manurhin mr73 design and construction

Notes:

[0] The frame of the MR73 is only slightly larger than the S&W K-frame. Yet it is much more durable, and so is the action that it contains. The size of its frame window, and that of the cylinder, falls between those of the .38 caliber S&W K-frame and the S&W L-frame, the latter dimensioned to equal Colt’s .41 caliber I-frame of the Python, itself patterned after the similarly sized E-frame of Colt’s Official Police. Too small a frame fails to contain Magnum stresses and strains, whereas too large a cylinder batters the cylinder stop with an extra rotational momentum.
    According to a test published in Cibles № 342, the rectangle of dispersion on a target shot at with the MR73 did not change after the test firing of 20,000 full power .357 Magnum cartridges. The writer concluded that it would take at least 300,000 rounds for the bore to begin to wear. The Manurhin factory museum exhibits an MR73 used by GIGN, with a round count of 96,000 full power .357 Magnums, a number unattainable by any S&W revolver.
    The top sideplate screw featured on early revolvers is deleted on subsequent variants, with sideplate retention ensured by the profile of its corner shaped as a wedge fitting in the frame. The four pins staked into the MR73 frame to serve as trigger and hammer axes, and retain the bolt stop and its spring, differ from it in heat treatment, resulting in the appearance of purple circles on its left hand side as a result of differential response to bluing salts.
    Serial numbers appear to be consecutive with the exception of special production runs, with prefixes as a rule indicating the original barrel fitment:

MC stands for a MR73 Silhouette in .357 Magnum with a 10¾" barrel mated with adjustable Match sights;
CK — a MR22 Silhouette in .22LR with a 10" barrel mated with adjustable Match sights;
MB — a 9" barrel with adjustable Sport sights;
MA — 8" and 5¾" .38 Special Match barrels with adjustable Match sights;
XA — a 6" .22LR Match barrel with adjustable Match sights;
Y — a 6" .32S&W Long Sport barrel with adjustable Sport sights;
N — a 6" .32S&W Long Match barrel with adjustable Match sights;
L — a 6" .357 Mag barrel with adjustable Sport sights;
V — a 5¼" .357 Mag barrel with adjustable Sport or Gendarmerie sights;
D — a 5¼" .357 Mag barrel with fixed sights;
K — a 4" .357 Mag barrel with adjustable Sport or Gendarmerie sights;
C — a 4" .357 Mag barrel with fixed sights;
H — a 3" .357 Mag barrel with adjustable Gendarmerie sights;
B — a 3" .357 Mag barrel with fixed sights;
G — a 2½" .357 Mag barrel with adjustable Gendarmerie sights;
A — a 2½" .357 Mag barrel with fixed sights;
HA stands for all MR73 models made by Chapuis.

    Click-adjustable rear sights are retained in the top strap by a solid transverse pin, with the elevation screw engaging a nut retained by a cutout in the frame. Sight adjustments are clockwise for down and clockwise for right. Gendarmerie rear sight blades are 2mm narrower than the 24mm blades fitted to Sport models, with corners rounded to minimize the likelihood of snagging on the draw. They are matched by a ramp front sight on the former and a taller upright sight sloping rearwards on the latter. In recent production, rolled pins replace the solid pins previously fitted to retain the sights. The early pattern rear sights have a tang 8mm wide, whereas the late pattern tangs are widened to 10mm. The Match rear sights protrude about 1" past the frame and feature replaceable blades with varying notch dimensions, complemented by replaceable front sights of varying profiles, retained with an Allen screw.
    Every Manurhin revolver is fired an average of 15 times at the plant. It must print five shots within a 20mm circle at 25 meters. Outstanding accuracy is not limited to the bench. Thus, using his service weapon, Christian Prouteau was able to cut in half a cartridge by shooting off hand at 15m.
    According to Manurhin, adjusting the rear sight by a click (⅛ of a full turn) on the MR73 is equivalent to the following correction of the point of aim at 25 meters, according to the model and barrel length:

Sport model in .357 Magnum / .38 Special:
    4" barrel, correction 7.7mm;
    5¾" barrel, correction 6.3mm;
    6" barrel, correction 5.7mm;
    8" barrel, correction 4.4mm.
Match model in .22 LR / .32 S&W Long / .38 Special:
    6" and 5¾" barrels, correction 5.2mm.

    In recent Chapuis production, the Swiss-made micrometer rear sight assembly has been replaced, first with an American-made adjustable Millett sight first used on the MR93 and MR96, then with a German-made Recknagel sight.
    The MR73 front and rear grip straps are grooved on the early models and smooth on their successors made after 1981. Additionally, the Police/Defense grip frames incorporate a lanyard retainer at the rear of the butt frame. Serial numbers appear on the butt of the grip frame of the early models, and on top left hand side of the frame window in the later ones.

[1] Traditionally, revolvers are issued in several versions: Compact, with a 2½ or 3-inch barrel, medium with a 4-inch barrel, and long with a 6 inch barrel. The MR73 added a barrel measuring 5¼ inches. The differences in performance between 6 inches and 5¼ inches are not significant, but the shorter barrel packs easier and draws quicker. Additionally, GIGN adopted the 8-inch revolver equipped with a scope and a bipod.
    Barrel-making technology comprises cut rifling, button rifling, and hammer forging. Cut rifling was invented in Germany in 1492. It employs a cutting tool that very gradually scrapes away the metal in a spiral pattern to form the grooves of the barrel until their desired depth has been achieved. It is a slow and costly process that minimizes the metal stress in the bore through reliance on highly skilled labor to run and adjust the machinery. The more efficient button rifling was perfected during World War II by Remington, who used the nearby facilities at Hart Barrels. In this process, a sharp-edged carbide button with the rifling pattern ground in relief into its hardened surface is attached to a rod and pulled or pushed through the lubricated bore to form the rifling. As the button passes through the bore, the raised rifling pattern presses into its softer surface to create the grooves in a cold forging process. This operation is very fast, taking about a minute per barrel. But it creates significant stress in the barrel, which must be stress-relieved afterwards.
    Hammer forging was developed in Germany in 1939. After a drilled barrel blank has been honed to a very fine interior finish, a tungsten carbide mandrel with the entire rifling pattern ground in relief into its surface is inserted into its as yet smooth bore. This assembly is then placed between two opposing power hammers, and rotated as they pummel the barrel into the mandrel’s pattern. The stresses produced by this method are relieved through heat treatment. Hammer forging yields the smoothest and most durable bore finish, with a work hardened surface. In Europe, hammer forging is the industry standard. It is the technique used in the making of the MR73. The factory claims its barrels, frames, and cylinders to be “machined from specially formulated, ordnance-certified alloyed steel stock procured from Aubert & Duval, an internationally-recognised specialist manufacturer of special steels.”
    Standard MR73 barrels are rifled with six grooves, with a right hand twist at a one turn in 476mm rate, replicating Smith & Wesson’s traditional rate of one turn in 18¾". Match barrels utilize five grooves at the same twist rate, with a bore diameter of 9.05mm. Up to serial number 39200, the bore diameter of standard MR73 barrels measures 8.96mm +0 / +0.03mm. After it, the barrels are finished to the .38 Match bore size, measuring 9.04mm +0 / +0.05mm. Handloaders can and should adapt their projectiles to the actual bore diameter. The barrel logo appears on the right in early production, then moves to the left in all its Mulhouse and Chapuis successors.
    All one piece MR73 barrels are screwed into the frame and pinned there. Match and Silhouette barrels, and barrel sleeves of the Convertible model, feature a full length, squared-off barrel lug that allows the use of an optional barrel weight that may be positioned and secured with set screws anywhere along its length. The standard barrel profile features a slimmer, rounded barrel lug that is undercut at the muzzle on 6", 8", and 9" long barrels, and extends to its full length on barrels measuring up to 5¼". The former can accept the GIGN bipod adapter with a Harris bipod attached to it. The factory method of scope mounting relies on drilling and tapping the barrel rib.

[2] Colt revolvers have their cylinder bolt offset, locating the notch in the cylinder in the thick web between chambers. Because of this the notches can be made deeper and better resist peening. Smith & Wesson revolver cylinders have their notch directly under the chamber, and as a result they are not made as deep. The same part, which S&W calls the cylinder stop, is made thicker but shallower than the cylinder bolt of the Colt revolver. Originally, S&W addressed this problem by milling a second and wider notch next to the cylinder stop cuts. They then press fitted and staked a hardened insert into this second slot. When the cylinder was finished, the insert was so finely fitted that it took a magnifying glass to spot it. As a result, many well-used antique .32 S&W and .38 S&W top-break revolvers still lock up tighter than their modern counterparts.
    What Colt calls a crane, S&W calls a yoke. The MR73 yoke, which pivots inside a cylindrical opening in the frame, results in a more robust construction than its S&W counterpart, supported by the frame in a partial arc cut away to allow its opening. The latter design, shared inter alios by Colt, Ruger, and Korth, is susceptible to springing and loosening of the yoke as a result of “bogarting”, flipping the revolver cylinder in and out after the purported fashion of a film noir protagonist. In addition to increasing the strength of its assembly, the yoke of the MR73 allows for additional clearance in manual loading and speedloader access, in virtue of having its pivot located to the left of the frame axis.
    In the S&W terminology, a cylinder stop is the semicircle protruding from in the bottom of the frame window, which clicks into the notches on the cylinder, and/or scores it with the unsightly turn ring. The bolt is the the rod in the center of the recoil shield, which allows the cylinder to swing open after the thumb latch has been pushed. The MR73 cylinder stop is likewise much wider and more robust than its S&W counterpart. By contrast, their bolts are similarly sized, with their diameter dictated by that of the ejector rod.
    The MR73 cylinder is milled out of a forging, and its chambers are roller burnished after drilling. The cylinder assembly chambering rimmed cartridges such as .357 Magnum, .38 Special, .32 S&W Long, .22 Magnum, or .22LR, is complemented by an extractor of conventional design, engaging the cartridge rim. The cylinder assembly chambering rimless 9mm Para cartridges utilizes an extractor based on the 1976 patent by Gilbert Maillard, overlooking similar contemporaneous art by André Pilorget. It engages the rim with a flexible annular ring that protrudes just past the contours of the extractor star. Whereas the S&W K-frame cylinder measures about 36mm in diameter, that of the MR73 measures about 38mm. By contrast, the cylinder of the S&W L-frame duplicates the dimensions of Colt’s E- and I-frames, measuring about 39.5mm in diameter. As discussed below, the MR73 cylinder uses two kinds of ratchets, pairing with two kinds of hands.
    In conventional single action solid frame revolvers and conventional swing out cylinder double action revolvers, the rearward movement of the cylinder is limited by the ratchet bearing against the frame. Wear at this point will be influenced by the actual bearing surface and its metallurgy. Single actions typically have more bearing surface than double actions. The double action ratchet must accommodate two modes of operation with differing hand movements in the two modes. In most double action revolvers, correcting this requires fitting a new ratchet, which is generally integrated with the extractor star.
    In most single action revolvers, the forward travel of the cylinder is limited by the cylinder neck bearing against the frame, with room for an ample bearing surface. However, the ensuing strength complicates maintenance, requiring excessive play at either end of the cylinder to be corrected by fitting an oversized replacement cylinder. By contrast, double action revolvers with swing-out cylinders compromise their strength by design. In a Colt V-spring revolver, the forward cylinder travel is limited by its neck bearing on the yoke collar, which provides more surface than is available in a S&W Hand Ejector. This results in slower wear under stress from firing. Unfortunately, the timing of the Colt V-spring action is far more sensitive to “end shake”, or fore-aft cylinder play, than that of the S&W. The hand and ratchet interface in the Colt V-spring action requires closer fitting than that in a S&W. By contrast, the coil-spring action of Mark III Colts configures its hand and ratchet similarly to the S&W, where the tail end of the yoke tube bears against the rear of the cylinder well, leaving a small bearing surface that is susceptible to wear but amenable to easy correction. There are two methods of correcting this wear. The factory technique stretches the yoke tube by impressing a groove therein with a device resembling a tube cutter with a rounded edge. The alternate method is to add spacing shims at the bottom of the cylinder well. In either case, if the problem is due to wear, the yoke tube tail and the bottom of the cylinder well must be dressed to restore full contact with a square profile. Typically, the S&W action will loosen up at the interface of the yoke tube tail and the cylinder well bottom sooner than at that between the ratchet and the frame. The MR73 action can be permanently shimmed at this point with Power Custom stainless steel cylinder endshake bearings sized for the S&W K-, L-, and N-frames, and available in 0.002" and 0.004" thicknesses.
    Once the cylinder has been properly fitted, the barrel-to-cylinder gap must be adjusted by grinding or filing off the rear of the barrel. If the gap is too large a new barrel must be fitted, or the existing barrel must have its shoulder machined down and be set back a thread. The Dan Wesson, with its two-piece barrel patented in 1967 by Karl R. Lewis, allows for a cylinder gap adjustable with hand tools. Jean Beltzer, the manager of Manurhin’s small arms manufacture, employed a similar construction in the 1985 MR73 Convertible system, which allowed for switching between centerfire and rimfire chamberings with an ingenious rotating recoil plate design that reoriented the frame-mounted inertia firing pin, protected by European Patent number EP0278795B1. Beltzer’s additional invention, protected by U.S. Patent number 4793084 and European Patent number EP0251935A1, allowing for the reduction of the barrel-to-cylinder gap at the moment of firing, remains to be commercialized.
    Manurhin extended its product line with single action bullseye “Match” revolvers chambered in .32 S&W Long, .38 S&W Special, and .22 LR, respectively in 1980, 1981, and 1987. Following a suggestion by Jacques Trausch, the cylinders of revolvers chambered in .32 S&W Long and .22 LR were shortened, with a corresponding extension of the barrel through the frame window, in order to limit the free travel of the bullet in the chamber.

[3] In spite of its superficial resemblance to the S&W design, the MR73 lockwork differs in several significant aspects. Thus the hammer fall angle on the Manurhin is a short 42 degrees, versus 54 and 58 for post-war and pre-war S&W, and 60 for the V-spring Colt. The extra mainspring weight required by the short action of the MR73 is limited by the efficient construction of its rebound slide, based on a 1977 patent by Gilbert Maillard. It moves back and forth on five rollers, propelled by a flat trigger spring externally adjustable for preload, rather than the small internal coil spring of the S&W. Four of the rollers serve to reduce the friction that impeded reciprocal motion, whereas the fifth roller bears on the end of the trigger return spring, to vary its mechanical advantage and stack the hammer-cocking leverage for a more linear double action trigger pull. In the course of pulling the trigger, the contact of the trigger return spring moves upwards on the central roller, with the horizontal component of the radial force exerted by the spring upon trigger through the rebound slide remaining constant throughout its travel. This arrangement is optimal for a smooth double action trigger stroke, ending in a sudden hammer release. Consequently, the trigger pull can be fine tuned on the MR73 by preloading the mainspring and the trigger return spring independently.
    The MR73 also differs in the quality of its construction, with every part machined from high strength steel forgings. S&W originally designed their lockwork to be made from low-carbon steel that was then case hardened. In GIGN service, none of their revolvers could handle the daily practice regimen of 150 rounds of Norma 158 grain .357 S&W Magnum ammo. The MR73 was tested with this ammunition. Its torture test was abandoned without appreciable wear after firing 170,000 full power Norma .357 rounds. Numerous published tests witness this capacity. By contrast, even the N-frame S&W .357 revolvers are generally good for less than a third of this life span.
    Colt double action revolvers are defined by the “bank vault lockup” of lockwork that uses an impinging hand pushing the cylinder against the bolt to lock it solidly in place at the moment of pulling the trigger all the way back. This method of construction is intrinsically more dependent upon hand-fitting, owing to the need for ensuring a nil clearance at lockup. It is also more maintenance-intensive, owing to the recoil forces thrusting against the impinging surfaces with each instance of firing. And yet it is unlikely to result in greater accuracy, given the results achieved by the looser S&W-style lockwork that allows the cylinder to rotate slightly when locked, with the chamber being momentarily aligned with the bore by the bullet entering the forcing cone. Here, for example, is a comparison between four .32 target handguns, a custom S&W M16, a stock Manurhin MR32, a SAKO TriAce, and a Walther GSP, published in the Swedish magazine Vapentidningen № 4 in 2007. Note that the Manurhin MR32, built on what amounts to an improved S&W Hand Ejector action, performs at least as well as fixed barrel autopistols. This suggests that the slight edge in accuracy sometimes seen in Colt Pythons shooting against their downmarket S&W competitors is more plausibly attributable to the quality of construction than intrinsic superiority of the “bank vault lockup”.
    The MR73 trigger action is of two types. The change took place in 1977, and cannot be distinguished without removing the sideplate. That was the switch from the “safety pin” music wire spring (le ressort genre épingle à nourrice) tensioning the hand in an early model of the MR73, to a flat spring that performs that function in the later models.
    American-style handgun shooting reached Europe in the Sixties with Raymond Sasia, a judo instructor employed as a bodyguard by Charles de Gaulle, who was sent to study the shooting techniques of the FBI Academy in Quantico, Virginia. He returned to France with an FBI certification and founded CNT, a shooting school in Paris that taught range officers, French nationals at first, then foreigners. The latter, upon returning home, taught new range officers. Thus the method “Sasia” promulgated FBI’s revolver shooting techniques throughout the Western world.
    One of the drills was the 7 meter fast response. It goes as follows: the gun is loaded with five .357 Magnum rounds and carried in a belt holster; in the pocket the shooter has 5 more loose .357 Magnum rounds. At the sound of a whistle, the range officers are given 25 seconds to fire the ten cartridges at the target located 7 meters away; the instructors have only 20 seconds. It turns out that in order to have the time to reload and fire the other five rounds in the allotted time, the first 5 rounds must be fired in less than 5 seconds to satisfy the requirements; no more than 3 to 4 seconds can be allowed for top placements.
    At this rate of fire, in the original MR73 design that tensioned the hand with a “safety pin” spring, the hand did not have enough time to return to the ratchet and rotate the cylinder, and consequently it slipped over the ratchet, causing the firing pin to strike the primer of the last expended shell. Owing to the inertia of the hand thrown backward by Magnum recoil forces, the music wire spring was not strong enough to return it forward in time to engage the teeth of the ratchet of the ejector and ensure the rotation of the cylinder. Manurhin’s engineers were slow to understand why this happened to some police shooters, because the factory testers never managed to replicate the malfunction. Shooters training with S&W M10, M13, or M19 under similar conditions never experienced this malfunction.
    Another action change took place in the Eighties, when the early needle-nosed hand that normally mates with an early “insular” pattern hexafoil ratchet, was supplanted by the late chisel-nosed hand that can only mate with a late “star” pattern hexacanthous ratchet. The cylinder hands tensioned by a flat spring are of two types, the needle-nosed hand designed to mate with the early hexafoil ratchet, and the late chisel-nosed hand that can only mate with a late pattern hexacanthous ratchet, as described above. The early pattern hand can operate with a late pattern ratchet; but the late pattern hand can only mate with a late pattern ratchet.

An early 6" MR73 L9530 fitted with a hexafoil ratchet, as shown in the middle, has been factory retrofitted with a 9mm cylinder of the Pilorget design, equipped with a hexacanthous ratchet, numbered en suite. A later production revolver L040980, fitted with a hexacanthous ratchet, can be seen on the right.
    The only change in the trigger action during MR73 manufacture is the switch from a tapered, forged trigger spring, to one of constant thickness, stamped out of sheet metal. The latter spring is stiffer and less liable to collapse as a result of over-tightening. In order to set the trigger pull, start by regulating the screw (#135) preloading the mainspring (#313) that tensions the hammer (#126) with a flat screwdriver. Your goal here is to ensure reliable ignition with every kind of ammo you use. Then adjust the trigger spring (#157) on the rebound slide (#114) by turning its adjusting screw (#120) with an Allen key. Your goal here is to make sure that the trigger resets after each shot. In this way, the double action trigger stroke can be readily adjusted externally, ranging between a light pull weighing under 7 lbs, and a heavier feel with a stronger reset allowing for a “live trigger” technique with faster cycling at a greater trigger cocking effort, as preferred e.g. by Jerry Miculek.
    The trigger on the models fitted with adjustable rear sights is equipped with an Allen screw that serves as an adjustable trigger stop. A factory trigger shoe is available for optional fitment.

[4] The MR73 has a uniformly dimensioned, compact grip frame in a true round butt configuration. There are two main variants of factory MR73 stocks. The original Police/Defense variant follows the contours of the grip frame except for filling the gap behind the trigger guard in the manner of the pre-WWII S&W grip adapter, and allows external adjustment of the trigger pull at the front strap of the grip frame. These stocks are correct for all early revolvers with serial numbers appearing on the butt of the grip frame. They are very comfortable to hold, but require a very firm grip for controlling the roll under recoil, and provide little feedback for a consistent handhold. The Gendarmerie/Sport, introduced a few years later, wrap around the front strap to create shallow finger grooves, and extend past the butt in a squared configuration, exposing the backstrap. They are more hand-filling and offer better indexing.
    Aftermarket stocks include custom-made anatomical Rink grips and a full range of service and target Nill Grips. The average hand is best served by the smooth, oil-finished, symmetrical, finger grooved, round butt, open back Nill configuration. Rubber grips for the MR73 were first furnished by Jacques Trausch. They could be had with or without a shelf at the bottom, retained by an oversized flat head screw that can be removed and replaced with a coin, and containing an Allen key for adjusting the trigger spring tension. Early models featured a slot for viewing the serial number on the butt. Chapuis took over their production following the death of Jacques Trausch in 2012. ARMT grips, designed and manufactured by Robert Talamoni, are backed by steel plates and retained by a conventionally sized flat head screw.

To sum up our analysis, the MR73 action represents a great improvement over its S&W prototype. The short action of the MR73 reduces lock time for improved accuracy in single and double action. Owing to its travel and tensioning on rollers, the rebound slide is much more supple on the French revolver, ensuring a linear double action trigger pull. The MR73 allows for much easier and more flexible mainspring and rebound spring tuning, easily switchable between the minimal tension ensuring reliable rebounding of the trigger in cycling the action, and a highly tensioned setting suitable for a “live trigger” double action rapid fire technique. The cylinder stop is much wider and stronger on the MR73, limiting the peening of the cylinder notches in rapid double action cycling. Lastly, the yoke pivot configuration eliminates the likelihood of opening up its support in the frame by swinging the cylinder open and closed.
    Most importantly, the dimensions of the MR73 frame represent an ideal midpoint compromise between the .38 caliber sizing insufficient to contain prolonged firing of full-bore Magnum loads, and the .41 caliber sizing that unnecessarily weighs down the revolver frame. The resulting sixgun remains unexcelled as a sidearm for serious social work. It is our good fortune to enjoy its ongoing production by Chapuis Arms.