" In fact, the model may have error less than 20% from actual.
I would expect a range of structural characteristics something like 20% (or more) for body tubes made from paper that is cheaply purchased, cheaply manufactured into tube form, and subjected to a wide range of storage and handling conditions. It's not going to be like aerospace-grade aluminums or anything.
More variables:
--local weather conditions, incl temperature & humidity, causing the BT to shrink or swell and changing the "grip" on the shoulder of the NC
--NC fit, incl manufacturing variation on diameter of the shoulder, surface roughness, intentionally molded surface protrusions (like those little ribs on some shoulders), unintended molded surface protrusions (mold flash), length of shoulder, and probably more
--age of BT and collective number of ejection events to which it has been subjected (abused?) already
--number, type, size/length, and spacing of external fins which may provide some degree of reinforcement
One of the bigger variables, over which we have no control or means to estimate, is the power/weight of the blackpowder ejection charge itself. Depending on exact components (carbon from one source does not behave the same way as carbon from another source), processing (were components powdered? to dust? were they mealed, corned, etc? how many times?), and a number of other factors (what color shirt was the chemist wearing?), blackpowder batches can have a tremendous degree of variation in thermodynamic performance, calorie content, energy release rate, and a few other things. Blackpowder manufacture today is still just as much about art as it is about science.