Forward thinking
Richard Atkins looks into how the M-word, momentum, will influence the future of shooting and the move away from lead shot
Anyone following the complexities of the ongoing move away from lead shot and introduction of ecofriendly wads will have noticed the widely held concerns that steel shot might not provide all the performance we seek. There is a seemingly common misconception that I feel may be worth addressing and that is the part momentum plays in the performance of cartridges we can buy in the UK.
In discussion groups and on shooting forums it is quite common to see pressure being mentioned as the limiting factor regarding what cartridge makers can load for legal sale in the UK.
This is often linked to the higher performance cartridges one can read about in the USA. The question is sometimes raised as to “when is the CIP (the body responsible for setting the levels to which guns here are proof tested and the cartridges must remain within) going to raise the pressure limits to match the American SAAMI rules”.
Ironically, the pressure limits that guns must withstand and cartridges comply with are already higher under CIP highperformance steel than the SAAMI limits. This may confuse some but that’s because there has long been a misunderstanding about the correlation between pressure and velocity among many who shoot.
Higher velocities do not of themselves necessarily mean higher breech pressures (the breech being the point of maximum pressure in a shotgun).
Pressures
Indeed, during many years of testing cartridges the highest breech pressures recorded on the proof reports on factory ammunition I send to the Birmingham Proof House laboratory have been 28g/1oz loads and not, as one might have thought, the heavier 34, 36g and above shot loads. This is another subject that could fill an article, so I will only give the essentials here.
The thing to remember is that guns are proofed, tested and marked to a certain pressure level and so all ammunition used in any shotgun must produce a mean pressure no greater than that appropriate to the level of the gun’s proof.
For a standard 12-bore shotgun, the proof level is 930 bar and the mean maximum pressure of the ammunition must not exceed 740 bar, and yet we have shot loads from 21g clay loads to 40g and more shot loads.
It is due to the incredible flexibility that the nitrocellulose powders can provide, hence we have burning rates to propel such a wide range of shot weights and all within pressure limits.
In a nutshell, velocities achieved are the
result of the total energy produced and not directly of higher pressure.
The higher shot load cartridges achieve their velocities by using slower burning propellants that maintain pressure output for a longer duration without exceeding the permitted maximum. Light loads require a faster burning propellant to burn efficiently under lighter load; hence the lightest shot loads frequently produce higher maximum breech pressures than heavier shot cartridges, but for a much shorter duration.
Relative energy can be depicted in a propellant burn graph. The total area under a pressure/time graph shows the total energy.
Faster propellants have a sharply rising pressure curve and equally fast pressure decay curve; this looks like a ‘spike’ on the pressure graph.
Slower burning propellants have a less steep rising pressure curve and elongated decay curve, and the result is a lower maximum pressure but a larger area enclosed under the graph.
The bulk of the pressure from a heavier shot load cartridge will be produced over twice the time period of the faster burning
powder, hence the less pronounced rise and fall shape of the heavy load pressure graph.
I mention this as clarification for those who fear that our guns – even some of our newer steel proof tested guns – may not be suitable for the velocities produced by American cartridges. As seen on shooting forums this is a belief held by some and, by posting, it becomes repeated by others and gains traction.
It may surprise some but, in fact, the pressure limits to which our CIP steel shot tested (and fleur-de-lys proof marked) guns are proofed is actually a little higher pressure than SAAMI.
The SAAMI pressure measurement methods are different and so it is not possible to precisely compare one with the other but information supplied suggests that while the SAAMI standard pressure limit for guns, for use with lead AND steel shot, is higher than the CIP standard proof for a typical 70mm/2¾in chambered shotgun, the CIP High Performance steel shot proof is at a higher level than the SAAMI limit (which makes no differentiation for steel shot).
CIP pressures for standard proof testing 12-bore 70mm chamber guns is 930 bar with standard ammunition, for lead or steel shot, producing a maximum of 740 bar.
The High Performance steel shot CIP test is at 1320 bar and allows use of cartridges produced with a maximum of 1050 bar (see chart for more detailed specifications).
Therefore it is not pressure that limits CIP steel shot performance; it is momentum.
SAAMI does not apply velocity limits and we have seen some extraordinarily high velocity steel cartridges made by American cartridge makers, some of them claimed to approach 1,700 feet per second.
This is presumably muzzle velocity, not at 2.5m from the muzzle as used by CIP, but is still very fast.
Why momentum matters
Momentum is used by the CIP in deciding the limitations for velocity and shot load that various calibres, shot sizes and shot loads are governed by. See examples in the chart.
Momentum is the amount of energy created by a cartridge. In essence it is the weight of the projectile (in kilograms) multiplied by the velocity (measured at 2.5m from the muzzle in metres per second), which gives the momentum figure in
12 Ns 13.5 Ns 15 Ns 19 Ns 9.3 Ns 11 Ns
X
B
between shot size, pellet energy and pattern density is germane.
Chokes are another topic in themselves. There is no single rule one can obey beyond restricting high performance steel to half choke, unless your gun makers specifically states tighter chokes are OK (as Fabarm does with its very long (97mm) Exis HP chokes).
The HP stands for Hyperbolic Profile, indicating Fabarm uses a hyperbolic profile, not a straight taper as most choke makers do. This allows Fabarm to stay within the maximum angle limits in the CIP specifications but with tighter than half choke restrictions. The particular difficulties come with lightweight English guns, with some having quite thin barrel wall thickness as a design feature for their superlative nimble handling and balance. These, along with some barrel types such as damascus must be treated with caution. The manner in which steel shot pellets press against each other during the high acceleration journey along the barrel creates much greater outward forces than happens with lead shot.
The softer lead pellets tend to deform slightly and sit into one another, much reducing the outward vectors to the forward thrust of the pellet motion. Retrieving fired lead shot from ballistic gel shows a golf ball like dimpled effect where this settling action is visible; the extent to which this happens is why premium lead pellets pattern more closely than softer pellets as they undergo less deformation.
The greatest risk to most guns is in the choke area. Here there can be a pressure surge caused by the choke restriction, the sudden extra resistance to forward motion combined with the pellets having to move in relation to each other to now progress through a smaller bore, creates a rapid increase in outward pressure on the chokes/ barrels at this point.
Ring bulges became more common in the USA, especially in the early days of steel shot. As this was mainly for waterfowl loads were often heavy and the combination of high velocities and hard shot saw plenty of guns with ring bulges in that area.
One hears less of it now. But here in the UK we have many older guns that are not steel proof and also have tight, fairly short fixed chokes and possibly quite light barrels. The ingredients are there for problems to arise if adequate caution is not observed.
The UK proof authorities do suggest extra caution with lightweight/lighter barrel guns with the suggestion of using IC (improved cylinder chokes) in them.
The CIP has included caution in its specifications and we need to understand why and proceed accordingly.