The Engineering profession by its very nature is cautious and conservative. Its not hard to “over-engineer” something where it never fails. Its much harder to use predictive data and approximations and design it to a mechanical safety factor of 2:1 or 3:1. Why is this?
The simple and short answer is the known unknowns. Inputs such as material quality, manufacturing quality, and mechanical design stress risers are the easy ones.
Material quality asks such questions as out of a thousand pieces of steel is there an internal flaw or defect and could you detect it? Magnetic particle inspection or ultrasonic testing can find flaws sub-surface depending on the technique. The product designer better have a long talk w his NDT Level III about process detection and capability! Did you orient your magnetic fields in the correct orientations and the ultrasonic wave type, intensity, and orientation on the inspection plan? Did you have the amperage set too high on the magnetic particle? You have a flaw, is it a reportable indication? What is the smallest flaw size you can detect? Now design for it because it is a known unknown.
What is the yield strength of your steel? Will a 45xx series steel work or do you need 86xx steel w a high carbon content? 86xx steel gives much more capability w less mass, how much more is the material and machining cost? Will the public be willing to pay 20-30% more for this part? Is it a highly stressed pressure component that is small w many mechanical features such as an AR bolt? What if I add a bigger compound radius or potential undercut radius? Will this buy stress margin?
Mean vs alternating stress! Do I use the ASME Elliptical, Goodman, Soderberg, or Gerber curve for alternating vs mean stress? Mean (average) stress is the easy one. What alternating stresses do I expect to see? Is there a particular frequency? Barrel vibration and bolt speed on an automatic? The alternating stress and frequency response drives many bridge failures for instance.
Here is the big one, high or low cycle fatigue! That firearm action will hold the pressure for a 1,000 cycles no problem, what about cycle 10,000? Is this design a potential 5-10 yr litigation issue when many of the guns hit the higher round counts?
Now your gun action design is complete! You had a product specification for pressures, cartridge SAAMI, and customer expectations. Now the manufacturing engineer gets the preliminary prints. They have been working w you all along but you made the tolerances too tight. That 86xx steel component you specified +/- 0.001” w its mating component w the same tolerances worked great in the stress analysis and tolerance stackup due to that alternating stress you saw in testing. They are seeing excessive tooling wear to hold that tolerance and the carbide endmill cost is HIGH! Can you switch to another steel easier to machine? What about that 45xx steel? Its fatigue factor was different.
Could adding S-110 or S-330 shotpeening help it meet life? What intensity, will 0.004-0.006” work? Can they over shotpeen it? The manufacturing engineer needs to run a saturation curve! Part masking is needed to prevent rounding to that bolt lockup surface. That is going to add cost! Is it better to buy that dedicated polyurethane masking or to add another operation to the part router? Are these cost swaps acceptable to accounting and marketing?
Assembly and heat treat technique? Can they overheat treat a lot? The original Springfield 1903 had a slight problem of embrittlement due to over hardening that caused high round cycle failures. Do I test every part or lot test batches for sampling? What test and range? Will a simple Brinell or Rockwell C indenter suffice? Heat treat penetration required? What samples do I send to the metallography lab? The lab is overwhelmed and wants to know if I can reduce the sample rate since the Six Sigma guru showed process capability and they have special process parameters.
Can they overtorque a part? Read about P17 rifles on Eddystone receivers. The original barrels were ok but existing flaws were magnified when over-torqued in ass’y and when re-barreled not detected by mag particle. Is there a false cut due to a tired operator on graveyard and doing a manual blend operation that causes a Kt stress riser in a highly stressed component? Is there a way to potentially mis-assemble this? We need to poke yoke this!
If you read this far now you can appreciate the known unknown things that keep a mechanical engineer up at night. These are many of the known unknowns from an ISO 9001:2008 company.
Now you consider publishing loads for Bubba to make at home w aftermarket products such as powder, primers, cases, and bullets of unknown origin. This is the greatest unknown variable of them all!
How susceptible is that powder to degradation? Will it significantly alter that pressure-time curve? How erratic was the standard deviations for pressure or powder position sensitivity in the cartridge case? What about pressure fluctuations w temperature? That new whizz-bang .300 XXX Magnum is the hot new ticket for Polar Bears and light African Safari cartridge for Plains Game! Ideal gas law PV=nRT is a good starting point but its been marketed as a heavy for caliber suppressor ready rifles!
Will the home handloader try to cheat the OAL and use a heavier for caliber bullet and still chamber the round? Is this the new military sniper rifle round that surplus cases are abundant and cheap? Does that thicker case reduce the internal volume available and raise the pressures another 3-5%?
That 90% starting load is a great caution for sure!
What is my long-winded point? Lawyers and insurance are the least of an engineer’s problems if they diligently follow their profession and best practices. Even if you do everything correctly you may still find yourself on the witness stand defending against product litigation and you followed every best practice. This is why licensure and design standards exist, its peer review at its best to establish minimum standards that attempt to solve society problems.
There is a professional organization called the Order of the Engineer. The folks that join take a solemn oath to respect the profession and always consider the higher ethical good when performing their branch of engineering. We put a lot of faith in an unknown engineer in the design or ballistics lab!
.
