I'll interject a little with my thoughts, though I know that's risky...
I consider the NSSA rules to not be guidelines for contruction, but only their guidelines for competition.
Dom chose to highlight part of the rules that I find quite useful in this conversation, but I think he highlighted the wrong part:
IMPORTANT NOTICE
In this, and in all other sections and sub-sections of these Skirmish Rules, the term
“approved” by the N-SSA or its designated representative(s) shall mean that the
item in question has been found to meet the dimension, configuration, mechanical
function, and authenticity criteria of the N-SSA.
It does not in any way imply, or
infer, or guarantee the safety or the integrity of any particular item.They make a point that they are trying to have as close to historical accuracy as possible, while having a reasonably safe time. They have a history of having uneventful (safety wise) matches. That doesn't mean they are safe. That's the point some are trying to make. In fact, the rules they set forth are written in such a way that makes it easy to
enforce using simple inspection techniques, rather than examining the actual safety of the gun, which would require much more in depth knowledge of the design and material than what anyone could hope to acheive in a field setting.
I would say that they are a good starting point, but since their rules don't really take into account the most important thing in a gun's construction, they have no hope but to present an ambiguous set of rules. What is the most important design consideration in a gun barrel? Simple...pressure. The second I would propose would be stress, though let's just deal with the first for now. The rules set forth by the NSSA don't mention any pressure or strength requirements for a barrel...save one - that the barrel meet the 1:1 ratio. This is an ambiguous rule - not because the NSSA used it, but because they use it without documenting it. This rule was around a long time before the NSSA, and was developed from experimentation rather than formulation, so it is by nature ambiguous. It is, however, a tried and tested rule of thumb that is quite useful in modern cannon design.
Douglas, you make this same point:
Actually they say "at least one inch". Minumum would be a better word, but "at least" implies it could be less. And again have you considered the working pressures involved? You say unsafe, but that is vague, unsafe at what pressure?
Just a side note - "at least" implies it could be more, not less. My real point - everyone here is talking about pressure, but the NSSA doesn't. Their rules make no claims as to what maximum pressures a cannon meeting their requirements can withstand, or what it's service life should be. If they were consistant in utilizing the 1:1 rule, that still wouldn't give any indication as to when it's necessary to build beyond their minimums, since they make no qualifying statement as to the bore ranges their rules apply to - except that they be chosen in accordance with the original piece.
The argument that is being made is that the NSSA rules are inadequate due to their ambiguity and and a few assumptions they make that are undeclared. They assume there's a limit to the overall bore of the cannon being shot at their matches, which is why they determined a fixed minimum size liner would suffice. I don't think it's suitable for all bore sizes, though it may be suitable in the situation it is recommended for. People choosing to build large bore cannon should definitely consider this to be a poorly written aspect of their rules, and adjust their design according to it's requirements.
That they don't mention any specifics as far as suitable materials is a problem, considering the vast differences between the three classes of materials they list - steel, iron, and bronze. Some alloys are ill suited for such use, though that becomes an issue of design as well - you can make a "safe" cannon out of a tree if you design it according to the material you are using. Of course, since their rules are focused on maintaining close historical accuracy by way of having similar dimensions, this is a moot point.
Citing their rules as a guideline for design of a barrel (though I've done it in the past myself) gives them more credibility than they profess to posess - they themselves claim they aren't guaranteeing safety, but as we recommend them to new builders, we claim they are ideal guidelines for a safe cannon. As I said, they are a good starting point, but they just don't give sufficient qualifiers for the requirements they set forth. I think the principles they use to establish their rules are sound, but overall, after examining them more in depth over the last couple of weeks, I agree that they shouldn't be used as a guideline for manufacture unless further conditions are placed on them, such as bore range, specified materials, etc.
Douglas,
One further note - it may seem that a few people are ganging up on you, but that is not the case. You have been using the best published source you could find for a reference, and there are some of us that see some issues with that reference. It doesn't fully invalidate it's usefulness, but simply makes the observation that those published rules weren't intended to be used in the manner some people are trying to use them, and that they could use some revision to better serve their own purpose.
