing of the exact angle we’re trying to measure:



Setting the cocking piece at the same angle for the endmill pass:





-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

Step 53 – Cocking Notch and Trigger Adjustment
______________________________________________

The bolt won’t stay cocked because there isn't a recess on the cam to hold it. Marking paint roughly outlines where the notch will reside (you can see the sear at 1:00 to the red):



Top view of the cocking sear. All of this still needs fine polishing:



Bolt body in the vice and milling the cocking notch:






Test fitting the trigger:



There was some faint bind on the sear when closing the bolt. A fine tooth file is used to slowly mate the surfaces. We also stoned each piece for seamless release:



On the back of the cocking piece, a hole is drilled for a retainer pin (especially handy when you unscrew the bolt assembly for cleaning):



The bolt in the cocked position:



The bolt in the fired position:



Three screws adjust the trigger. Referencing the “Jewell 1.5 oz BR” picture above:

• Screw 1 is turned with the action cocked until it fires. The screw is then backed out 3/4 of a revolution
• Screw 2 is adjusted for the desired amount of over-travel
• Screw 3 sets the pull between 1.5 and 3 ounces. I went with the lightest amount possible.



-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

Our Shop
________________________________

This thread shows close-ups of the machines we’ve used on our action. I thought some may also be interested in seeing the tight quarters in which we work.



Dad’s main shop is in a small room off the side of his garage. We have more machinery at my place just seven houses away (one basement room full and two single car garages). Nonetheless, his “favorites” are strategically jammed into this 15 x 17 foot square.

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Mark Terry]

I am amazed. Thank you for the education. Some of it I actually understand.

On about page 6, there is reference to "blanking". I think in reference to mainspring tension or force with what I thought I read as higher spring force being attributed to "blanking" (I can understand the logic that higher spring force would tend to shorten lock time). Can you explain "blanking"?

Also, in the video of the gentleman shooting benchrest, I was surprised to see him align the scope and break the shot without watching through the scope. Is this how it's done?

Thanks again,

[Lee Martin]

Mark – blanking is just another term for pierced primers. They usually result from an oversized firing pin hole, too sharp a tip, or a weak spring. Pair any of these with high case pressure and you get primer flow.

Blanking in the PPC is a different animal. When the round took hold in the 1970’s, loads were pretty passive. Then the surplus powders came into benchrest and pressures rose. Some even eclipse 70,000 PSI. Standard small rifle primers, including the BR variety, aren’t designed to operate like that. Of course, they’ll hold as is common with proof testing. But primer strikes become more pertinent. Strong springs increase the pin’s energy transfer on the breech side while 70,000 PSI pushes hard on the cup side. That thin piece of primer wall can only withstand so much. When it fails you get piercing or even worse, complete rupture.

So back to the PPC. In the early 80’s stronger springs were used to reduce lock-time. Many of these ran 35 pounds plus which is stiff. Primers soon pierced. Not all the time of course, but enough to prompt a fix. Shooters could either reduce the spring load or drop down on powder. Lock-time isn’t as important in benchrest compared to other forms of shooting. Node tuning is, so heavy charges quickly trumped heavy springs.

The second part of your question related to the video. Reading wind flags is an art. Most folks, myself included, look through the scope with one eye and peripherally check the flags with the other (this may or may not involve lifting your head. It really depends on the height of your scope rings). When conditions are sporadic you can actually come off the gun and time the trigger touch with the read. I’ll discuss the various techniques at length when I get into wind management. Hopefully this explains some of what you saw above.

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

Step 54 – Installing the Bolt Stop
_________________________________________

There are various bolt stop styles to choose from. We’ve tried a couple, but ultimately want these characteristics. One, it should be simple and easy to clean. Two it should be adjustable. And three, it should minimize the amount of steel removed from the sidewall. A threaded-ball plunger meets these requirements.

Ours is 1/4" x 20 and 17/32” in length. The spring load is a solid 10 pounds:



A locating mark is put on the left side of our action:



Spot drilling the mark:



We also milled a flat over the hole for a set knob:



Tapping the hole for 1/4" x 20 threads:



The ball plunger was screwed into the receiver and tested:



A groove has to be cut into the bolt. The ball detent rides in this recess and sets the stop location:







Once the line is milled in our Bridgeport, a Riffler file polishes the channel:



The benefit of a screw plunger is its adjustable nature. By rotating the body in or out, you can fine tune the stop force. We found by bottoming the screw and backing off a quarter turn, the drag is just right. Under normal cycling, the detent positively stops the body. A straight, even pull with a little more effort overcomes the 10 pounds for removal. This is good because you want to run the bolt fast without over-shooting the plunger. On the other hand, it should be easily removed with moderate pressure. The 1/4 turn above bottom gets us both.

Inside view of the ball plunger:



-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

Step 55 – The Gas Port
___________________________________

Blow a primer and the gas has to go somewhere. To vent it away from the shooter, most actions have an escape passage in the bolt body. When in battery, that passage aligns with a hole in the receiver. Mine will exit opposite the loading port.

The action is centered and drilled first. Our gas port is just behind the lug raceway:







A punch marks the bolt body via the action’s gas exit:



The bolt is then drilled.

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

Step 56 – Heat Treating the Action
______________________________________________

Heat treating gets a lot of attention on internet forums. Well, I should clarify. It gets a lot of attention indirectly. The designations for the alloys used in gun building are commonly cited. 17-4 PH and Carpenter 465 get most of the buzz. But dropping the commercial code doesn’t tell us much about their properties. Neither does a Rockwell C scale reading. Hardness is only one quality to consider. Strength, both tensile and yield, along with ductility, elasticity, and impact resistance are all at play. Only when measured in their entirety can we compare material performance.

Heat treating isn’t purely about making the alloy harder. It’s about altering the crystallites or grain structure. Those lattices govern all of the mechanical attributes listed above. Allotropy or polymorphism is the transformation that occurs to these crystallites when temperature or pressure is increased. Our action will be changed by heat alone using a furnace, oil bath, and tempering stage. Together these form the heat treating process.

I chose 4140 chrome moly for the reasons outlined on Page 2 of this thread:

• 4140 provides more than enough tensile and shear strength for our PPC
• 4140 is less susceptible to galling (the chromium oxide layer in stainless makes it prone to friction adhesion – aka, galling)
• 4140 free machines better than most stainless steels when annealed (and I speak from experience on this one – we’ve done Ruger cylinders out of 416, 17-4, 304L, and C 465. While they machine well, none turn as good as 4140)
• The only downside to 4140 is its heat treatment requires oil quenching.

When we heat treat an alloy, the metal undergoes three phases: 1) heating, 2) cooling, and 3) tempering. All of these must be precisely coordinated with respect to temperature and duration. Otherwise you’ll over or under shoot the desired result. And the results themselves are all about compromise. We can heat treat 4140 to 70 C scale and have tensile strength in excess of 300,000 PSI and a yield over 250,000 PSI. That sounds impressive until you study the impact resistance and ductility of the steel at 70. It’s very hard but very brittle. Sudden impact force can cause it to fracture or just plain fail. An example you all can appreciate is a Ruger cylinder heat treated to 70. The shear and tensile strength each approach 300,000 PSI. That would lead you to believe a 100K proof load would be plenty safe. Not so. The impact resistance, or toughness, at 70 C is low enough that the cylinder wall would likely stress fracture. So while it would be very hard to deform the steel linearly or coaxially, it’s more susceptible to fracture.

Most of the alloys used in firearms perform well in the Rockwell C range of 36 – 44 (barrels are less – my earlier posts outline why). Tensile strength, yield strength, impact resistance, and ductility are suited for pressures well above 100,000 PSI. But remember pressure is denoted per unit area, or square inches. That’s why a Colt Peacekeeper cylinder can’t consume pressure like a 5-shot Ruger. Mass matters.

Heat treating my action – we want the end result to be 45 C scale. As shown, that puts tensile strength at 210,000 PSI and yield strength at 195,000.



Step 1 is to coat the receiver with a thin oil film. Unlike production furnaces, ours isn’t vacuum capable and we don’t have oxygen-reduction catalysts. Instead, we use oil on the part which burns off when the temperature rises. That burning consumes much but not all of the chamber’s oxygen. In commercial heat treating, vacuum along with high purity protective gases minimize surface oxidation. Our oil prep does a decent job in preventing scale.



The action is placed in the largest of our three furnaces:





Temperature is raised to 1,600 degrees Fahrenheit and held there for 30 minutes. This furnace has a small peephole through which you can see the amber colored steel:



Here’s a better shot of the heated action:



After the 30 minute hold phase, the receiver is pulled and immediately submersed in an oil bath. Video of that step:



It is then placed in a steel jar for cooling and to drain the oil:



The surface has a fair amount of scale which is removed with brake cleaner and a brush. This is done while we’re waiting for the furnace to cool to 800 degrees. The cleaned-up action:





Prior to heat-treating, every edge was polished and smoothed to remove machine marks.

What we have now is a very hard piece of steel. The rapid cooling from 1,600 to room temperature in oil makes a brittle, martensitic structure. This brittleness is reduced through tempering. Tempering steel lowers internal stresses and defects by transforming some of the martensite into cementite or spheroidite. Higher ductility and better fracture resistance follows (simplified definitions – martensite = hard steel crystalline, cementite = highly brittle iron carbide lattice, spheroidite = ferrite matrix with high ductility, lower strength)

To temper it, the action is placed back in the furnace and drawn at 800 degrees. The metal is held there for 90 minutes before we unplug the oven and walk away.

In summation, heat treating changes the spatial arrangement and grain size of our 4140. When you add carbon atoms to iron they tend to reside in the middle of the crystallite. Primarily in places where they fit easily. This is the body-centered lattice shown below. The carbon atoms have little influence on the relative strength of the steel. However, if we shift the carbon atoms to positions in which the iron atoms can’t move readily, strength increases. It becomes more of a face-centered lattice. Heating above the critical temperature causes the transformation. Rapid cooling, or quenching, allows the carbon atoms to stay locked. Because this structure is quite brittle, tempering is done to stress relieve the crystallites.



BBC = body centered cubic
FCC = face centered cubic
The black atoms are iron, the red atoms are carbon

A deeper dive would involve physical chemistry and thermodynamics. Those disciplines are intertwined with metallurgy but are also great sleep aids. I do hope this reinforces the importance and dynamics of heat treating though. It is often mentioned when discussing the inherent strength of firearms. It surely can’t hurt to know the underlying process.

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"