John,
 
I sent this to you via e-mail, but I thought I'd present it here at THL as it goes to the issue I was discussing with CAfrica in "Rookie question about bullet weight and powders".

QuoteI've been wrestling with a new idea of late, and I'm going to redo a great deal of the MAI load workups. I've attached a pdf file regarding "Optimal Barrel Timing" have a look at it. In addition, have a look at this URL, http://practicalrifler.6.forumer.com/viewforum.php?f=4&sid=f4f52b11d92c68b6bad5b4d967d8be7c and look for the sub-site that deals with "Optimal Charge Weight". I'm going to use these ideas as the basis for the new MAI workups.
 
The primary reason I'm passing this on to you now, is that after doing some paper-whipping, I'm not sure 26" is the "optimal" barrel length for the MAI if you want the maximum velocity AND the maximum precision. Here's how things shake out on paper.
 
Bullet Weight..Exit Time..Muzzle Vel....Press. kPSI....Optimal Exit Times....Optimal MVs
 
160...............1.25............3195.........57789..........1.304 & 1.168........3062 & 3418
175...............1.31............3048.........61000..........1.388 & 1.304........2876 & 3062
200...............1.42............2822.........55461..........1.524 & 1.388........2630 & 2886
210...............1.47............2733.........57241..........1.524 & 1.388........2637 & 2894
 
The Optimal Exit Times, are those times at which an acoustic wave (that reflects back and forth from muzzle to bolt face) is at the bolt face as opposed to the muzzle. (The pdf has the gory details.) So half way in between these times, is the worst time for the bullet to leave the muzzle. You'll note that the 175, 200, and 210 have the same optimal values. This is because the propagation speed of the transverse wave is solely dependent on the barrel steel, and the length. Given that all barrel steel is essentially the same, (especially considering the speed of propagation of an acoustic wave), length is all that matters. So let's look at the data we have in hand.
 
I'll start with the 200 since that's the bullet you designed this rifle to shoot. Right now, at an average chamber pressure of 55,000 PSI and an average muzzle velocity of 2822 f/s, the bullet exits the muzzle at 1.42 milliseconds after ignition. The eigth and ninth "boltface nodes" occur at 1.495 and 1.388 ms. 1.442ms is when the wave is at the muzzle. The 1.42ms departure as a result of the 2822 MV at 55kPSI is closer to the "bad" time (a "muzzle node"), of 1.442ms, than it is to either of the "good" times (a "boltface node"), of 1.495ms or 1.388ms. The question is, which way should I go to find a "boltface node"? The answer is probably "Up". I think I can get to 2881f/s with and increase of less than 0.4 grains of powder. This shouldn't increase the chamber pressure too far above the 55kPSI we want to keep this bullet at. BUT, I know that I enlarged a primer pocket not too far above this charge weight. If I can't get down to an exit time around 1.388ms (with an attendent MV of about 2881), I'll have to go down to get to the other nearest boltface node at 1.495ms and 2675f/s. So, how could we change this situation without having to loose the 2822 MV we know we can have at 55kPSI? The answer is... change the bbl length.
 
Keep in mind that node timing is independent of anything but the length of the bbl and bullet velocity. Therefore, by shortening the barrel, (or lengthening it, I just don't know how to do that as I don't have a barrel stretcher - but maybe you do ), we can change the timing of the nodes. So let's say we want to keep our current exit time of 1.42ms, because we like the 2822 MV and the 55kPSI chamber pressure. How short would the bbl have to be to move the boltface node to 1.42ms? The answer is 24.5" or 1.5" shorter than it is now. So what would we loose in MV by shortening the bbl by 1.5". Based on our recent examination of bbl length vs MV, I'd guess no more than 30 f/s, leaving us with 2792 fps. (In truth, I'd expect the change to be on the order of 20 f/s, and probably even less.) Of course when we shorten the bbl and keep the charge the same, the exit time will have to be slightly less. I think that's a relatively insignificant detail.
 
Let's look at the 175. Again, we've got two choices - "up" or "down". However, in reality, we really can't go "up" very far. There's not much 'headroom' WRT chamber pressure. So, we can likely drop from our current 3048f/s average MV down to 2881f/s. A pretty big drop. OR... we can change the bbl length. Let's see where the nodes "go" when we shorten the bbl by 1.5", (keeping it "optimum" for the 200 grain bullet at about 2800f/s). The nearest "good" node is now at 1.322ms at an MV of 3021f/s. We're almost there already at 1.31ms and 3048f/s. So, shortening the bbl means we would drop the 175 load just slightly to lose about 30f/s, and the pressure would go down from 61kPSI, and that's good too.
 
Here's what happens for the 160 and 210:
 
The 160 would have to go up to 3284f/s, or down to 3035f/s, and
The 210 would have to go up to 2826f/s, or down to 2624f/s.
 
Hard to say if either could "stand" to go "up", but they might. Of course that makes the "ideal" weights (for the 24.5" bbl) 175 and 200. I'm guessing the 185 Nosler will fit "nicely" in that range.
 
Of course in reality, these paper numbers can't predict exactly what bbl length should be, but they certainly indicate not only the direction of change (shorter), but also the magnitude (about 1.5"). Fine tuning is then accomplished by seating depth and small (0.1grain) changes in charge. It'll be interesting to see what a 2" reduction in length means when I test my MAI.
 
So... the question is, do you want to shorten the bbl, or do you want me to reduce the charges to move the bullet's exit times to the "boltface nodes"?
 
Paul

So gentlemen, let's look at OBT in the context of adjusting a barrel to a specific bullet once a charge has been found that gives the maximum velocity for a maximum desired/acceptable chamber pressure. In concept, starting with anticipated/estimated muzzle velocities and chamber pressures from some program like Load From A Disk (which I have found consistently, and significantly "conservative" by the way), one could calculate what an optimal barrel length would be and specify someting in excess of that when ordering the barrel manufacture. After actual testing, during which the charge that generated the maximum "allowable" chamber pressure ws determined, the barrel could be "trimmed to length" so that the maximum velocity/maximum pressure could be maintained. Then, final tuning for the greatest shooting precision could be accomplished via seating depth adjustment and small changes in charge.
 
Paul
 
PS
 
I thought I'd add some M-95 Milsurp ammo data, as it is interesting WRT OBT.
 
The 206 grain milsurp bullet averages 2385f/s at 39505 kPSI chamber pressure and exits at 1.21ms after ignition. The theoretical optimum node timing for the 19 & 11/16ths barrel is 1.205ms. Just what the ammo exits at. The next node ocurrs at 1.14ms with an associated MV of 2538f/s. I'm confident I can get the 205 Hornady to that 2500-ish MV without excessive (45kPSI) pressure. The interesting thing, at least to me, is that the milsurp MV/exit timing is the precise value that gives the theoretical "best" accuracy.
 
Ya reckon they knew sumpin' we're jus' larnin'?
 
Paul

Hey buddy,
 
Well, the cob-webs cleared the ol' brain cells sometime after 1PM but have been a tad busy researching RSI data, prices--etc. Also looking for a lap top that I can afford...looked at the site you gave me on surplus PC's--most interesting!
 
OK, back to the topic you posted. The OBT and OCW data is most interesting and possibly this under-educated ol' guy is getting the gist of it, so bear with me and the many possible questions that will come later.
 
I am wondering about the temp differentials we will experience from you being in Alaska (cold) and me being in Texas (hot). The load data you are collecting will most possibly need to be reduced when I start my experimention and load development here. I am definitely going to have to get a laptop when I begin testing at 200 & 300 & 600 yards (+/-). Am I going to need to obtain other pertinent WX data such as barometric pressure, etc.?
 
Hope to start getting stuff built for the loading shack and shooting bench this wednesday....gots to get enough PVC to build the target frames and start clearing the firing lane......plus make the pics to post later as progress continues......all this while accomplishing my necessary ranch chores!:D
 
As I said, as soon as the new FFL arrives...you will be the "second" to know.
 
Thanks for all you efforts and work......greatly appreciated, amigo!:D :D
 
Ol' John..

Paul,
 
Just as a secondery thought..........Using the VV-150 poweder we had the secondary pressure spike......reducing the barrel length to 24.5" do you think this would get rid of this unwanted spike? Inquiring minds want to know..duh!:confused:
 
Ol' John;)

Gitano,
 
I have found this whole discussion (and the material you sent me) fascinating. The one major "problem" that I have (and maybe you can shed light on it), is the assumption that the oscilating hormonic stays IN THE BBL.
 
To my mind, the returning "wave" would go into the ACTION and only turn around at the end of the action.  That means that the whole calculation should be based on the length of the BBL AND THE ACTION.
 
What am I missing here?  What is the premise upon which the assumption is based that the wave turns around at the BBL/ACTION joint?  Does it have something to do with the point at which the "beam" is supported?  
 
Regards.
 
C

Quote from: LLANOJOHNHey buddy,
I am wondering about the temp differentials we will experience from you being in Alaska (cold) and me being in Texas (hot). The load data you are collecting will most possibly need to be reduced when I start my experimention and load development here. Prolly so, and that's a good point WRT the barrel length too - not the temperature differential, but change in charge. I am definitely going to have to get a laptop when I begin testing at 200 & 300 & 600 yards (+/-). Am I going to need to obtain other pertinent WX data such as barometric pressure, etc.? I really don't think that will matter John. A temperature probe - easy and cheap to come by - is probably a good idea, but unless you intend to shoot competitively at 600yds or more, I think incorporating barometric pressure is "esoteric" or... anal .
 
Ol' John..

Quote from: CAfricaThe one major "problem" that I have (and maybe you can shed light on it), is the assumption that the oscilating hormonic stays IN THE BBL.
 
To my mind, the returning "wave" would go into the ACTION and only turn around at the end of the action. Where is that "end" exactly? That means that the whole calculation should be based on the length of the BBL AND THE ACTION.
 
What am I missing here? What is the premise upon which the assumption is based that the wave turns around at the BBL/ACTION joint? Does it have something to do with the point at which the "beam" is supported?

The simple, truthful answer is, "I don't know." I too wondered about this, but 'resolved' it for myself, at least until I can get to the range and run some experiments.
 
First, what shouldn't be allowed to fade into the background, is that the Optimum Barrel Timing theory apparently works. (I try never to let theory over-ride observation.) However, the only "proof" we have that OBT works are Long's assertions and some independent coincidences. (One of which for example, is the M-95 milsurp bullet weight and charge/MV.) Speaking for myself, I find his arguments and the circumstantial evidence compelling. Since he started his theorizing with the barrel modelled as a cantilevered beam, AND that appeared to work as a foundation for the model, I accept it while still wondering about the wave's response at various junctions. I don't think Long would argue that "things" dont get "complicated" at the barrel-action interface. The question is whether or not these "complications" have any material impact on the OBT model. Apparently they don't, but the verdict isn't completely in yet I don't think. Here are the elements as I see them now:
 
1) The barrel IS a cantilevered beam. In classical mechanics, by definition, such a beam would be supported ONLY at its proximal end. In riflry terms, that means a free-floated barrel. However, even if the barrel isn't free-floated, the differential between the speed of propagation of an acoustic wave in steel and wood is so great that the wood would likely act as a "perfect" reflector. Add to that the differential of the moduli of elasticity, and I think "free-floating" isn't an absolutely necessary condition for treating the barrel as a cantilevered beam. However, were there substatntial (>5 lbs or 2.5 kg), pressure applied to the barrel at the tip of the forearm, I think matters would get "complicated" for the OBT model.
 
2) Again in classical mechanics, a cantilevered beam is attached to an object (the "load") that is "large" relative to the largest cross-sectional dimension of the beam. Therefore, even if the materials are identical, the "load" again acts as a reflector - not "perfect" this time, but close enough to permit the calculations to "work". Unfortunately, I, (and it appears you too), don't particularly see the action as "large" relative to the barrel. In fact, barrels usually weigh more than actions. However, there is a decoupling of the beam with the "load" due to the threads, or however the barrel attaches to the action. How significant this decoupling is, I cannot 'divine', and I think it can probably only be determined experimentally.
 
3) "2" above seems significant, but I think there's more to the action than "2" alone addresses. Specifically, the action is very seriously bonded (coupled) to the stock. This creates a "large" object from which the beam is cantilevered - large mass and large area. By the same token, going back to "1", the differences in moduli of elasticity and speed of propagation of an acoutsic wave, applies here too. What's good for the goose is good for the gander. However, the coupling of a barrel to a forearm is considerably less than the coupling of an action to a stock - or at least is should be. Even in a non-free-floated barrel, there's lots of "air" between the barrel and forearm.
 
4) Finally, the action has a very complex shape and structure. I strongly suspect that impulse-induced waves get muted very quickly as they reflect off of the myriad edges and interfaces in an action. If so, the action then acts as an infinite sink. Again this allows the model to treat the beam as cantilevered. Surely, some energy gets tranfered to the action by the traveling wave, the questons are: How much? and Does the transferred energy get reflected back into the barrel? Personally, I think the answers are: "Some", and "No".
 
These are just my thoughts, and by no means would I even imply, that they resolve your question without doubt. But, if we are to believe what we see, and what Long postulates, then something must account for the model's ability to predict outcome.
 
I'm still working on "pieces" to this puzzle. It's frustrating that I can't find a way to get to the range to do some experimenting. Lotsa work to do here, but in the end, I think (and hope) it will have been worth it.
 
Paul

OOPS, forgot to address John's question about the secondary spike.

Quotedo you think this would get rid of this unwanted spike?

In a word... "No". I've been reading more on this subject by several that are trying to "prove" that the secondary spikes are in fact real (these are rather rabid RSI supporters), as opposed to those in the Oehler 'camp' that argue that the secondary spikes are artifact of muzzle blast. Anyway, the RSI guys have cut off barrels to drastically short lengths, and the spikes remain. HOWEVER, don't jump to the conclusion that this proves Oehler's suggestion. On the contrary, attempts to prove it IS muzzle blast have equally failed. Personally, I believe - let me restate that so there is NO confusion - I BELIEVE that the "issue" is in fact artifact, but it is an artifact of the strain gauge and what it is 'sensing', NOT muzzle blast. I could support that argument at length, but the operative word is "length". I'll spare y'all. Suffice it to say that I think there are surface waves traveling the barrel, and that the strain gauge is responding to that phenomenon, NOT actual chamber pressures.
 
But, I want to add to my response to CAfrica's question too. I just read some more from Long, and it goes right to this question of "effective bbl length" as opposed to "actual barrel length". Here is a quote from him. I have highlighted what I feel is relevant to our discussion.
 

QuoteI also found that my 6.5-284, which is built on a MAK tube gun receiver sleeve concept, has an effective barrel length about 1.5 inches longer that the measured barrel length. The barrel is 28.5" finished length (not the 29" as per my web site). I have validated this effective length very carefully using QL, Oehler 35P, and a PressureTrace strain gage. The exit times that generate optimal performance are right at what a 30" barrel would produce. I was astonished by this, and went on to validate this observation over many loads and conditions. I believe that is because the sleeve is very tightly coupled to the barrel (100+ ft-lb torque), is very stiff (0.250" Al tube epoxied to a Remington 700 receiver), and there is no stock and epoxy bedding to dampen the wave reflection. In other words, the wave travels farther back into the receiver before turing around. Of course, this is not a clean reflection point, but a distributed reflection. I am only talking about the effective reflection point. The bedded barrel/receiver is "shorter". Another data point...

The good news is, the time between the reflection from the muzzle and back again is fairly "long" in terms of bullet speed. Therefore, the OBT model doesn't require a terribly exact bbl length measurement. Once the load is determined that puts the bullet out the muzzle at the proper theoretical time, fine tuning with charge increments of 0.1 grain, and seating depth adjustments, will ultimately fine-tune the load to the effective barrel length.
 
Paul

Gitano
I tend to agree with your idea of the artifact occuring at the strain guage. In fact I believe the artifact is created by the barrel oscilation. No actual proof, just what makes sence.
 
This is all very interesting, it also sheds light on the mystery of "load development." I know this is one small point in a huge list of variables, but it seems to me that is COAL is maintained, changing powder weight and increasing or decreasing accuracy is really tuning the load to the guns harmonics. Looking at all the load data I have, this seems to be able to create a significant increase or decrease in accuracy. If you work that backwords it could suggest that barrel harmonics is something to worry about. I know the BOSS works, and basically we are trying to accomplish the same thing in another way.
 
Ron

For the most part, you're absolutely correct. However, let me just make sure that the difference between what the BOSS does and what OBT addresses is understood by everyone.
 
The simplest way to illustrate the differnces in wave types is to use the ol' ruler-on-the-countertop example. The BOSS addresses the longitudinal wave harmonics analogous to "twanging" the ruler on it's end. This type of wave causes the ruler to move up an down, and is clearly visilble if the ruler is "twanged" hard enough. The OBT model addresses the transverse wave, analogous to hitting the end of a ruler with a hammer. There is no visible manifestation of this wave.
 
Paul

Mmm, some of what you say Gitano, went through my mind too.  I was wondering for instance if the position of the "recoil lug" in the action might affect the point where the major portion of the wave "turns around" and whether the "returning wave might be slightly diffused because of the shape of the action.
 
One of the things that crossed my mind is that certain rifles in certain calibres just seem to be more accurate than others.  Might it be that the particular shape of the whole and the specific points of attachment etc. are just more in "tune" with the "standard" factory loads than those of the next manufacturer's rifle?
 
These musings open up a whole new area of research into the "accurate rifle".  I like your concept of adjusting the bbl length to the optimal load for instance.  If the appropriate predictability can be accurately developed (i.e. factoring in the action), then it would surely work.
 
Regards.
 
C

Gitano
OK, I totally understand and agree with your comment on the BOSS. So, if we are only worried about length, and not pressure, why not coarse thread the end of the barrel and sleeve it with a piece of larger barrel. You could then adjust the bbl length to tune for your load. Then, when your happy, have a barrel built to the final length and mass indicated by the experiment. Now your rifle will be built around your pet load. The chamber and leade might be different, but with todays barrels, I don't think you would have to worry about the new barrel being bad. Any other loads would have to be developed for the rifle, but your pet load would have it's best leg forward.
Ron

QuoteOne of the things that crossed my mind is that certain rifles in certain calibres just seem to be more accurate than others. Might it be that the particular shape of the whole and the specific points of attachment etc. are just more in "tune" with the "standard" factory loads than those of the next manufacturer's rifle?
 
These musings open up a whole new area of research into the "accurate rifle".

We're definitely on the same "wavelength" here.
 
Noyb72,
Disregarding any problems with machining, I think what you suggest could be done. However, one of the foundation principles of engineering is to predict what will happen without having to fabricate something new for every instance. There's a long history of how slavishly adhereing to this principle usually leads to 'problems', but a generalized model that provides a starting point, and works, would save a lot of time, money and effort.
 
Paul

Couldn't we use a deliberatly short barrel and then put on an extension that fits with your model that started this whole thread as a starting point?
Ron

I s'pose.
 
Paul