JonB_in_Glencoe
> We know that as the pressure increases in a ladder test of castboolit loads, the amount of obturation to the castboolit increases.
> As that obturation increases, I suspect that friction increases.
Yes, that is what we've always been told and we've heard it so often it's become "what we thought". But do we really "know" that? Or are we just repeating and believing someone else's assumption? Obturation is defined as swelling to close or obstruct. If our cast bullets are .001 - .003 over groove diameter are they swelling to close? No they aren't. The cast bullets we shoot almost always are over groove diameter. Truth is they are swaged down to go through the barrel. Additionally with lubed bullets they are swaged down to ride on a layer of lube. Thus there is no frictional contact between the alloy and steel. Also the lubed cast bullet is swaged down within one length of bearing surface and begins riding the lube within that one bearing surface of travel. There is no "friction" between alloy and barrel after that. Yes, old BP bullets were dependent on swelling to close and were supposed to obturate as those bullets were smaller in diameter than groove diameter. The over groove diameter bullets we use today do not "swell to close".
Conversely. the jacketed bullet also can be swaged down or even truly obturated if undersize and remains in contact with the steel throughout its travel in the barrel.
Over 12 years ago after I began broaching the subject of the adverse affect RPM had on cast bullets or even jacketed bullets I began serious testing to find out. I received a lot of criticism with "bbbbwwwwaaaahahahaha you changed what you said....." commets from numerous knowledgeable people here. Yes I did change many of my "beliefs" that were based on all the "expert" opinions I'd read for years in books and magazines by well known and respected writers. I, like all of us, accepted them as "fact". As I actually tested what occurs to cast bullets I found many of those assumptions and theories were only based on opinions and guesses at what was happening and were repeated so often by other "experts" and all of us that they became myths and old wife's tales. Frankly I made quite a few enemies on this forum because many self styled experts did not like the truth getting in the way of their own beliefs. I seek the truth and if it proves I am wrong then so be it.
Now to the point here; Do I actually know the answer to the OP's question? No I don't. My own supposition is based on results from other testing that have this far led me to make those suppositions as stated in this thread. Recently I would have said that a GC'd bullet would give more pressure with a given load (all other things being equal) than an identical PB cast bullet. I ran a test on that very question and found there is basically no difference in pressure between the two bullets with the same load. I was wrong and am man enough to admit it and learn. Guess I could have run off to another forum but I've not. So it is with this proposed test. I think I know the answer based on other test results but I have not specifically tested for an answer to the OP's question. If my suppositions are proven incorrect and wrong I will learn from the results and not take any of it as a personal affront.
What if that obturated lubed cast boolit friction surpasses the friction of a unlubed J-word?
My theory on a pressure test comparison of castboolits and j-words:
Light loads will yield results showing J-words creating higher pressures.
Increase the loads to the point that obturation is creating enough friction to surpass that of J-words, then results will start showing castboolits are creating higher pressures.
At what pressure do you think this might occur? In the 44 magnum for a "heavy load" if I load both the 240 gr 44 XTP over the max listed load of 21 gr 2400 and the 429360 (242 gr fully dressed) over the same is that enough pressure? For a lower load I intend on loading both over 8.5 gr Unique. I'm not sure I really want to exceed a published max load for the XTP bullet(?) attempting to find a "point that obturation is creating enough friction to surpass that of J-words" if necessary, do you?
In the 30-30 I was intending on testing a 170 or 150 gr jacketed bullet over a max listed load (probably with H4895) against the same load with a 170 gr 311041 or 150 gr 30-150-FN.
In the 45-70 I was going to use a max load with 400 gr Speer bullets and 400 gr 457483 cast bullet over a max listed TD level load with 4759, a max lever action load with 3031 and a max Ruger #1 level load with RL7.
Any other suggestions?
One great problem of this pressure test, centers on what I have previously stated in this thread [years ago], All things can not be equal, OR BETTER PUT, all "other" things can not be equal.......... One huge issue of "all other things not being equal" is having two projectiles that have the same shape[nose ojive] and same volume and same weight...So when they are seated in the case the same, there will be the same jump to lands (or lack of jump), and same case capacity, so there is a same burn pressure rise. Also the projectiles should be the same diameter, so the case tension is the same.
Those "variables" can easily be adjusted for. Let me say this based on my past experience from other testing indicates unless there is a long free bore to the throat a difference of a few thousandths in bullet jump makes little to no difference in measured psi. What does make a difference in smaller capacity cases is a difference in seating depth (not to be confused with the cartridge OAL). Variations in seating depth in smaller capacity cases (mostly handgun cases) can indeed have a measurable effect on pressure.
My plan in the 44 magnum is to seat both bullets to the same seating depth which will still allow proper crimping. In the rifle cases the bullets will be seated to the crimp groove and properly crimped.
The 44 magnum will be tested in a Contender pistol barrel, the 30-30 in a Contender Carbine barrel and the 45-70 in my Siamese Mauser bolt action rifle.