Over/Under Mechanism Design
* NEW for 2025 *
ExpulseTM System
The epitome of functionality for the Over-Under
Ergonomic design and functionality have been perfected by way of the patent-pending ExpulseTM system, a dual-mode expulsion system offering a no-compromise solution between the environmentally-friendly use of extractors with the time-saving, automatic ejection of shells. ExpulseTM offers the shooter the chance to extract or eject after every shot without having to switch between modes as both are always active. Upon barrel opening, the mechanism cocks both hammers at 40o where it comes to rest on a well-defined detent position maintaining the barrels open. At the detent the shooter may manually extract the shells, load the gun and continue shooting. In the event automatic ejection is required the same opening procedure is observed to the detent position and then continued past to the end-of-stroke, hard-stop at 45o where ejection takes place. Do what you want when you want. The ExpulseTM system is accomplished employing only one component housed within the fore-end iron where simplicity rewards reliability.
Another important point of doublegun design which is seldom, if ever, discussed is that of the communication between battery and ejector work, or everything associated with the opening/cocking of the weapon. During this phase we want to operate a gun that is smooth to open with the lowest possible force required so as to keep physical fatigue at bay, especially in the shoulders. To accomplish this ExpulseTM employs cocking rods that contact each hammer at the furthest distance away from said hammer´s axis of rotation. This reduces the force required with the tradeoff of a longer stroke. This means that the cocking rod stroke is greater than average, which in turn implies that the cocking geometry on the forend side must be capable of providing said stroke by way of novel camming. This issue is overlooked in many designs and the result is an opening of the weapon that is ¨gritty¨ with higher forces required to actuate.
* For those manufacturer´s building side-lock over-unders ExpulseTM offer the opportunity to differentiate your guns from the rest of the competition. For those manufacturer´s that are trying to break into BEST quality gun status ExpulseTM is a must-have feature. Set yourself apart from the competition or be destined to failure. Never forget, fortune favors the bold.
The Mattias TM range of over-unders begins with the Pegaso TM (Pegasus) line of dedicated fowling guns for instinctive shooting. Pegaso´s TM have been designed specifically to match, if not exceed, the lively and composed dynamics the venerable side-by-side is renowned for.
The inherently low action height of the side-by-side barrel layout combined with optimum weight distribution afford this configuration the quick and smooth yet controlled swing characteristics required for the instinctive shooting of wild-flushing game. Under such conditions, fast and stable mounting of the weapon are requisite for timely and accurate shooting. The side-by-side sets the bar for wing-shooting very high indeed, leaving a standard of excellence to which all other variants must be measured. This high standard was the fruit of decades of design iterations at the hands of passionate gunsmiths who, unlike today, were brave and aggressive in their pursuit of excellence. By the end of the 19th century the side-by-side had reached the pinnacle of double gun performance, especially reknowned for their prowess in the game fields. The best quality over-under has not experienced a similar development path. The Great War and WWII did their part to degrade both talent and demand, ushering in a dark age in development among the upper echelons of the gun world. This was further exacerbated by an apparent lack of interest in perfecting the designs once markets improved. The current state-of-the-art in over-under design is much the same as that brought forth by Boss over a century ago with almost no significant changes, if any at all. The same hefty actions and angled firing pins, among other compromises, that existed in the noteworthy designs from both Boss and Woodward are still present in best quality over-under´s today. As such, traditional, sidelock over-unders seldom see heavy use as owners are reluctant to subject the crown-jewel of their collection to certain deterioration, relegating them to the role of mere exibition pieces. The few intrepid owners who put such guns through their paces, so to speak, learn full well the shortcomings of these designs. In fact, over-under guns costing one-tenth the price are usually more reliable.
Acceptance of the over-under by many game-gun aficionados has met with resistance on account of the increased weight and less than ideal handling this barrel configuration is typically associated with. Original over-unders during the late 19th century had locking mechanisms so cumbersome that it was commonplace to see guns over 7-1/2 lbs. instead of the 6-3/4 lb., or thereabout, ideal for a 12 gauge field gun. It was not until the early 20th century that Boss patented their design for an over-under who´s lock-up hinged on trunnions mounted on either side of the action instead of the early, side-by-side inspired under-lug. The results did much to finally make the over-under a contender for the hearts and minds of wing-shooters albeit never taking over the throne occupied by the side-by-side, a reign which it enjoys to this very day. Even best quality over-unders today fail to match the weight distribution and mounting characteristics of their ¨sibling¨.
Designing a firearm destined for hard use in the gamefields and/or competition circuits with utter reliability all the while incorporating new solutions to cure old ailments meant starting from zero. This ¨fresh sheet¨ design did not allow one, single concession on any of the critical mechanisms/functions associated with over-under guns. Including all options offered (barrel selection, Hybrid-TriggerTM, ExpulseTM ejectors), the design incorporates no more than 12 moving parts, instilling maximum levels of reliability, performance and safety, all for a fraction of the price sidelock over-unders trade for today. Among the multitude of improvements employed in all Pegaso´sTM the following represent the key features which distinguish the design from the field. When combined, the result is a robust weapon that can be handed down from one generation to the next, paying homage to your shooting heritage with a fowling gun composed with excellence in mind.
WEIGHT DISTRIBUTION
Weight and it´s distribution were carefully mapped out on the lowest frame possible for the gun which was ensured by a patented lock-up that requires no more height required than that of two shells stacked vertically. Each gun is built on a scaled action frame conferring in all PegasoTM guns the lowest action height in their respective gauge. All remaining functions dealing with cocking and ejection are thereby relegated to the lowest point on either side, perfectly aligned with the sidelock while also providing ballast for the weapon. Additional steps include designing the forend iron to have the metal exposed on the underside or lowest point, and ensuring it´s switch-gear are behind the leading hand. In short, the only steel allowed beyond the leading hand is that of the barrels while the only components allowed above the mid-point of the action frame are the aptly named top strap, top lever and top barrel, nothing else. Although these changes make little difference in and of themselves together they combine to differentiate a merely “good” handling gun from a sublime one. A case of The Aluminum Lugnut Syndrome if ever there was one.
6- POINT LOCK-UP DESIGN
The same lavish attention to detail lent upon the weight-distribution has also been furnished on the mechanical design ensuring that the guns will remain active generations from now as all PegasoTM guns, albeit lightweight, are built on heavy-duty frames based on eccentric-arc raceways capable of handling their 20,000 psi (1.320 bar) proof cartridges indefinitely (10×7 cycles) without coming off the face as a result of mechanical sag due to repeated, shock loading. This can only be accomplished employing the finest tool steels available. This ensures the gun´s operation with ultra-high-pressure, Steel-shot cartridges in the event lead shot is outlawed in the future. The patented lock-up provides a total of 6 locking surfaces whose combined area amounts to approximately four (4) times that of a standard Boss inspired designs.
Apart from the increased robustness the novel lock-up offers it´s key design feature is the eccentric-arc raceway geometry responsible for locking and hinging, separately. Standard, trunnion mounted barrels (representing almost 90% of over-under´s to date) employ the small surface area of the actual trunnion for both hinging and lock-up. A weak point of the trunnion design is the adhesive wear inherent in such a high-pressure point after repeated opening/closing which is then exacerbated by the fact that this same area is responsible for a portion of, if not all, the recoil forces. All in all, it is too much responsibility for too small a surface area. The end result of which is a loosening of the joint due to pin wear and plastic deformation over time. The Lastra lock-up´s key feature is in the separation of locking and hinging functions. The barrels hinge on the arc raceways using the concentric/parallel surfaces (A) and (B) to guide the motion. As we can see in the diagram below, once the barrels brake open the locking surfaces (C) lose contact with the action where no sliding equates to no wear. It is the separation of these two functions (opening & lock-up) that distinguishes this design from the competition. Wear on the ¨opening¨ surfaces (which can only be the result of deliberately abusive maintenance practices) would not alter the ¨lock-up¨ surfaces, i.e. the gun would still be safe to fire. If one straightens the arc upward one will come to see that the design wedges the barrels into the action when closed thereby converting the barrel-group and action into one, solid structure capable of the even distribution of recoil forces but more importantly nullifying any and all vibration.
IMPROVED BATTERY & SIDELOCK DESIGN
Sidelock over-unders employ locks copied directly from side-by-side designs and result in a multitude of trade-offs which keep the guns from enjoying optimum levels of reliability in the field. Most notably are the acute angles employed to ensure primer ignition. The sudden impact the firing pins endure represent the worst-case scenario for a steel component as the high rate of loading (strain rate) reduces the materials ability to shrug off sudden fracture, with no warning whatsoever. This issue is exacerbated by the side loading the strikes provoke. The result of which are bending loads that are responsible for crack propagation leading to inevitable breakage and downtime. Yet another issue associated with traditional designs is the dragging of the pin tip that is experienced as it is struck. This drag induces adhesive wear on the pin resulting in the loss of material and the sharpening of said tip. The result of which are light primer strikes and perforation in some cases. All in all, these are unacceptable features in what are the most expensive firearms one can find on the market today.
PegasoTM guns are built on an improved sidelock design allowing both firing pins to be kept in-line with the primers and thus avoiding all of the aforementioned shortcomings. To accomplish this simple transfer-bars are used to relay the hammer strike to the firing pin. In the case of the bottom barrel the hammer swings from the top of the sideplate ensuring efficient use of main spring energy. This arrangement also allows for a controlled stroke of the pin (something impossible in traditional designs) which ensure that after firing the pin does not remain in a buckling condition (bending load) as it is caught solely between the hammer and the primer. Rebounding hammers avoid this issue but introduce other shortcomings, specifically, the larger hammers required to maintain sufficient energy that necessitate taller sideplates and action heights. In the Pegaso TM design the retraction of the firing pins after ignition are the responsibility of the top-lever spindle acting as a cam to mechanically seat said pins by directly actuating them. Naturally, this design feature is facilitated by the alignment of the firing pins. In this fashion whenever the gun is opened the pins are pulled into the frame positively while offering direct feedback in the event excessive fouling begins to affect their travel as opposed to mere return springs. This arrangement also alleviates timing issues that can arise due to wear and/or deformation of the cocking-rod mechanism that would affect pin retraction during opening of standard designs. As such, leaving the cocking-rod mechanism to the sole duty of arming the hammers, where overdraft ensures operation in light of wear. Yet another advantage this arrangement offers comes by way of passive safety in the event the top lever does not close fully. This can happen in the event foreign material, fouling, etc. does not allow the gun to be fully closed, compounded by a shooter that pulls the trigger, unwittingly. In this scenario, the firing pins are blocked by the spindle if the top lever is not closed to at least 75%, avoiding primer ignition. The sidelocks will be completely pinless with both being released via the SAFE button being actuated in reverse a la Perazzi. Pegaso TM provides the best of all worlds with no trade-offs whatsoever.
No discussion of sidelock design would be complete without making mention of the mainsprings and their design considerations. The mainspring of any hammerless shotgun is the component that suffers most. Hammerguns, by virtue of having more space internally (i.e. longer spring length) aided by the higher mass the external hammer affords do not require the spring to be so highly stressed and generally last much longer as a result. Mainsprings on hammerless guns, however, are a different matter altogether. They are typically employed with a hardness of between 50-54 HRC and operate at stresses of approximately 150-170 KSI. Most steel components have endurance limits of only 100-110 KSI, meaning that the mainsprings will eventually fracture due to fatigue. As such, the design of the mainspring is of critical importance, so much so that the sidelock needs to be designed around an appropriate spring for the gun, not the other way around. Only then can you arrive at the best solution possible in ensuring a long interval between changeouts. The majority of mainsprings on sidelock over-unders will last between 15-30,000 cycles (shots) due to wear and/or fracture. PegasoTM guns use V-springs made out of Uddeholm-Boehler´s outstanding 460 VMR at 55 HRC which are properly pre-stressed and shot-peened to ensure a minimum fatigue-life of approximately 150 KSI conferring a minimum life span of 350,000 cycles (shots) per spring, by far the greatest replacement interval on any over-under on the market. The V-springs have been designed specifically for the gun to be carried open, even stored in the compressed state, without having to worry about potential loss of spring force due to mechanical sag (low temperature creep).
TRIGGER OPTIONS
Bringing the battery into play is the responsibility of either a single, mechanically operated trigger (barrel selection optional), a traditional dual trigger or a Hybrid TriggerTM employing a dual trigger scheme whereby the front trigger acts as a single trigger operating both barrels, if one so desires. Triggers can be specified in either field or target specification. Field triggers offer a 1mm take-up to a wall where a further 1mm @ 4lb pull fires the gun. Target triggers offer no take-up requiring the same 1mm travel @ 3lb pull to fire. Detachable trigger groups are available upon request.
BARREL GROUP (Uni-BlocTM System)
The barrel group is based on the patent-pending Uni-BlocTM barrel assembly process which employs a single forging, including the chamber, for the upper barrel which is then silver-soldered unto a lower barrel assembly composed of a barrel/mono-bloc assembly. Using this method a hard tool steel can be used for the lock-up on the lower barrel while maintaining an unobstructed barrel contour on the upper barrel. Uni-BlocTM barrels represent a win-win for both aesthetics and mechanical integrity as no concessions are made on the strict criteria a properly constructed barrel-group demands. This is in stark contrast to the marketing gimmicks that constitute ¨billet¨, one piece barrel-assemblies. Barrel material used is heat treated, rotary-forged, double-vacuum (VIM/VAR) melted CrMoNi Steel pressure-tube forgings with high nickel content, as barrels should have. All barrel bores are chrome lined under a nickel base and are approved for steel shot. All ribs are professionally laser-welded, ensuring they will never come apart at extremely high temperatures. All guns proofed at 20,000 psi (1.320 bar) by the Spanish Proof House.
DESIGN THEMES
Mechanically, the design has been based on purely visceral principals, that is to say that all components and mechanisms have been designed as if Nature would have done so, where simplicity and efficiency are paramount. Aesthetically, the firearm should be presented in elegant fashion paying homage to natural forms maintaining balanced proportions throughout. In the case of an over-under variant the action shape or ¨detonating¨ should be rakish & grave, commensurate with the tool´s intended use. PegasoTM has been designed to offer satisfaction while shooting, being carried or just admired late at night during the off-season.
TECHNICAL SPECIFICATIONS
Gauge Action Height Average Weight (28¨ barrels, field forend, pistol grip, full length top rib)
12 54.5 mm (2.15¨) 3.00 kg (6 lbs. – 9 oz.)
16 52.0 mm (2.05¨) 2.90 kg (6 lbs. – 6 oz.)
20 50.0 mm (1.97¨) 2.80 kg (6 lbs. – 2 oz.)
28 47.0 mm (1.85¨) 2.60 kg (5 lbs. – 12 oz.)
410 cal. 46.0 mm (1.80¨) 2.55 kg (5 lbs. – 10 oz.)
METALLURGY
All main components on PegasoTM guns are milled out of double-vacuum melted steels of either VIM/VAR or P-ESR forged, billet rod stock. All components are professionally heat treated under vacuum and quenched under 20+ bar of nitrogen. In the case of critical ballistic-tolerant components (action, uni-bloc, barrels & locking bolt) the heat treatment is conducted in 3 distinct cycles, 1) stress-relieved after rough machining, 2) hardened & triple-tempered after final machining and 3) plasma-nitrided or case-carburized.
Proper selection and treatment of steels is of great concern to the maker and no expense is spared in sourcing only the best materials from the finest makers in the world. In particular, the action and uni-bloc are made of proper tool steels instead of the usually employed low-carbon, construction steels used by other makers. The difference in using proper tool steels for critical components is the difference between having a gun that will wear itself away during heavy-duty use and one that will not. Below is a brief list of some main components and there mechanical properties (hardness).
Successful use of tool steels for safety-critical applications demand tight control of all heat treatment parameters that can only be accomplished with modern equipment operated by seasoned professionals. At the very top of that list in western Europe is HAUCK Heat Treatment (www.hauckht.com) out of Euskadi (Basque Country), the only heat treater I trust completely. You may find another outfit as good but none better.
Component Steel Grade Manufacturer Hardness (case & core)
Action (silver) 400 VMR Boehler-Uddeholm 68 & 43 HRC
Action (case color) FNDW Aubert-Duval 62 & 43 HRC
Uni-Bloc 460 VMR Boehler-Uddeholm 68 & 56 HRC
Barrels FDMAW Aubert-Duval 40-43 HRC
Locking bolt Aermet 100 Carpenter Tech. 52-54 HRC
Lock parts K340 Isodur Boehler-Uddeholm 70 & 62 HRC
Forend 460 VMR Boehler-Uddeholm 68 & 56 HRC
Mainsprings (V) 460 VMR Boehler-Uddeholm 55-56 HRC
Firing pins K455DE Boehler-Uddeholm 57 HRC (Tip & Butt) 45 HRC (Body)
TECHNICAL CONSIDERATIONS
This section is devoted to the technical considerations and decision-making process for choosing appropriate materials for the main components of a best-quality shotgun, specifically the action and barrel-group. As such, this section will be of a highly technical nature aimed at engineers, designers and passionate enthusiasts of these wonderful weapons. Here we will explore the pros and cons of design variables ranging from failure criteria, load characteristics, mechanical design to material selection.