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How close to 125 power factor should I get

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Just came across this thread. Better to use the standard deviation to determine your load. For a 147 gn bullet you need 850 fps to make minor. Just add 2*STD to get your desired velocity. You stated your STD was 20 fps, so at least 890 fps will give you a high confidence of always making PF. See JAN/FEB 2015 FrontSight for explanation or PM me for details.

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I still see a lot of people with the mindset that an additional 5 points of PF should be enough. Just don't be surprised when you fail to meet PF at a match. The STD should always be used to determine your load velocity. I explain why in the article.

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OP, you could be chrono'd on the first stage, or the last stage, of a Level II Match. Velocity is a function of air density, ie, temperature, atmospheric pressure and humidity. What you chrono at 100F, 85%RH at sea level will not be the same as 65F, 35%RH and 6,000 ft. Match chronos are arranged in three's so it is an average of averages, not just deviation from one unit. Better to be closer to 135 than 125. You go sub minor for no score. Besides most guns are also more accurate at a higher floor.

Edited by 9x45

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Let me elaborate somewhat on my original post:

Pick whatever load you plan on using, chrono ten, if the slowest goes 130PF then you are good to go. If not, load a little hotter til the lowest velocity makes 130PF.

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Somewhere between 130-135 I call it good. Plus, it will knock down steel better, or so I tell myself.

This.

It absolutely gives you a little margin of safety on low struck poppers.

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I bump up .1 grains at a time. Shoot 10 and if my slowest is over 125 that's where I stop.

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I prefer a more scientific approach. If you ignore STD, your chance of failing PF could easily double and you would never know. Here is an example. Using a PF=130 for a 147gn bullet corresponds to a velocity of 884 fps to make minor. This corresponds to a chance of failing PF of 15% (see article mentioned above). As long as the standard deviation of your ammo remains at or below 17 fps, there is no problem. But if something changes and the STD of your ammo is actually say 25, then your chance of failing PF just jumped to 30%.

That is why I recommend adding a minimum of 2*STD to the velocity required to meet PF for your bullet weight. This corresponds to only a comfortable 10% chance of failing PF. Use 2.2, 2.5 or even 3 times STD depending on the range conditions and other unknowns you are dealing with.

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I'd bump it up some as well. Riding right on the edge isn't that big an advatange, and if you don't make chrono it really sucks for the match. My .02 cents.

Coach.

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Somewhere between 130-135 I call it good. Plus, it will knock down steel better, or so I tell myself.

This.

It absolutely gives you a little margin of safety on low struck poppers.

This is what I do.

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I prefer a more scientific approach. If you ignore STD, your chance of failing PF could easily double and you would never know. Here is an example. Using a PF=130 for a 147gn bullet corresponds to a velocity of 884 fps to make minor. This corresponds to a chance of failing PF of 15% (see article mentioned above). As long as the standard deviation of your ammo remains at or below 17 fps, there is no problem. But if something changes and the STD of your ammo is actually say 25, then your chance of failing PF just jumped to 30%.

That is why I recommend adding a minimum of 2*STD to the velocity required to meet PF for your bullet weight. This corresponds to only a comfortable 10% chance of failing PF. Use 2.2, 2.5 or even 3 times STD depending on the range conditions and other unknowns you are dealing with.

^^^This.

Within reason, most of the suggestions to simply bump the load up until the Power Factor is 5 or 10 above the minimum will work. But it all depends on how much variation your load gives. Too much variation and that 5 to 10 point buffer might not be enough.

I use the twice-standard-deviation method listed here. I will take the average velocity, subtract twice the standard deviation, and use that adjusted velocity to calculate my power factor. Using this method I'm confident that 95% (or more) of my loads meet the power factor requirement.

I've entered a formula into my spreadsheet that automatically calculates the power factor after subtracting twice the SD.

post-42299-0-83269700-1450129465_thumb.j

Edited by Blacksamwell

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Weigh your bullets to know the real weight. Fire 5 shots over the chrono, delete the highest and lowest velocities, get the average of the remainging 3 and calculate the Factor. If you are between 5 and 7 points above minimum you will always be fine.

Do not discard any value, because they are very relevant. You may go to a match and find that all of the loads are on the low side and you're DQ'd.

Run 10 rds over the chono and you will have a better idea how your loads are doing. I run chrono at some major matches in the NE.

Strive for 133 - 135 and no lower, and then you can be sure that a trip isn't wasted.

You don't get DQ'd for failing chrono.

If you are major and go below 165, you shoot minor.

If you shoot minor and are below 125, you shoot for no score.

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I still see a lot of people with the mindset that an additional 5 points of PF should be enough. Just don't be surprised when you fail to meet PF at a match. The STD should always be used to determine your load velocity. I explain why in the article.

that makes sense if you want to always make pf with the first 3 rounds. since in uspsa, you actually get 6 rounds, and take the highest 3, I don't worry about STD as long as the average is around 130. In general standard deviation isn't going to kill you. A low average velocity, or a HUGE STD and bad luck might cause you to go under.

fwiw, I do chrono at least once and sometimes twice before any major match, just to make sure nothing has changed in my materials or procedure.

Edited by motosapiens

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I bought a chrono because I didn't like sweating at chrono stage and having a chronograph is something a reloader should have, I bought the sane one used at Nationals, so I hope that helps, I live an hour from Universal Shooting Academy so conditions will not be much different.

At IPSC Nationals I was 170.2

At USPSA Nationals I was 166

Same loads, loaded at the same time, two different guns, both STI Edge .40 S&W.

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Weigh your bullets to know the real weight. Fire 5 shots over the chrono, delete the highest and lowest velocities, get the average of the remainging 3 and calculate the Factor. If you are between 5 and 7 points above minimum you will always be fine.

Do not discard any value, because they are very relevant. You may go to a match and find that all of the loads are on the low side and you're DQ'd.

Run 10 rds over the chono and you will have a better idea how your loads are doing. I run chrono at some major matches in the NE.

Strive for 133 - 135 and no lower, and then you can be sure that a trip isn't wasted.

You don't get DQ'd for failing chrono.

If you are major and go below 165, you shoot minor.

If you shoot minor and are below 125, you shoot for no score.

In IDPA you will be shooting for no score regardless.

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Moto - yes, the intent of my analysis was to pass PF first time every time with a quantifiable confidence level. You only get two attempts. The important thing to note is that you can remove many unknowns that impact velocity by using your measured STD to set your desired average velocity. With only 3 samples, HUGE variations can exist, with 6 samples, yes there is less variation but who knows if it will be enough.

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RePete is correct, do not toss out any values, that's the ever present variation which is quantified by your standard deviation. And it is the best metric which determines your chance of failing. I agree, if you are closer to 8 to 10 PF points, you are probably okay but it depends on all the unknowns and your STD.

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On ‎12‎/‎8‎/‎2015 at 11:54 AM, jwhittin said:

I still see a lot of people with the mindset that an additional 5 points of PF should be enough. Just don't be surprised when you fail to meet PF at a match. The STD should always be used to determine your load velocity. I explain why in the article.

Just went back and read your article!  great job explaining this concept, will certainly use in my reloading!   thanks!!

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Found it interesting up above,, long drawn out formula,,, to arise at 5 pf over. I mean really. If ur Standard deviation is over 20 you probably need to look at your loading techniques or components.

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I’ve always had my ammo make 135 on the nose at every major match I’ve been to. Very low SD, very predictable at the chrono. One sectional I shot last year my ammo read 130. Went home and it shot 135 over mine and my buddy’s chrono. Moral of the story is a little extra cushion never hurts. I might have really been sweating that day if I had been loading for 130. 

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Completely agree^^^

I liked to keep mine at 135 even with a tight SD to avoid any irregualrites in chrono equipment. 

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14 minutes ago, Adamj said:

Completely agree^^^

I liked to keep mine at 135 even with a tight SD to avoid any irregualrites in chrono equipment. 

 

And if someone needs convinced of that? Load ammo to 130 and to 135. You will not be able to feel the difference, with the same bullet and powder. If you can, it’ll be extremely slight.

 

Heat it up a bit. It doesn’t hurt.

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1 hour ago, MemphisMechanic said:

 

And if someone needs convinced of that? Load ammo to 130 and to 135. You will not be able to feel the difference, with the same bullet and powder. If you can, it’ll be extremely slight.

 

Heat it up a bit. It doesn’t hurt.

Exactly. I used to scrape the power factor floor and try to get is as light as possible like that would give me some advantage. I have since realized that such a small change doesn't affect my shooting. My bullets just get to the C zone a little faster now. 

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I load for a minimum of 5 over desired power factor with my nominal bullet weight. My projectiles typically come in heavy so I'm usually 5-10 over desired PF. Plenty of room for error for me. Three last thing I want to do is travel to a major match and not make declared power factor.

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3 hours ago, MemphisMechanic said:

Heat it up a bit. It doesn’t hurt.

 

It usually helps my accuracy to keep it a little hotter, up around 135PF.  I don't really notice a difference at all in recoil between say 127PF and 135PF.  But, when I shoot groups at 25 yards the 135PF loads group better and are more consistent for me over the 127PF loads.  The powder seems to burn cleaner at the slightly higher pressures too.

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