Can Somebody Explain "SWEETSPOT" ?

[Jimbo357mag]

I know what it is and I know how to get there but when loading a cartridge (other than lead bullets) why is it usually slightly below max. pressure and what is happening? Considering a test barrel why is the accuracy usually better as the pressure increases, to a certain point, for any given powder? Why are some powders more accurate than others for any given cartridge. I know we are talking pressure/time curve and velocity/time curve and barrel harmonics etc but most times accuracy is predicted at slightly below max. I don't get it? Why isn't a slow bullet just as accurate as a fast one?

edit to add: I should mention that I have noticed that faster powders seem to have good accuracy at lower pressure and velocity. I guess that is why W-231 makes a good plinking load.

...Jimbo

 

[Ale-8(1)]

Without going into the technicalities, it depends on the bullet.

Some are "happier" and stabilize better at higher velocities, others at more modest velocities. The classic 148-grain lead wadcutters used in .38 Special target loads are supremely accurate at what are considered very slow velicities . . . at relatively short ranges, as well.

Each bullet will find its "sweet spot" at some velocity/spin rate unique to it. Thus are there a zillion loads listed in the manuals.

 

[sturmenater]

ale-8 mentioned some great resons for acuracy and inacuracy

also i will mention case capacity and how much powders fill the case compared to others or air volume left in the case plays into the mix.

you have to play around to find the right combo

 

[Snake45]

Every gun and ammo combination is a law unto itself.

Finding any gun's "most accurate" load is as much a matter of sweat, luck, and art as it is a matter of any kind of science.

ETA: A good place to start is with loads that have worked well for other people--the more other people, the better. But be advised that there is NO guarantee that just because some given load has produced tack-driving accuracy for everyone else who's ever used it, that YOU will have the same result with it.

 

[Ale-8(1)]

Absolutely, Snake. That's why reloading is so popular to serious shooters.

 

[gregs45auto]

You also get into barrel "harmonics". The bbl will vibrate like a tuning fork in a figure 8 pattern. Where the bbl is pointing when the bullet exits. Just how fast the velocity is vibrating bbl. etc etc etc. It is by varing the powder charge to find accurate loads. I am not a benchrest shooter. They take this to the extreme!! hth instead of muddying up the discussion!! greg

 

[Sig685]

My belief is that each bullet has velocities nodes that work well for it. One needs to discover where these nodes are. This is why one hears of someone shooting a specific bullet caliming insane accuracy at a specific velocity and other finding equaly insane accuracy a few hundred FPS faster or slower. So the guy with the fast velocity says "push it hard" and the guy with the slower velocity says "I like them accurate more than I like them fast."

The truth is, they are all correct. They just found different velocity nodes.

So this is the way I go about finding the node that I want. I pick my the bullet that I believe will help me achieve my goal. I realise many of you are tired of hearing about long range shooting, but that is a very demanding discipline. My goal is to have the bullet arrive at the 1000 yard line at greater that Mach 1.2, because I believe the transoninc disturbance are begins to affect the bullet between Mach 0.95 and 1.20.

So, I pick the bullet that has a BC and a weight that will allow me to have the minimum velocity at 1000. I always discount claimed BC a little it, more depending on the manufacturer. I run the numbers on JBM until I figure out the minimum MV required to achieve my goal.

If you are a hunter and your goal is a PBR of 350yards or some such, you can run the numbers with the bullet that you want to use for its terminal ballistics and come up with a minimum MV.

So let's say we are looking for a minimum MV of 2750FPS, these means we need to look for an accuracy node that is higher than that velocity. In my case, an insanely accurate node with 2600 FPS MV is useless to me. If the PBR you want needs a minimum MV of 2750, 2600 is not going to do it for you either.

I prepare a bunch of cases and the bring a press, bullet seating dies and powder measure (scale) to the range, along with a chronograph. I did my homework at the powder manufacturer website and I know the minimum load needed for the velocity we are looking for. I load cases and shot one at a time over the chrono. In a few rounds, I will have found the load that meets the MV I am looking for.

Here is a strange observation for you, I can actually feel when an accuracy node is reached, the report is minimal, the disturbance in the rifle and the barrel is non-existent. You have to see it to believe it. The more scientific approach is to load 3 cartridges at .5 grain interval and either run them over the chrono for ES and SD or shoot for group. If you can do both at the same time, good for you. You should be able to find the upper and lower limit of the target accuracy node and I find the best accuracy will be in the middle of these limits. You can then play with .1 or .2 increments, but I usually call it good enough.

When I reach that, I then load 5 or 10 cartridges with that load and shoot for a good group. You have to be able to say; "good enough for me."

At that point, you can mess around with bullet seating length if you want.

 

[Jimbo357mag]

Thanks guys, interesting thoughts all, especially the looking for node peaks response.

...so let me see, for a given gun, cartridge, primer, bullet combination there is probably an ideal velocity for best accuracy. But depending on which powder is used there may be a need to make some adjustments to velocity to attain the best accuracy with that powder and ultimately some powders will be more accurate than others.

...aaah, it is still a little mysterious to me how this whole thing comes together. It does seem like incremental trial and error still plays a big part in finding a good load.

...Jimbo

 

[firescout]

My belief is that each bullet has velocities nodes that work well for it. One needs to discover where these nodes are...

Hillbilly terminology: Node = was aware of :mrgreen:

 

[Jimbo357mag]

My belief is that each bullet has velocities nodes that work well for it. One needs to discover where these nodes are...

Hillbilly terminology: Node = was aware of :mrgreen:

I "node" that !!! I wern't born yesterday. :roll:

...Jimbo

 

[sasu]

One parameter is the powder burn rate and it is affected by the chamber pressure. Most powders are designed to burn most efficiently at the pressure levels they are typically used at. That is why you get more uniform pressure curves at near maximum loads.

A more uniform pressure curve probably accelerates the bullet more softly and also lets the barrel vibrations be more repeatable for each shot.

This is my speculative, unscientific, unconfirmed five cents. Feel free to tear it apart.

 

[Sig685]

(snip)
...so let me see, for a given gun, cartridge, primer, bullet combination there is probably an ideal velocity for best accuracy. But depending on which powder is used there may be a need to make some adjustments to velocity to attain the best accuracy with that powder and ultimately some powders will be more accurate than others.

...aaah, it is still a little mysterious to me how this whole thing comes together. It does seem like incremental trial and error still plays a big part in finding a good load.

...Jimbo

I don't agree. I said that a given BULLET has accuracy nodes in velocity ranges. I do not believe it has anything to do with barrel or primer or cartridge or powder; it's the bullet that sets these nodes.

What the powder and primer, etc will do is make sure that once the sweetspot is found for your bullet and velocity, that velocity is repeated over and over again. The barrel will be needed to aim the bullet properly.

So, your first task is to identify these accuracy nodes for the bullet that you want to use and then work on making the ammunition all the same.

Let's say you found that for your bullet, there is a nice accuracy node at 3000FPS. As long as you can reproduce the 3000 FPS exactly, for every cartridge you shoot, you will have a good load. A longer barrel can let you find a faster accuracy node and/or reduce the load for the same MV.

That's my theory.

 

[WESHOOT2]

You also get into barrel "harmonics". The bbl will vibrate like a tuning fork in a figure 8 pattern. Where the bbl is pointing when the bullet exits. Just how fast the velocity is vibrating bbl. etc etc etc. It is by varing the powder charge to find accurate loads. ........

This point is often overlooked, but it remains a critical component for 'accuracy'.
All barrels do it.

Also consider the size and shape of the (normally) unfilled case, and how it, too, can affect powder burn rate.


But MOST IMPORTANT: Are you good enough to notice? :mrgreen:

 

[Jimbo357mag]

excerpt>
But MOST IMPORTANT: Are you good enough to notice? :mrgreen:

Oh sure, at 50 yards 1/16" this way or that ....and with my handguns, well I don't want to brag but I have done pretty well in some of those RugerForum shooting contests. ...yeah right. :roll: :roll: :roll:

...Jimbo

 

[Lost Sheep]

One general definition of "Sweet Spot" can be found at http://en.wikipedia.org/wiki/Sweet_spot

Partially, it says a "combination of factors suggest a particularly suitable solution".

Control for all factors (powder, pressure, primers, barrel length and weight, etc) and then optimize the performance results you want. Pure accuracy, velocity/powder ratio, etc. When one (or a mathematical combination) of those parameters is optimized: that is the "sweet spot".

How you find it is the stuff of miles of bookshelf space, both published books on internal and external ballistics, and even more of privately compiled load and performance data collected by individual shooters/reloaders.

Good luck, good shooting.

Lost Sheep