gunderwood
Active Member
- Joined
- Jul 7, 2009
- Messages
- 31
Kirby, that is the real discussion here. Too often people take models like QL and either consider them as Truth or they consider them as completely useless. Models are models, not reality; approximations. F=ma is an approximation, a model of sorts, that works under certain conditions to a certain degree of accuracy. We know that Newton's laws of motion are wrong, but unless you are going very fast (relativity) or very small (quantum mechanics) it is close enough (note that relativistic effects are still there even at small velocities, but are so small we ignore them). We still teach it today because it works so well to explain a lot of the simple physics we naturally encounter and observe. However, it if very important to know when that model breaks down or else you could be in a world of trouble.
I started out the post discussing how/when to model and some pitfalls. The real world example I gave was to be a Monte Carlo model. An incredible useful model, when used properly. One of the first things I do when using QL is to find data I trust and is complete enough (barrel lengths, COL, etc.), such as Lapua, and run QL against it. In my original post I did this for Lapua's .300WSM loads and none matched well. Velocities were all wrong, but pressures seemed appropriate for what I have seen in the past across many cartridges. I.e. MAP, MAP-10%, MAP-15%, etc. More or less what you would expect a reloading manual to publish with some safety buffer.
Playing around with the model can be fun, but doing so to match your desired results is useless. Garbage in, garbage out. I did do this with barrel lengths when they were not specified (Hodgdon data), but that is a fairly minor tweak. Even for that though I was pointing out that it was mostly junk. The best data I compared was Lapua's because it was complete enough.
I compared Lapua data and QL .308Win and the match was great, but everything I did with .300WSM and light bullets was nearly always off in velocity by 200 fps +-50 fps. Running Richard's actual, recommended max 4000fps load consisting of 75gr of N550, etc. the velocity is short in QL by approx. 200fps and the pressure is 68.5k PSI. No argument, it is hot. Does it shows pressure signs or not? Richard gave you brass that has loose primer pockets. That is a pressure sign in my book. I don't like to run anything that hot. I like 5 reloads at least.
If you were to trust the QL velocities and try to find a load in .300WSM that breaks 4000fps the pressure would have to be well north of 70k PSI. However, understanding that QL seems to be calculating the velocities of all .300WSM loads with light bullets wrong (even factory) and the correction factor is around +200fps, you would find in QL that 75gr of N550 does push the 125gr Nosler to 4000fps when corrected. Again, no argument. The load is hot, brass life in the 1-3 reloads I would believe easily.
Is the pressure right? Nope. Is it close? I think so or at least within the margin of lot variances, environmental conditions, gun spec, etc. I have no lab data to back this up. However, all the Lapua loads were right were you would expect factory data to be and this was true also for the .308Win which matched great (QL vs. published Lapua). Take it for what it is worth.
I was hoping that by providing an example, more people would run factory data to get a understanding of how good the QL match is to reality for particular cartridges/bullets/powders. Still have to work up slow, still can't trust QL completely. Compare the published loadings and QL with actual measured data (most of us only have chrono's). Using all three methods, I now have a much better picture of what is actually going on. Too often people want QL run and want max loadings run. Potentially asking for trouble, the data I posted above shows why.
