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Light loads and the first few microseconds


pat

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Evening all,

whilst discussing load development I happened to mention that I would always target a bare minimum of 85% fill ratio, with the rationale that anything lighter can be dangerous. I hadn't previous given it much thought as to why that should be the case, and when asked to justify this, I had to say "let me get back to you on that one". I hadn't needed to understand it at the time, and so didn't have an answer on hand, but I have had a chance to think about the internal dynamics and I think I have been able to tie a few things together that form an overall coherent theory that may explain some interesting phenomena - including why a light load can be an issue.

Thinking about a loaded round, we can make some observations.... the case has a given volume, which is reduced by the seating of a bullet.... powder kernels are solid (be they ball or extruded).... powder has a bulk density which is different from the kernel density (there is some air in between the balls / or kernels). These should all be axiomatic and require no further proof.

Thinking about the way that smokeless powder burns, there are a number of salient points.... powder turns from a solid into a high temperature gas... the rate at which this happens is affected by the pressure in the case.... the rate at which this happens is affected by how much of the powder is already burning (current mass fraction). The second and third are ostensibly conjugate - the more hot gas there is to set other kernels alight, and the higher the pressure the more those gas molecules are likely to bump into an unlit part of a kernel the faster it lights, whilst the higher the percentage of the powder that is already alight, the faster it will release gas, and so it goes on.

It is noteworthy that as the kernels burn, they "create volume" that gas can occupy. At the initiation, a large percentage of the available case volume is occupied by solid powder and so is not available for gas to expand/compress into.

Now that we understand some of the mechanics of what is going on we can think about what must happen in order for things to work the way they are supposed to. Given the powder's desire to exponentially burn and build pressure, if it was fully contained then we'd end up with an actual explosion eventually (that is how a bombs work, though it is notable that the term "bomb" initially refers to a confined space). The fact that the bullet is "not" confined (it is "free" to move) is what allows the pressure to stabilise - as the bullet moves forward, it is adding more volume which the gas can occupy - applying Boyles' Law or the Ideal Gas Law shows us that increasing the volume decreases the pressure (which, in turn moderates the burn rate).

Most smokeless powders use a base which has pretty much the same burn rate, and so the difference between a "fast" powder like Bullseye and a "slow" powder like H50BMG comes from the use of retardants and the surface area:volume ratio - but fundamentally the powder, if in the same shape and not coated, would all burn at the same speed. Fortunately we do have retardant coatings and different shapes to control the burn rate.

Now, an interesting point was made - "if X amount of powder only generates Y pressure, how can 0.5X make more than Y pressure?" - now I was a bit slow on my feet at the time and a sensible retort might have been "well, try putting 120gn of Bullseye in a 50BMG, stand well back, and let me know what happened!" ;) Given that you can get 240gn of an appropriate powder in there safely, I think this demonstrates the point that it is not just the amount of the powder that is important. Hold that thought!

In order to push the bullet out of the case, we need to overcome that static friction between the neck and the bullet - we need to reach a certain amount of pressure. The amount of powder that needs to be burnt in order to achieve that pressure depends on the free space in the case to start with. Remember that powder is not a monolithic solid that fills the case, it is a collection of balls or kernels with air spaces between them. The amount of space available for gas to expand / be compressed into is a function of how much powder is in there to start with (kernels are solid and "can't" be compressed like a gas, so any space they occupy at the beginning "does not exist" for gas to occupy). If the case it very full then there will be only a small amount of "free space" - therefore it doesn't take much powder to burn before the required pressure is reached to unseat the bullet. But now let's imagine we get rid of half the powder. Rather than"doubling" the free space in the case, we may have increased it by a factor of 20 or so! It stands to reason that much more powder has to burn, in order to generate the same pressure - it is after all filling a much bigger space!

Now, the burn rate of the powder depends on the mass fraction that is already alight. And now the problem reveals itself. Far too much of the powder is already alight, and evolving gas at a rate far too high, for the nascent motion of the bullet to effectively relieve. If we think about the original scenario where the case was full, there was only a small amount of "free space", so any small motion of the bullet would enlarge that space significantly, but when there is already a large volume, then a small movement of the bullet will only have a tiny impact on the pressure relief, and hey presto, things have gone rather wrong.

In order to not end up with a ridiculously enormous single post, I'll make some further observations in other posts :)

Cheers,

Pat.

 

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As a further aside to this I was contemplating the interesting effects that we see during load development. We can tune the load in three main ways - powder type/charge weight, seating depth and barrel tuners. With further thought it would make sense to include things like neck tension.

One thing I found interesting is the appearance of velocity nodes. Whilst working up a load, we can get to a point where the increase in MV begins to level off a bit, before it starts climbing again. This didn't make a huge amount of sense. But it did make me wonder if there could be a similar phenomenon at play here.

It is generally the case that giving a bullet more jump before encountering the lands causes a reduction in pressure and speed (QL will indicate the opposite, but that is another story). This happens, I believe, for a few reasons : 1) a bullet seated deeper leaves less "free space", ergo less gas needs to be produced to overcome the neck tension so the mass fraction alight at the point it starts moving is lower, 2) because there is less "free space" to start with, then for the same forward movement the volume increase ratio will be higher thus moderating the pressure more and 3) a bullet which is already moving will engrave / obturate more easily (static friction is normally higher than dynamic friction - this also explains why you need to be careful when seating into the lands - QL recommends increasing the Shot Initiation pressure substantially and for good reason).

Taking that into consideration, and not messing with the jump, but only the charge weight, it should become apparent that something similar can happen. If we add more powder, then this reduces the "free space", which in turn reduces the mass fraction that must be alight in order to start the bullet on its way, which will then moderate the gain from the additional powder. But of course that should "always" be the case whenever you add more powder. And perhaps to some extent it does happen - it would be interesting to try using an inert filler material in lieu of actual powder to see what would happen if the mitigation of the reduced volume was eliminated - I digress. I'm not entirely sure "what gives" with regard to why this effect is more pronounced at some loads than others. It is possible that some sort of hoop-stress shear wave (c.f. Chris Long / OBT) is interacting with the bullet, or it could just be down the the mechanics of the engraving / obturation (perhaps there is a partial "stall" going on which is widening the pressure / speed plateau).

It is interesting to note that the above mechanism which is responsible for the reduction in pressure / MV as we shorten the round must also affect the barrel time, and hence it should come as no surprise that it is possible to tune the load that way. Moreover it is likely possible to tune different aspects of the load - that is to say that there are some barrel motions which are "slow" relative to the exit-time-variance, and so we may observe a drift in POI as the charge is changed (as well as a possible group size reduction), whereas within reason we may be less likely to observe a general POI shift as seating depth is changed.

In terms of group size, I found it quite interesting that a small change, say 5 thou, in seating depth can make or break things - but with the extra mechanisms at play that is much more "understandable" than what happens with barrel tuners. Here again, we can see a substantial difference in group size with just a small change in location. Many tuners have rather fine pitch threads and a whole revolution may be only 40 thou, and yet we can see enormous differences within 1/4 turn, which would again only be 10 thou!

As I see it there are two (not necessarily mutually exclusive) possibilities for why any given bullet's POI is not exactly where the crosshairs were (assuming a correct zero, of course) : 1) the barrel wasn't pointing in the right place or 2) the bullet got nudged off course.

By 1) I am referring to barrel motion ("harmonics") - if we have a small variance in exit time and the barrel is in a "rapid" part of the harmonic motion, then it should stand to reason we should see some stringing, in the direction that the barrel is moving at the point the bullet exits.

By 2) I am referring to one of two mechanisms - a) Chris Long's idea that a hoop - stress shear wave is at the muzzle at the point of exit, which has enlarged it to the point that the bullet is no longer in contact with the entire circumference of the bore - then given the variance in centre-of-mass from bullet to bullet, the bullet will exit when it is touching some "random" part of the bore, and hence we see random scatter, and b) acceleration of the muzzle in a direction normal to the bullet path causing some "tip off" - that is to say imagine the barrel moving sideways (say it's on a train, pointing out a window sideways), but the bullet is also going sideways, both are going sideways at the same speed and so there is no relative motion between them, ergo the exit will be "clean", but what if the train applies is brakes ? well, whilst the bullet is still in the barrel, it will slow the sideways motion of the bullet, but that is only possible because the bearing surface is in contact with the bore - but what happens as the leave the muzzle ? when the bearing surface is half way out - 75% of the way out - 95% of the way out ? The barrel will still be slowing down, but it is only touching the bullet behind its centre of mass, and so it would try to "tip" the bullet. Delete the train from the picture and replace it with barrel harmonics. If the bullet exits at a time that the barrel is accelerating in any direction normal to the bullet flight, you may end up with "tip off". We know that aerodynamic jump causes deflection in "unexpected" directions, due to the angle between the axis of rotation, the flight trajectory and the ambient wind, so it stands to reason that any deviance, however caused, may have an influence.

If the group size was purely down to the harmonics themselves causing the bbl to point in a different direction then two things "should" be true : 1) we should see some sort of Lissajous pattern emerge in the impact locations (but they appear random) and 2) it would have to be a very high order harmonic - given a total barrel time of 1.35ms (will vary, just a suitable example) and given that a substantial change in bbl time would be quite noticeable in its effects on MV (delayed ignition aside, ahem, err), we should not really be seeing exit-time variances much longer than 0.025ms....which would be 20kHz if we're thinking peak-to-trough - and so far no one has suggested / simulated harmonics that high (think Varmint Al went up to 1.5kHz is). That's not to say this isn't possible.... indeed perhaps there is some ultrasonic excitation from the primer detonation.....  worthy of further thought :)

My gut feeling is that sizes are down to a mechanism proximate to harmonics but not necessarily directly related. Changing the "length" of a barrel from 26000 thou to 26005 thou is not going to change its resonant frequency by much - it's a 0.019% change - equivalent to putting 50.0096gn into a case when you "should" be putting in 50.0000gn. Whatever the tuner is affecting, it's much more sensitive than that!

I am minded to think about wave theory again - the very notion that "the speed of light is too slow" is fascinating... that you can apply and measure several thousand volts to a "dead short" just because the "dead short" is finite in length and it takes time for that voltage to appear at the other end of the "dead short" "wire" means that for a (admittedly rather brief) period of time, this situation is "fine". Indeed if one times it well, so that when the reflection comes back from our short (the opposite voltage is reflected to cancel out the original) we are outputting the opposite as well, then it actually looks like an open circuit - pretty crazy, and all because the speed of light is so slow ;) That being said, the same should be true in steel as well. A shear (transverse) wave applied to one end will travel to the other, and when it gets there it will reflect, but it could wiggle things about "vigorously" whilst there. Trouble is that the wavelength in steel will be somewhat longer.... 20kHz would be around 16.25cm. And we're talking about moving something a few thou. Mmmmm :D

 

Cheers,


Pat.

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10 hours ago, Andrew said:

I can't wait for the next installment. ~Andrew

🤣🤣

My laptop blew a fuse (fuze for you chaps across the pond)

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24 minutes ago, Andrew said:

Fuze is a soft drink here.

Fuse is how educated people spell it. ~Andrew

😁  I'm edukated two

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