What's your Power Factor margin of error

[leam]

As I've only been reloading for a couple years my technique is still pretty basic. I measure out a few times from the RCBS Powder dispenser and the RCBS 505 scale keeps me honest. If there's a question I scale for a tad over vice under. The 4.1 grains of Clays has given me good solid service and I can't complain. Just found a note with some chrono results; I'm shooting a 180 PF! That's a good bit of safety margin.

I'd not mind backing it down a tad but am not sure what is a good margin. "Good" being defined as what I can do repeatably without lots more work and tools to make it even more accurate. In October I'm shooting the GA state match, after that it's back to local club shoots. That is, one light round won't cause any great heart burn but i'd prefer to shoot in local matches what I'd shoot at State.

Thoughts on how to trim down the PF and still keep things right?

Leam

[TM262]

Major 169 - 172 with single digit SD

Minor 129 - 133 with single digit SD

[Chris Keen]

Chrono'd 175 at Nationals today, but keep in mind I am planning on shooting the IPSC US Nationals, where Standard is 170pf. So I am a comfortable 5pf above the minimum.

[Sarge]

I shoot production and am working on a load that will get me close to 135. +10 should give me one less thing to worry about at matches.

[Jim Rusert]

Shooting a Glock 35 in Production division, PF 145 not primarily for a margin of error but because my gun runs great at that PF!

[lugnut]

I shoot for 135 PF in Production but my Glock likes it that way.

I was a bunch over 170PF last time I shot Major with my G21... probably close to 175 but I want it around 171-172.

[bkeeler]

I would go 8-10 over the PF needed. So If you need 165PF I would load to 173-175PF. I currently bumped up my load recently, chronoed at a match and it was a little too close for me(167PF @ home it was 171PF). Your not going to feel the difference between 172-175 PF under match conditions. Get the PF you desire and practice with it and do some Burkett Timing Drills.

BK

[steel1212]

I tend to want it to be 170PF for major and 130ish for minor. I run 1911s so I can spring my guns to work well with the minor. My loads have been consistent, when I've chronoed them, at every match I go to. Same loading procedure for every major match I've been to. Once fired brass, winchester primers, JHP bullets, same OAL, etc.

[BMartens]

Major 169 - 172 with single digit SD

Minor 129 - 133 with single digit SD

+1

Plenty of cushion.

[BritinUSA]

175... Past few years I have factored locally at 172 and only made 165.n at Nationals.

Edited September 18, 2009 by BritinUSA

[G-ManBart]

175... Past few years I have factored locally at 172 and only made 165.n at Nationals.

Pretty much what I'm looking at as well.

Last year my one batch of ammo went 172-174 at several big matches, was always in that range at home and went 166.1 at Nationals....the exact same batch loaded at the same time. Changing absolutely nothing on the press, and using the same lot of powder (bought a bunch) and bullets I was again in the 173-174 range at home and went 172 at Nationals. The cases were from a different lot, but both were new and the same brand etc. I did switch primers (who hasn't?) but in testing they chrono'd the same.

I can't figure out why it was so low last year (temps were even close to what I'd tested at) and so close this year. This year I actually got a light bullet (115.1 vice 115.3 or 115.4 which is more common) so that accounts for most of the difference I saw between home and Nationals. It may have just been a coincidence of three low readings last year, but that would be long odds since it would have been three at the low end of the ES that is typical for that load.

If I'm not getting a consistent 173+ at home, I'm bumping up the load a touch until I am....it's not worth the risk. R,

[Chris Keen]

I actually got a HEAVY bullet weight this time at Nationals ...... 165.9 When I weigh my bullets at home it's generally 165.1 or 165.0

[BritinUSA]

I actually got a HEAVY bullet weight this time at Nationals ...... 165.9 When I weigh my bullets at home it's generally 165.1 or 165.0

That was probably the same coating of dust that was on my car, camera, camera bag, magazines, gun, shooting glasses, hair, shoes, socks, underwear (how the heck it got in there I will never know). That's the extra weight, right there.

[Chris Keen]

[vluc]

Shooting a Glock 35 in Production division, PF 145 not primarily for a margin of error but because my gun runs great at that PF!

Bingo! Let the gun tell you what works best and don't get hung up on a number.

Run a G34 and my PF is the 136-139 range because my pistol just seems to run better at that. Same when I use that exact load in my G17. Range drops to around 132-134 or so.

Edited September 18, 2009 by vluc

[Graham Smith]

Shooting a Glock 35 in Production division, PF 145 not primarily for a margin of error but because my gun runs great at that PF!

+1 In trying to find a load that gave me a consistent PF with a low SD and performed well, I found I had to get up to about the same place with my G35. Any lower and the SD started going up and the performance went down. It's taken me time to realize that any load recipe has to include my gun. Which is why it's important to chrono your loads on a regular basis.

Also, consider this, if you allow for a total variation of 10% (± 5%) you will have to have a mean PF of 135 or risk falling below the 125 minimum. So, the more you can tighten up your variance, the lower your average PF can be. Also, a low variance should give you tighter groups.

[Duane Thomas]

Whether or not a tight SD means tighter groups can be greatly dependent on bullet weight. I was discussing this awhile ago with Bruce Gray, and commenting on the fact that the Federal 165-grain Hydra Shok in my testing had some of the highest ES I'd ever seen out of any ammo, handloaded or factory, absolutely horrible SD....and was consistently some of the most accurate ammo tested in any gun I put it through. Bruce commented that differences in velocity really don't seem to make that much difference, accuracy-wise, with light, fast bullets. Remember the gun is moving as the bullet travels down the bore. With slow, heavy bullets, differences in velocity can have a profound effect, especially on elevation, because the bullet is traveling down the barrel for a long time, relatively speaking. Whether that bullet is moving faster or slower means the barrel will raise less or more, respectively, before the bullet is out of the gun. By contrast, with a fast, light bullet, the bullet is gone before the muzzle can move much, one way or the other. Therefore, velocity variations make much less difference in potential accuracy with light, fast bullets than slow, heavy bullets. Or so sayeth Bruce Gray, and my experience with the Federal 165-grain Hydra Shok would tend to support that.

[zhunter]

171-172 at home

My loads always seem to chrono higher at matches tho

Last years SS Nats my load was 183 on their Chrono, but I was used to it as I use it all the time

[Duane Thomas]

It may have just been a coincidence of three low readings last year, but that would be long odds since it would have been three at the low end of the ES that is typical for that load.

This is why it's so important to know not just your load's power factor, but its standard deviation (SD). SD, for those who don't know, is a measurement of how close you can expect any particular shot out of that gun, with that load, under those environmental conditions, to be to the average velocity. If I can believe the info sheet that came with my CED Millennium chronograph, how SD works is that, statistically, it's been proven that 99.7 percent of rounds fired will fall within three standard deviations above or below the average velocity, 95.4 percent will fall within two standard deviations above or below, and 68 percent will fall within one standard deviation above or below. This is important information to have.

Just to make the math simple, let's say we're firing a 130-grain bullet, trying to make the 125 power factor in USPSA/IPSC, our goal is a 1,000 fps average for a 130 power factor, and we do in fact achieve that. Is this going to guarantee we make power factor on match day? (Granted the environmental conditions are the same, which is why we obsess so much about things like whether or not a powder is temperature sensitive, the effects of altitude on bullet velocity, etc.) Well, that depends on the SD. Let's say our 130-grain, 1,000 fps, 130 power factor load has an SD of 10. That means that we can have every reasonable expectation that on match day 68 percent of rounds fired, basically two out of three, will be within one SD (10 fps) above or below the average velocity, in other words between 990 and 1,010 fps (128.7 to 131.3 power factor). 95.4 percent will be within two SD (20 fps) above or below, or 980 to 1,020 fps (127.4 to 132.6 power factor). And we can pretty much bet the farm that almost every bullet we fire out of that gun, 99.7 percent, is going to fall within three SD (30 fps) above or below, or 970 to 1,030 fps (126.1 to 133.9 power factor).

So, with a 130-grain/1,000 fps/130 power factor load, if we have a 10 SD, we can be confident that even if every one of our shots is toward the lower end of what comes out of that gun, which is extremely unlikely but still possible, we're still going to make power factor on match day. If we can turn out ammo that gives us something like, say, a 6 SD, our comfort factor increases even more. If we have a 20 SD, on the other hand? That means if all three of our rounds come out of that gun traveling toward the low end of what we can expect (940 fps), we're only going to post a 122.2 power factor. We cannot have confidence that that load, with that SD, is going to make power factor on match day, even though it still has the same 130 average power factor as the load with the tighter SD. At that point we have two choices, (1) we can change the load, go for a different bullet, different powder, or whatever other changes we think might tighten up the SD, or (2) we can bump the load up to give us more of a cushion even with the sloppy SD.

Bart, it sounds to me like your ammo, on that day, gave you the "unlikely but still possible" performance of three rounds in a row at the low end of the scale. But because you had paid attention to your SD, you still made power factor, albeit barely.

[BayouSlide]

Run a G34 and my PF is the 136-139 range because my pistol just seems to run better at that. Same when I use that exact load in my G17. Range drops to around 132-134 or so.

Ditto. You saved me some typing. Your numbers are exactly the same as mine.

When I tried running lower PF numbers, the gun felt sluggish.

Curtis

[al503]

I go for 168-170 so long as none of the rounds chrono'd (I usually run 10-15) go below 165. My chrono is on the slow side anyway.

[G-ManBart]

Bart, it sounds to me like your ammo, on that day, gave you the "unlikely but still possible" performance of three rounds in a row at the low end of the scale. But because you had paid attention to your SD, you still made power factor, albeit barely.

That was great stuff Duane....very well put.

I absolutely do track SD (and ES) as well as keep record of the weather when I chrono. That particular load with new cases typically turns in an SD around 8 or 9. On the low end I've seen it at 6 and the high end was 14 (smallest sample size, by a lot, on that one). I'm pretty sure it was just a coincidence of three on the low side in a row, but being my first nationals it had my pucker factor going!

The particular powder (N105) is a fairly long stick and doesn't meter as well as some others such as 3N38 or 3N37. I get mid single digit SD's with 3N38 and a reliable source here told me he's seeing mid single digits with 3N37 (I have some I haven't tried yet)...probably at least partially because they meter so well.

I think the other thing that enters into play is simply variation caused by lighting and eventually I'm going to get around to building a chrono box and probably switch to IR....if you get a number with that sort of setup, it should be pretty reliable. R,

[Duane Thomas]

That was great stuff Duane....very well put.

Thanks! I appreciate the compliment.

I have the infrared skyscreens for the CED Millennium, myself. They're great. For one thing, as you observed, they totally do away with the dependence on natural light, variations in natural light, and the "everything stops because the sun went behind a cloud" effect.

[TreblePlink]

Temperature clearly has an effect. But I strongly suspect that chrono error is an equal factor. The timebase of these things should be very accurate, but particularly with only 24 inch skyscreen spacing, transducer error (bullet leading edge detection) is likely to be present. This can be caused by uneven illumination, as is well known, but ideally should be a very thin plane the bullet passes through. I've not seen this on any of the low cost units.

And when two units are used why do they cascade them with narrow spacing instead of increasing the spacing to 48" and staggering the skyscreens? Start/Start >>>>>>>>> Stop/Stop. Double the distance cuts transducer errors in half.

It would be very interesting for someone to do a controlled comparison of the popular units.

[kaiserb]

One thing I found to eliminate the errors from differing light conditions, is to build a box for the chrony. Inside the box the chrony is always under the same lighting condition, no direct sunlight, shadows, clouds or other items to fake out the chrony and limit the accuracy. My consistency of loads shot across the chrony on different days and times has improved greatly.