Paul, 450-366,

if it has been too technical so far, maybe it is helpful to concentrate just on one well-known fact: Modern propellant is extremely sensitive to heat and pressure. Cordite, which didn't have today's additives to tamper that effect somewhat, was a prime example of this - African heat was enough to create dangerous pressure spikes, that's why Nitro Express loads were given such low pressure limits, (which aren't necessary anymore with today's powders). That also answers whether problems existed in the past - if rifle makers had reason to lower pressure that drastically, then there must have been a lot of problems in the early days.

Now, nitro-cellulose powder creates the expected level of pressure and energy with an ever-expanding 'boiler room' - during the burning of the powder, the barrel is really the extension of the cartridge case, so while the bullet moves, the volume of the confined compartment (case plus barrel) up to the position of the bullet constantly expands, which lowers pressure dynamically, allowing the powder to burn in a controlled and expected fashion. However, once you increase resistance to the movement of the bullet, be it by an obstruction or a bullet with excessive resistance to engraving, you are making the 'boiler room' smaller then it should be at that point in time (thereby raising pressure beyond the design criteria of the powder).

Here we have the main misunderstanding by people who think that by now the bullet and the inside of the barrel can do something to each other. They erroneously assume pressure at this point to be either static or to to be in a linear relationship with the force of resistance that bullet/rifling can pose. This would be the precondition of bullet and rifling staying in contact with each other, and without contact they cannot damage each other. The relation between the volume of the 'boiler room' and resultant pressure is however not linear but exponential. That means that before an oversize stainless steel bullet could damage the inside of a barrel, its temporary slowing of the 'boiler room' expansion raises pressure. However, if exposed to confinement, modern powder burns even faster, creating further pressure, which in turn makes the powder burn faster yet again. What you have is a cascading, self-feeding process that approaches detonation. This process happens a lot faster then the time a bullet needs to damage a rifle. The barrel either bursts outright before that happens, or the barrel over-stretches, releases the bullet, thereby lowering pressure again by expanding the volume in which the powder works, and then afterwards showing symptoms that people call OSR.

OSR, as most people seem to understand it, namely the bullet itself doing damage, can however only happen when two conditions are met:

a) The bullet is harder or at least approaching the hardness of the barrel.

b) You drive that bullet through the barrel with a dowel, thereby eliminating the stretching of the barrel that the dynamic combustion process causes. Now, bullet and rifling can stay in contact and potentially damage each other (if condition a is also met). But who launches their bullets with a dowel?

Now to the important point: What are the practical implications for the shooter?

If you have a modern rifle with correct barrel dimensions and know how to reload, there is no issue with 'hard' or mono-metal bullets, you have a lot of safety margin even if bullet size or hardness is a bit out of whack. In general, things go only wrong when there are several factors at work. If that was not the case, nobody could shoot Barnes bullets without screwing up.

On the other hand, if you are a novice to reloading, have an old gun with thin barrels and have not even slugged that barrel to see whether it has the correct internal dimensions (old .404 barrels can be significantly tighter then newer one, for example), then don't add another risk factor - harder bullets DO make the pressure curve steeper (unless undersize, but then, your barrel might be undersize as well). The same goes for people who do not properly clean their barrels, leave oil in the chamber, never remove carbon deposits around the chamber shoulder or the carbon ring in the barrel, and so on. Something relevant as well: If you shoot bronze slugs, steel jackets and normal bullets on top of each other you can weld yourself a new alloy inside your barrel which can significantly increase friction and thereby pressure.

So the conclusion is quite simple: You have a valuable vintage double? No need to understand OSR, stick to Woodleighs and traditional loads and never look back. I myself, on the other hand, own a modern Holland & Holland .470 double, and shoot mono-metal bullets and loads that approach that of a .500 NE. - and I have zero problems. I do however know how to control and adjust a pressure curve, use mathematical modeling for it and cross-check with a pressure gauge that, amongst other things, displays my pressure curve graphically at any point in time during the launch of my bullet. That way I catch even secondary or tertiary spikes as a result of inefficient and potentially dangerous powder/case/bullet mismatches and can readjust. In other words, I do my homework, and can therefore enter somewhat more 'risky' territory.

As a final word: What damages rifles is in many cases not peak pressure as such, but how fast you get there (the steepness of the curve). Therefore, even a load with a 'normal' peak pressure can be damaging under certain circumstances. Mono-metal / hard bullets simply contribute to a steep rise in pressure more then traditional bullets. If other things go wrong, they can be the tipping point, that is all you need to know.

... Forgot one thing: The reason why the described damages tend to show up in the forward section of double rifle barrels - it is the result of an intended design 'flaw'. The principle behind a chopper lump barrel is to be without seams, and to be strong at the breech, and progressively weaker towards the muzzle. Recall the early days of cordite with its pressure spikes. Manufacturers could not control WHETHER a gun would blow up, but HOW. They chose to make the barrel comparably weak towards the muzzle, so the barrel would stretch / burst as far away from the shooter as possible. The drawback i, that you get damage in these barrels (towards the tip) much easier then with other types. But better the gun gets damaged then your face. Besides, I would never shoot an old gun period as I don't know what the previous owners did to them. - I have been asked whether I have seen damages in guns that look like what people call OSR. Yes, as I spend much time with my gunsmith. All damaged rifles that he gets, bolt, doubles, drilling, have one thing in common. They are usually over 30 years old.