5-Shot Conversion - Documented

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dingode

Single-Sixer
Joined
Oct 21, 2012
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140
Thank you for the update! I recently ran across a set Youtube videos Ruger put out on how they go about making different guns. That got me thinking about your post, and I was going to find it and see if there was an update. Looking forward to your next installment.
 

dingode

Single-Sixer
Joined
Oct 21, 2012
Messages
140
Thank you for the update! I recently ran across a set Youtube videos Ruger put out on how they go about making different guns. That got me thinking about your post, and I was going to find it and see if there was an update. Looking forward to your next installment.
 

Lee Martin

Hunter
Joined
Dec 18, 2002
Messages
2,313
Location
Arlington, Virginia


Before I begin, I want to cover cylinder-to-bore alignment. Conventional wisdom says line-boring is the most precise method to index the cylinder throat and barrel's bore. I'll flatly tell you it isn't. That's no knock on the process or any of the fine gunsmiths that use the technique. If I built custom revolvers for sale, I'd damn sure advertise line-boring. It's universally accepted as the gold-standard. It sells.

Line-boring is done in a horizontal mill using a frame/cylinder jig. A spud is first screwed into the frame which acts as the drill guide. The gun is held in battery and a centering drill spots the bore's centerline. You can either drill one pilot hole per chamber and pull the cylinder or progressively increase the diameter. Regardless, with line-boring you can't complete the pilot hole; otherwise you'd run into the breech face. Line-boring also requires a bearing block to wedge the bolt and a tightly fit basepin. That ensures indexing is consistent chamber to chamber. It supplements how the cylinder is kept stationary while cocked for drilling. Most of the work though is done by a jig which secures the frame and cylinder. While line-boring is quite precise, it presents some challenges:

• The frame jig must hold the action dead perpendicular to the mill's head. If you decide to get into a line-boring, you'll spend some time getting this right.
• The centering bit and pilot drill cannot be the exact diameter of the guide spud's hole. If they were, they'd drag the edge. So they must be undersized by a thousandth or so. Alignment much past three zeros becomes difficult.
• The pilot holes must be re-indicated with a centering device to complete the bore (cylinder outside of the frame). Thus you're drilling in two passes. This introduces the chance for slight misalignment upon re-centering (even as the drill follows the pilot hole)
• Frame and cylinder jigs are very rigid. However, they do not compare to the leveling capability and positive grab of a 100 pound, 3 or 4 jaw chuck.

By now, I'm sure some of you are shaking your head. Fine enough, but we've both line-bored and cut cylinders with the method outlined below. My conclusions are based on actually shop time, not conjecture.
__________________________________________________

Line Indexing

We didn't invent the process, but my dad switched to line-indexing on his own in the 1990's. After dozens of conversions, we found it easier and more accurate than line-boring. It all starts with sound cylinder fit. As illustrated in step #6, the bolt matches the frame slot and notches so there is absolutely no side-play. We also use the same oversized pin for indexing mock-up. Dad machined a frame mounted shank, made with extremely tight threads. It was center bored and honed to 0.2500" on the nose. Next he cylindrically ground a pointed rod, once again 0.2500" diameter. Key takeaways: 1) The point was indexed and zeroed to absolute center, and 2) The rod is so tightly fit there's resistance when pushing it through the hole (unlike line-boring shanks which are a thousandth or two larger than the drill).

The homemade frame spud.



The shank is threaded into the frame and the rod is inserted. With the gun in battery, the rod is tapped and the fine indication point marks the cylinder face. This is repeated five times.



A Hartford Super Spacer is mounted on our Bridgeport. The chuck weighs 100 pounds and is accurate to seconds of a degree. The chuck had already been leveled to the table.



A Baker centering gauge is installed and the point is put in the indentation. Each mark on the gauge represents 0.0005". The mill is turned on a very slow speed and the dial indicates misalignment. The table is adjusted in tiny graduations until the needle doesn't move as it spins in the mark. Once it stays on the same dial mark, you're indicated to 0.0000".



The mark is spotted with a starter bit. This is followed by a 1/4" drilled hole front to back.



I then enlarged the hole with a 31/64" bit (0.4844").



A 0.510" reamer makes the final pass. This reamer was ground by my dad and matches our .500 Linebaugh pilot; also home ground.



I purposely reamed the throat under final spec. We prefer honing the throats to the desired diameter, approximating the bullet. This allows us to achieve tighter tolerances and produces a superior surface finish.

Video of me reaming a throat. Forgive the shakiness, I was holding the camera as I feed the reamer.

https://youtu.be/h2HyYkNx6DQ

The process is repeated 4 times after the indexing head is rotated 72 degrees. Integral to proper alignment is re-indicating each mark with the centering gauge.



Checking throat-to-bore alignment – saying one method is better than the other means nothing without empirical evidence. The best way to do that is with a centering device. You start with a frame shank sporting a hole that exactly matches the throat. For example, if the throat measures 0.5100", the shank's bore also has to be 0.5100". The cylinder is fit to the frame, the shank is screwed in, and the gun is cocked while held in a vertical mill. Using another centering device that rides the surface of the shank's hole, the headstock is slowly fed down and zeroed. As it transverses the cylinder gap, it enters the throat. Misalignment, or lack thereof, is shown on the dial. Meaning if it stays zeroed in the throat over 360 degrees, you're dead nuts on. If there's runout, it'll show-up on the dial.

Bore alignment – line indexing or line-boring are tied to barrel installation. Remember, we're talking throat to barrel bore alignment here. If you thread the blank and lathe the cone with any runout to center, you're nixing the precision throat. A lot of guys index on the outside of the blank when adjusting the chuck. Forget that. Even the highest quality blanks exhibit a thousandth or two of O.D. runout to the rifled bored. And most of the pistol blanks we use show 0.002" – 0.004" runout over the entire length. I'm a stickler on this, but you really need to center the blank in the chuck by zeroing across every groove. When I barrel this thing, I'll detail those steps.

A couple of things to back my point. Magnum Research uses line-indexing on their BFRs, albeit in a more automated fashion. We all know how well they shoot. And earlier this year I was talking to a friend who is one of the most talented gunsmiths I know. He doesn't commercially smith, but has done everything from custom built benchrest actions to Ruger conversions. For a living he builds engine balancing machines that he and his brother designed….that speaks to his engineering prowess. Anyway, we were talking about line-boring Rugers when he said there's a better mousetrap. I didn't say how we did ours, instead giving him the floor. After five minutes of explaining his approach, it turns out he uses the same method we do to indicate bore centerlines. We're not alone on this one.
 
Joined
Apr 2, 2014
Messages
3,204
I'm not arguing with your method and I'm NOT a gunsmith, but just a couple of thoughts.
After the cylinder was chucked in the 3-jaw I'd vertically "sweep" the center bore to confirm
it to the quill travel ( remember it's a Bport ). And somewhere between last drill and before
reaming I'd pass a short carbide end mill down the holes.
Dave
 

Lee Martin

Hunter
Joined
Dec 18, 2002
Messages
2,313
Location
Arlington, Virginia


I start by flipping over the cylinder and re-indicating each hole. A centering device sweeps all 360 degrees of the bore and the table is adjusted to zero runout. Again, this must be done on every throat.



A 0.610" endmill was selected to cut the rim recesses. Starline .500 Linebaugh rims mic 0.603" – 0.605" in diameter. I always want a little extra space to prevent grit from binding headspace. Upon cutting the first 0.060" recess, the endmill proved dull. It was re-sharpened on a tool and cutter grinder.



Milling the rim recesses. A depth stop was set at 0.060" in from the face.



The case measures 1.400" in length. Average rim thickness is 0.0602", so the reamer is fed a minimum of 1.338". I always go a little deeper and chose 1.360". Another depth stop is set on the quill.



Before chambering, all five holes are again swept using a centering gauge. This confirms zero runout. You'll recall, the chambering reamer is home ground and the pilot matches our throater. The finisher is held in a floating reamer holder.



Testing a case.



The fully chambered cylinder.



Next time I'll harden it.
 

Lee Martin

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Dec 18, 2002
Messages
2,313
Location
Arlington, Virginia
Step_16_Tagline.jpg


Before I show the steps involved, it is important to cover some basic metallurgy. This'll go beyond the normal talk of cylinder hardness, alloy types, and case pressure. If understanding the 'why' and 'how' of these isn't important to you, read no further. This is dull, but explains how cylinders absorb, and withstand, discharge forces.

The alloys commonly used in gun manufacturing are either quench hardened or precipitation hardened. Quench hardening works by quickly cooling the metal once its critical temperature is reached. Precipitation hardening, also known as age hardening, works by rapidly heating the metal to a defined temperature after a fast cooling process. This is best done in inert atmosphere. Following quench hardening, the metal must be tempered. That's because in the hardened state, the crystalline structure is dense and finely arranged. Or put another way, it is brittle. Tempering increases toughness and reduces the brittleness. The resulting alloy is more ductile, and therefore more stable.

Strain hardening and quenching increases strength and decreases ductility. Tempering decreases strength and increases ductility. When it comes to revolver cylinders, the term 'hardness' does not tell the whole story. What we're after is a compromise between the two. In fact, we must consider three variables: tensile/yield strength, elastic moduli, and fracture toughness. The below chart illustrates all three in unison:

Stress_Strain_Curve.jpg


The Y-axis represents the force or stress imparted on the object. In a revolver, that's the pressurized gasket, aka the brass. Our X-axis depicts strain, or the deformation of the metal through bending, flexing, and breaking. Yield strength is our first tipping point. The yield strength of an alloy signifies the transition from elastic to plastic deformation under stress. Quick definitions:

• Elastic deformation - temporary deformation due to applied stress (like a cylinder wall flexing and contracting at discharge)
• Plastic deformation – permanent deformation after applied stress (like a bulged chamber)

Strain hardening occurs beyond the metal's yield strength. Work hardening is another way to describe the phenomenon.
At some point, enough strain is applied to reach the ultimate tensile strength. This is nothing more than the maximum stress tolerance before failure.

Yield strength is an important part of structural design. Yield strength is set to prevent plastic deformation under a pre-defined amount of stress. For example, a proof load which is 50% higher than SAAMI specifications. Secondly, the part must be large enough in area to absorb the defined force. So both hardness and tensile strength are indicators of a metal's ability to resist plastic deformation. The two are correlated, but not by simple mathematical linkage. That said, yield strengths are usually 75% - 90% of the metal's ultimate tensile strength. To recap, yield strength measures the stress at which elastic to plastic deformation occurs. Ultimate tensile strength is the most stress that can be withstood before failure. This is where toughness comes into play. Toughness specifies the amount of energy the material can absorb per unit of volume before fracturing. Clearly cylinder mass, whether it be chamber or bolt notch thickness, matters big time. Mass increases toughness exponentially. Toughness is the combination of the metal's strength and ductility. The latter is the degree to which an object can deform plastically before fracture. Now we see why hardness isn't the only thing to consider when discussing cylinders. That's because strength and ductility are inversely related. Strong brittle metals can fracture easily. Weak ductile metals bend easily. Tough metals can flex elastically without fracture. Toughness and resilience improve durability and that's what we want in a cylinder.

I chose 416 stainless for this .500 Linebaugh because I had some on hand and it machines beautifully. We've done cylinders out of 4140, 416, 17-4PH, and 465 and they're all viable alloys for the job. Yes, some like 465 and 17-4 have higher tensile strengths than 416. To be exact - at Rockwell 40, 416 provides just under 200,000 PSI (or 1,350 mpa). 17-4 can be hardened to 230,000 while a workable 465C is 250,000 PSI. Do these numbers really matter? They sure do on the internet when folks play armchair metallurgist. Alloy types are always dropped with discussing proof loads; none more prolifically than Ruger's 6-shot 454 Casull. I certainly don't blame Ruger for selecting 465 for that gun. They proof the entire cylinder at 90,000+ PSI. A 4-series stainless failed at that level, prompting the change. But how many of us shoot proof loads? I sure don't. In fact, I don't overload period for a given cartridge and a given platform. I've only proofed one of our cylinders, and it was inadvertently in a 6-shot .401 Bobcat (416 SS). It happened when testing Unique under a 265 gr cast bullet. Things seemed fine as I went up in half-grain increments. That is until I opened the gate and the primers fell out when extracted. The pockets on brand new .220 Swift brass got big….way big. To our surprise, not only did the cylinder hold, the chambers didn't bulge. How much pressure did it take? I can't say for certain, but Swift brass is designed operate at 62,000 PSI. No doubt I was way above that.

Hardness tests are done on a host of scales to include Rockwell, Vickers, and Brinell. In most, a diamond ball is pressed into the metal and the size of the indention (either depth or area) is converted to a hardness figure. For this 416, we'll harden to Rockwell 40 – 41, C-scale.

Step 1 – the cylinder is belt sanded to remove surface scratches.

5_Shot_Build_104.jpg


Our small furnace is set-up and plugged in. We have a larger unit for bigger items, such as rifle actions.

5_Shot_Build_106.jpg


The furnace is adjusted to 1,750 degrees Fahrenheit.

5_Shot_Build_105.jpg


Once it hits 1,750 F, it is held there for one hour.

5_Shot_Build_107.jpg


Then we pull the red hot cylinder and quench it in oil. This rapidly cools the metal. I let my dad do this part for two reasons: 1) It is tough to hook the cylinder in such a small furnace, and 2) I'm better than him at videoing with my Smartphone. Video below:

https://www.youtube.com/watch?v=9-mFWpfD518

Post cooling, what we have is a very hard piece of steel. However, ductility is low and the cylinder is quite brittle. Firing it at this point would likely induce stress fractures. So we'll temper it at 400 degrees for one hour. Then the furnace is unplugged and we walk away.

Next time I'll confirm hardness on a Rockwell tester, hone the basepin hole and throats, and polish the entire exterior.
 

VernTMG

Single-Sixer
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Aug 30, 2009
Messages
288
Location
Glendale, Az.
That is amazing your dad looks like he has fun dropping that cylinder in the oil I enjoy watching the whole process thanks again for sharing.
 

hittman

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Wow!

Thank you for sharing this. I've never seen such a detailed, comprehensive documentation of custom gun making.

If I ever had any thoughts that I could customize one of my single actions ... all hope is lost now! :lol:
 

Lee Martin

Hunter
Joined
Dec 18, 2002
Messages
2,313
Location
Arlington, Virginia
Step_17_Tag.jpg


We never finish the throats with a reamer. While they work fine, they're limited by: 1) Reamers can walk, which compromises concentricity, and 2) Reamers leave machine marks which abrade the bullet.

This cylinder was reamed undersized at 0.5100" – 0.5106". Final dimensions will be done using a Sunnen Hone. For those not familiar with hones, there's no better way to achieve throat concentricity. Hones use an abrasive stone and a self-truing process. Unlike grinding, which allows wheel imperfections to be imparted on the object, honing follows a complex path. The stones simultaneously enlarge the hole radially while they oscillate axially. And since the stones erode towards the desired geometry there's no need to true them. So what does that mean? Basically the honed component holds better concentricity than the machine tool that created it (ie, the reamer). Better yet, our Sunnen Hone is accurate to 0.0001" and can return varying textures to include near-mirror finish.

This gun will see 0.511" cast bullets so the desired throat is 0.5115". That extra half-thousandth allows the bullet to pass without material upsizing or constriction. Using a Brown and Sharpe Hite-Icator, a dial bore gauge is set to 0.5115". These height indicators are accurate to 0.00050".

5_Shot_Build_109.jpg


5_Shot_Build_111.jpg


5_Shot_Build_110.jpg


All throats were checked against the indicated gauge. The dial at the top tells you if you're off or not. If the hole falls to the left of the zero, it's too small. If it falls to the right it's too large. All of our throats zeroed dead on which means 0.5115". Now you may ask why we don't use pin gauges. It's simple. Pin gauge sets are to the thousandth. In other words, they don't indicate where I'm landing in between 0.511" and 0.512". This gauge does and like the hone is accurate to the ten-thousandth.

5_Shot_Build_112.jpg


There's more than one way to throat a cylinder and honing isn't an end-all. And I'm not suggesting reamers walk as much as drill bits. They hold form pretty well. But reamers can wear unevenly and won't bore a throat as concentric as a hone. As for the machine marks, they can be fire lapped with abrasive compound (I like Clover brand), soft lead bullets, and low velocity. They can also be polished out but that may impact concentricity and it's harder to maintain uniform diameter across 5 - 6 holes. Honing provides the desired surface finish from the hit.

Our Sunnen hone:

5_Shot_Build_113.jpg


Here's video of dad honing a cylinder a few years ago. It was too hard to hold the camera as I honed this 500 today.

https://www.youtube.com/watch?v=Orppwq02CbY

Then I used a speed lathe and some sandpaper wrapped around a mandrel to lightly polish the chambers.

5_Shot_Build_114.jpg


The front and rear of the cylinder are polished using a mandrel between centers. A flat file and emery cloth removes the oil discoloration.

5_Shot_Build_115.jpg


As you can tell, the oil quenching creates a lot of surface reaction. After the front and back are polished, the exterior is sanded.

5_Shot_Build_116.jpg


5_Shot_Build_117.jpg


Unrelated to step 17, here's a pawl jig we made. I forgot to include this in the timing write-up. What this allows us to do is easily measure the legs relative to the top edge of the jig. A simple depth mic gives us our measurements.

5_Shot_Build_118.jpg
 

Lee Martin

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Messages
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Step_18_Tag.jpg


I'm using leftover blank from my .50 Alaskan BFR for the barrel. It's 6-groove, 18-twist stainless Pac Nor, measuring 0.510". I start by cutting a piece 7-inches long. The final barrel will be 6-inches, but I want extra to work with when it comes to truing the ends, gapping the cylinder, and lathing the crown
.
Before centering, each end is faced in our 13" Colchester.

5_Shot_Build_119.jpg


The ends are spotted for centers.

5_Shot_Build_120.jpg


A lathe dog drives one end on center, while the tailstock portion free-spins on another.

5_Shot_Build_121.jpg


The outside diameter is 1.100". I'll reduce this to 0.800" with no taper.

5_Shot_Build_122.jpg


Here's video of a pass. Word of warning, and something I learned the hard way. Always wear socks in a metal shop. One of the hot chips you see flying off landed on my foot. They can burn right into the skin.

https://youtu.be/RbkF_gOEavY

Because I couldn't cut beyond the dog, the blank is rotated and the other end is lathed. If you're doing a banded barrel, this is when the step is machined. It really isn't hard to create; the trick is timing it to the ejector housing.

5_Shot_Build_123.jpg


The rough turned blank is locked between centers and sanded. This removes the machined texture, but isn't final finishing. That'll happen after the threads, cone, sight tapping, crown, and ejector hole are complete.

5_Shot_Build_124.jpg
 

DougGuy

Single-Sixer
Joined
Jul 21, 2014
Messages
171
This is a very nice documentation of even nicer work!

I like that you mentioned the advantage the Sunnen hone has when it comes to finishing cylinder throats. I use a Sunnen LBA 666 hone for all my throat finishing as well, it makes an appreciable difference. For final polish I find that the 800 grit flex hone does a really good job following the Sunnen hone. I keep a drill on the side of the Sunnen and run the flex hone in the chambers using the oiler from the Sunnen to keep oil flowing through the chamber while the flex hone is spinning.
 

Lee Martin

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Dec 18, 2002
Messages
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How the bore aligns with the throat is vital. Even though forcing cones guide the bullet, we want to minimize that task. Line-boring and line-indexing do that on the cylinder end. But the second leg of the journey, namely cylinder-to-barrel transition, is often overlooked.

The cone is the first thing the slug sees beyond the throat and gap. I'll cover forcing cones at length when I make mine. This step focuses on cutting the barrel threads and cone dead-on to the bore centerline. To repeat a point I made earlier:

"Line indexing and line-boring are tied to barrel installation. Remember, we're talking throat-to-barrel bore alignment here. If you thread the blank and lathe the cone with any runout to center, you're nixing the precision throat. A lot of guys index on the outside of the blank when adjusting the chuck. Forget that. Even the highest quality blanks exhibit a thousandth or two of O.D. runout to the rifled bored. And most of the pistol blanks we use show 0.002" – 0.004" over the entire length. I'm a stickler for this, but you really need to center the blank by zeroing across every groove."

The front of the frame was checked and does not need trued. Next the blank was put in a four-jaw chuck and spot indicated using a Sharpie (lathe running). That got me close enough to indicate the lands with a pin gauge. The bore is 0.500", but I don't want to force a tight 0.500" pin and scuff the rifling. Instead, I lightly lubed a 0.499" and slid it in. Mind you, this is only preliminary centering. The precision work is done on the grooves with a horizontal dial test indicator.

5_Shot_Build_125.jpg


A horizontal dial test indicator is used to true all six grooves. These have 0.0005" graduations, but splitting the marks gets to 0.00025". The probe is run into the rifling and the 4-jaw chuck is slowly rotated. As the finger rides over the lands and back into the grooves, the dial moves. Low spots are grooves, high measurements are lands. Once a groove is found, the dial is zeroed. Using the chuck key, the headstock is gently moved 180 degrees. Runout is noted by how far from zero that groove is relative to the opposite side. The chuck jaws are finely adjusted until both opposing grooves read 0.0000". This procedure is repeated for the other two pairs of opposed grooves. There is no more accurate way to indicate a bore. Every barrel we do, whether it be a Ruger conversion or benchrest rifle, gets this treatment.

5_Shot_Build_126.jpg


5_Shot_Build_127.jpg


I shot a quick video to better illustrate the technique.

https://youtu.be/3P54ysShrMo

So are these measures that necessary? They certainly don't hurt accuracy, but can't guarantee it either. Truth be told, our nerves, respiration, and eyesight are way more critical to shooting revolvers well. My dad and I do this stuff because all it costs us is time (plus I truly believe it matters). Up next, I'll cut the threads and lathe the forcing cone.
 
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