The answer to all dedicated, high-performance, precision competition rifle design considerations is tied up in the perplexing technical question of why the .308 Winchester calibre is the ballistic performance benchmark and not the .30-06 cartridge. We will explore this along the way.
My approach/doctrine: First, design (select) the bullet and cartridge, then design and build (assemble) the gun – fit for purpose and dedicated to the specific task and required performance parameters (for your dedicated small arms system). Most of us buy a commercial rifle and then try to match a ballistic solution to it, with perhaps a degree of modification to the small arms platform (rifle), such as different chassis, free-bore dimension improvements, etc, in the pursuit of “fit for purpose” (dedicated purpose parameters). Historically, we commit a great deal to conventional precision reloading as an accuracy substitute. Before we start off with cartridge (bullet/performance/potential) selection, let us first consider the individual precision matrix elements in which this bullet has to perform, such as the barrel:
What is a “HUMMER (of a) barrel”? It is a nickname for a mythical, super-magic, “accurate” hole-in-one barrel. You might come across it perhaps once a lifetime if the thunder gods see fit to bestow such grace on you. But mostly, it is merely an urban legend, something we all hear of, but which sadly evades us common sport-shooting athletes. Fact: There is no magic barrel that can accurately shoot any bullet, irrespective of design, weight, length and speed. Principle: Unlike bullets, barrels are remarkably sensitive.
We need to think of a barrel as a very sophisticated pressure vessel. The design, materials and manufacturing process should be determined by the dedicated purpose. There is no general barrel for precision sport shooting. Every aspect should be very carefully considered and selected for the dedicated purpose, weight, twist rate, yield strength, thermal load and “fatigue life”, which is a major consideration in South Africa regarding legal barrel replacement for seasonal competition league durability.
Yield strength properties: Considerations of the barrel material at operating temperature, as the various materials, such as 4140/4150 steel, chromium-molybdenum alloy, the popular precision choice 416 (SS) stainless steel and custom 17-4PH, act quite differently. The conundrum of the two competing properties: Better “ultimate strength” required for pressure and less “elongation of failure” is the trade-off on reduction in fatigue life. The material based on operating temperature for a machine gun (short bursts), ELRS rifle and a big-bore hunting rifle is vastly different. Treatments: Examples are nitride-treated barrels (melonite, isonite or QPQ considerations) as opposed to a chrome lining, which also drastically increases the lifespan, especially in semi-automatic rifles.
Barrel “thermal load” properties and capacity are a complicated subject and generally misunderstood. The heat generation tempo of the barrel inside surface is considered 1 000 degrees Fahrenheit per millisecond or 1 million degrees Fahrenheit per second! It translates to 555 500 °C or 555,5 °C per millisecond. In terms of 1 000 rounds barrel life, at 1 millisecond per bullet – 1 second total barrel life time. Speed kills barrels; to generate high speed, higher pressure and thus higher heat is required. High BC bullets are the main culprit. In my opinion, exceeding 3 000 fps has an unacceptable exponential barrel life degradation rate. Fouling is also directly related to heat, melting bullet surfaces and leaving a residual layer.
Small rifle barrel calibres start with an oversized blank metal rod (blanks) and a process of deep-hole drilling (so-called “gun drill”). Drilling a long, thin hole (deep-hole drilling) in a steel tube has its challenges, and various specialised techniques, such as counter rotation, are used. The bottom line is all barrels are drilled skew in an arch to some degree. I am 110% convinced that the “reverse” gun drill process is critical – to drill from the muzzle in the direction of the chamber and not from the chamber side. The actual chamber should be aligned and reamed with the actual hole (bore) section at the intended neck and free-bore sector, and certainly not the barrel centre-to-centre average alignment. I do not see manufacturers doing this reverse-direction drilling as a standard manufacturing protocol and process, and most gunsmiths are simply clueless. The long (drill start), straight section should be the muzzle end, and the arch should be clocked in a twelve o’clock high position when seated to ensure muzzle pointing precision. With the conventional drill direction from the chamber, the discontinuity of the whole straightness (curbing/arch) will result in a lateral load increase, with bullet movement during acceleration altering the plain vector, i.e pushing the muzzle off point (aim).
On a different “node”, it is easy to understand – in the lateral movement (plain vector) context – why, for example, standard deviation (SD) in bullet speed plays such an important role in “accuracy”. The reverse process is critical for barrels over 24″ for further rifling: Single-cut point, button or broaching, but cold hammer forging seems less affected, and perhaps the reason why hammer-forged barrels are traditionally perceived to be more “accurate”. I prefer a broached manufactured barrel.
How crooked is a too-crooked barrel? A barrel may measure up crooked, by all means, but the shooting results of a “crooked barrel” in terms of a specific application are very dependent on the type of design geometry and actual material of the bullet, for example, a hunting-calibre barrel in .30-06 out to 200 m. It will shoot acceptably with one particular bullet (construction) only. It is important to understand that composite bullets (with plastic inserts, air pockets and cavities, light core materials, etc, to change the centre of gravity and pressure point) are less affected by smaller barrel inaccuracies. It is obvious and practically observable from average sport-shooting improved results compared to just 10 years ago. The jury is still out on the performance of monolithic bullets due to the high-variability performance rate to precision barrel dependability. I do not prefer monolithic bullets; only as an alternative.
Regarding land and groove dimensions, rest assured that the groove dimensions are usually OK and will leak pressure, period. The dimensions and consistency of the lands, which imprint (plastic dispersion) on the bullet, are critical for bullet stabilisation, or else the bullet will “wobble” – the transfer motion of the bullet off-centre. There is a major relationship between the lands and bore width aspect ratio, the principle being that the wider the land, the less possibility for the bullet to “tip in bore”. It is obvious that a slower twist rate would resolve in a lower angular rate of muzzle yaw, resulting in better groups. Standard square, equal number 4 or 6 grooves are just fine, with the driving surface sheer vertical to the ID (internal diameter) of the barrel. Polygon-grooved barrels have shown potential for smaller groups, faster speeds and definitely less fouling, but this remains a complex discussion.
Constant twist, meaning the barrel has the same twist rate from the start (forcing cone) to the end (muzzle), is standard for small calibres but questionable for small calibres with excessively long BC bullets. Interestingly, in medium calibres (14 mm to 30 mm-plus), most barrels are of the gain-twist type, by some mathematical model, where the twist rate increases towards the muzzle to overcome the start torque spike, especially in bullets with drive bands. The problem with the latter gain twist is more wear on the bullet-bearing surface, which could be problematic with laminar flow and drag at ELRS (Extended Long-Range Shooting) distances entering the transonic and subsonic speed range.
“Barrel wip” (I like to call it the “torque twist effect”) scientifically refers to the “bending mode”, i.e. the (i) Fundamental Frequency and (ii) 2x Harmonic modes. The bending frequencies practically never line up with the bullet spin rate in usable barrel lengths for a particular calibre diameter. Given a general straight barrel profile in a .30 calibre, the barrel must be at or exceed 35″; impractical and ballistically unsound. Therefore, in my summation, the controversial statement “Harmonics” is (actually) not a factor, thus “nodes” are practically non-existent. The actual issue is “forced vibration” due to lack of bore straightness when the load on the bullet in the barrel causes barrel vibration. If the barrel bore centre line is not drilled coincident (and/or at an arch) with the OD (overall diameter) of the barrel, the bullet creates a bending moment, where the bullet is off the centre line, causing a lateral load displacement of the bore (in that direction) as the bullet passes on, the barrel moves back and forth, i.e. “forced vibration”. Forward barrel support points, like a full stock on a .303 Lee-Enfield, are exactly the attempted countermeasure for forced vibration. Competition barrels have to be free-floating, potentially drooping. Think of gravity as a result of curved space-time and not a force that pulls or pushes your barrel or bullet down, just to spice things up.
Muzzle brakes, barrel tuners and silencers all create a bending moment of the barrel, period. If fitted correctly, it should not influence grouping, only the point of impact. I prefer a small OD recurve silencer, and a good argument can be made for a silencer rear end firm fit pressure on the barrel to control forced vibration. Interestingly, air rifle sleeves (barrels) are sometimes contained and tensile-strained (pulled straight) between the two ends in larger-diameter, hollow tubes of aluminium or carbon fibre.
What about wrapped barrels? The only reason (factor) why you would consider sacrificing the advantages of a solid metal barrel material for an overwrapped composite barrel is weight. Of course, light weight directly influences recoil. Let me explain bending stiffness: The composite wrap effectively has the density of plastic and the elastic modulus of aluminium. If the composite material does not have the same bending modulus as the metal tube, it will result in targeting concerns effecting pointing consistency (variability). Also consider the thermal load properties and yield strength of the various combined materials. The barrel metal main tube must be insulated from the carbon (semi-conductor) wrapping to prevent corrosion.
Barrel geometry considerations: Perhaps the most important for the end user are free-bore diameter and length and forcing cone angles; the shallower the angle, the higher the (peak) push forces required, but it has considerably lower push force variability. This is critical to precision (internal ballistics) repeatability because repeatable (start-up) pressure is the holy grail to making the pressure time curve repeatable, which renders consistent muzzle velocity (bullet speed) and consistent forced vibration – critical for any small rifle calibre. The preferred modern angles of forcing cone lands angles are between 1 and 1.5 degrees. The idea is to run bullet seating spaced between 0.003″ and 0.006″.
Consider the thermal dissipation of the carbon fibre wrap with a very low coefficient of thermal expansion, as opposed to the metal. Carbon fibre is not magic; compositions with carbon nanotubes, graphite and graphene should be explored. Composite material has a low impact resistance, which should be a consideration in a tactical, military or active sport-shooting environment, such as PRS. Please understand that the composite material is not an additional, supernumerary contributor to the metal qualities but an auxiliary substitute to replace the reduced (lost) barrel metal in order to regain some of the original properties and performance. For example, the stiffness of a 416 SS barrel (30 mm diameter, .300 calibre) does not increase by reducing the metal to 20 mm and overwrapping it with 5 mm of composite materials. For a 12-16 mm hunting rifle barrel, you need to retain a low weight but want to increase better 10-shot grouping regularity; the correct composite overwrap can assist, best perhaps in a varmint rifle configuration. On the hunt, the first two shots are usually all that counts, but in match shooting with longer shot strings, a heavy barrel is preferred, and a heavier, steady gun is usually better.
Overwrapped barrels make a lot of sense in high-end air rifles, such as the Walther LG 400 series, with an incredibly thin-walled tube sleeve that has to be stiffened. The lead pellets or modern slugs practically do not temporarily bulge/deform the barrel as they pass. I think overwrapping techniques have a design and function application where purposefully small(er) diameter barrels are manufactured because it is easy to remedy centre-line bore straightness after rifling and then wrap it up to maintain straightness.
Practical barrel life fatigue – percentage wear versus deployable distance – on my rule-of-thumb scale: 0.5%+ wear < 3 000 m / 1%+ wear < 1 500 m / and- 1.5%+ wear < 1 000 to 500 m of the original optimal barrel precision at application (deployment) distance. It looks implausible, but remember that our conventional ballistics reference and experience are with hunting rifles benchmark at 100 m hunting to 300 m and 1 000 m Bisley and everything in between. When you consider the actual number of competitive precision available shots, it makes sense to use a lube like HBN2 to extend barrel life by 100% or more. As a reference: I shot and have seen MillSpec .338 LM at 2 000+ shots with every load and bullet possible, and cleaned perhaps 5 times, it would cut 2 holes and a reasonable 5-shot group at 100 m. However, at 1 000 m problems are evident, and at 1 mile 2 shots under 1 MOA and 3 shots exceeding 2-3 MOA. As opposed to commercial tactical rifles used in competition, shot with mild loads and well maintained, doing excellent at LRS (Long-Range Shooting) past 1 500 shots at 1 mile, sub-1 MOA. The absolute ability of the barrel (lands) to spin the exceptionally long ELR/ELRS bullets is vitally important for forced bullet stabilisation at distance since these high BC bullets are inherently dynamically unstable.
Barrel failing modes: Perhaps the most common and re-occurring is “forcing cone wear” (referred to in Afrikaans as die keel erodeer/brand uit), extending the free bore. Symptoms: High dispersion and drop in muzzle velocity because as the bullet unseats from the brass case, there is gas blowby, losing pressure before the bullet engages the lands and probably tips the bullet. The practical shooting remedy is to recalibrate the seating depth to the lands with every reload cycle. HBN2 is a great solution to reduce start-up friction and the start-up pressure spike. Secondly, actual metal fatigue in terms of critical crack-depth cycles (just before the barrel bursts open). In a modern hunting rifle or competition-style barrels, the actual safe lifespan exceeds precision, thus barrels are usually replaced well within safe critical-crack-depth range. The warning should be extended to the exceptions such as old firearms and highly (over-) pressurised modern firearm barrels. Of course, the biggest problem for ALL barrels, perfect Hummers or not, is that it does not exist and functions independently, and it is highly dependent on the ancillary components (action, trigger, stock, cartridge, etc.), a harmonious relationship that largely defines precision transferring into accuracy down range. The message: The best Hummer barrel may not be the right combination. In the next edition of this series, we will discuss precision matrix elements such as twist rate, bullets and brass. Till next time, safe shooting!