Power factor margin of error

[GunBandit]

Quick question. What margin do you give yourself for power factor when you are reloading?

I ask because today I chrono'd some .45's I loaded and the AV was 746 fps. That comes to a PF of 171. I shot them over the chrono in 25 degree weather so that could mean they are reading a little lower than they would in warmer weather, I just wondered if that was cutting it too close. Appreciate your input.

[ttolliver]

When I was casting about to answer that question myself, I found that a lot of people tended to consider 5 points over safe enough. Those threads that had people loading to only 3 points over often had other people saying that might be a bit narrow and surprise you when you least expected it.

As you already pointed out, one consideration that could work either for or against you is the temperature sensitivity of your powder.

And you'll see comments here about keeping ammo out of sunlight in extreme heat conditions, etc. Or firing the first manually loaded round into the berm when you get to the chrono. I figure those people who dial it down to 2 or 3 points over minimum PF have to take those precautions pretty seriously. The rest of us less so.

[Sarge]

I like 7 over minimum. So 132 or 172. You are probably good at 171.

[MikeRush]

It depends how sensitive your powder is to temperature as well (as well as the direction of that sensitivity). I had a load barely make it (169) on a sunny SoCal day go 179 on the cool crisp morning I chronoed at Nationals. Same charge, bullet, everything- less than a week apart.

[ZackJones]

I like PF minimum + 10. I know that's a bit higher than others but when you consider travel costs plus match fees for an area or state-level match the last thing I want to do is not make my declared PF. Also, I don't know that my body is conditioned enough to tell the difference between a 165, 170 or 175PF so I'd rather deal with a little extra recoil than risk it.

[johnmac]

Keep in mind, it's not only the "average", but the variability of your load speeds, and the process called out to determine power factor, that can influence whether or not you pass chrono. For instance, to paraphrase the USPSA chrono process from Appendix C2, it appears you get to shoot three, and if the average of those three does not make PF, you get to shoot another three, and the top three of the six total are averaged. For IDPA, it appears two out of three shots must be at, or over, PF. And for IDPA, you get two chances to do this. These technicalities are not an issue if you're way over required PF, but might make all the difference if you're close. When working up loads for PF, I'm most comfortable shooting a few strings over the chrono, and making sure no weird, but likely combination of low velocity shots could bite me, and not just looking at the average.

I've always taken for granted that the advertised bullet weights on the boxes I buy are reasonably correct, I probably shouldn't. Although, with the crimp I put on my cartridges, I really don't know if anyone could pull them to weight them at a match (I cannot pull them with my "hammer" type puller once I apply a heavy crimp).

All than being said, I target developing loads at least 7-10 over PF, so I don't have to sweat some weird environmental factor causing me to tank on chrono.

Edited January 26, 2014 by johnmac

[Hi-Power Jack]

one consideration is the temperature sensitivity of your powder.

firing the first manually loaded round into the berm when you get to the chrono.

Yes, depends on your powder type - some are more sensitive than others.

I believe firing the first round into the berm is more re: accuracy & POI than velocity, though.

I fire 20 shots over a chrono and take the one lowest number - if that is good, I'm happy.

[mgardner]

I like 10 over PF. I'll use range brass for "lost brass" shoots and the velocity can vary enough to get me in trouble if I'm close.

[deerassassin22]

I also like 10 over PF means I never have to worry about the chrono stage ever.

[chirpy]

GunBandit, you should chrono them at higher temp or you might keep them in a heated container until just before you shoot them. Some powders read LOWER at high temp believe it or not and people will advise you to keep your ammo cool before chrono. As to your oc, I think 5-10 would work most of the time, but then again one never knows.

FWIW

Richard

PS: johnmac, never take bullet weight for granted when it is so easy to weigh a representative sample!

Edited January 26, 2014 by chirpy

[TwoShot]

I want every round out of a sample size of 10 to be at least 7 over pf. Chrono is the last thing I want to worry about at a match!

[Hi-Power Jack]

I want every round out of 10 to be at least 7 over pf.

That seems a little high - what PF do you Average for 20 rounds?

I like every one of 20 shots over the chrono to be over the necessary PF,

but not 7 over...

[Sarge]

I want every round out of 10 to be at least 7 over pf.

That seems a little high - what PF do you Average for 20 rounds?

I like every one of 20 shots over the chrono to be over the necessary PF,

but not 7 over...

So you are saying if you shoot 20 rounds and each one is 125.5 you call that load good?

[Nik Habicht]

I used to chrono ten percent of what I loaded for matches -- literally set aside ten random rounds of each 100 loaded, everytime I added Primers to the press.

In order for me to consider the other 90 rounds to be good enough for the match, the ten round sample had to do three things:

Average 132 pf for the string of ten.

Each Round had to exceed 1000 fps for 125 grain heads, i.e. exceed the required power factor

And at ten yards they all had to group in the upper A-zone on the target beyond the chrono -- easier to accomplish than you might think, when avoiding hits on the skyscreens/supports.

I never worried at Chrono.....

[TwoShot]

It may seem high to some but I have heard too many horror stories like, "My ammo made 170 at home, at the match it was 164.9!" With everything 7 over power factor, I usually run 7-10 over at a match. When I pull a sample of 10 and they go 7 over like I want, that is not a guarantee there is not a couple 167's in the box. Just trying to do enough so that I don't have to worry or get any rude surprises!

[TH3180]

9mm I'm about 135pf, 40 S&W about 171pf. With TiteGroup for 9mm and WST for 40S&W. I have a SD of about 10fps for both. I'm good with that.

[Hi-Power Jack]

I want every round out of 10 to be at least 7 over pf.

I like every one of 20 shots over the chrono to be over the necessary PF,

if you shoot 20 rounds and each one is 125.5 you call that load good?

I would, but it's never happened, unfortunately.

I usually get a range of pf 126 - pf135, or so, for 20 shots, and as long as that lowest number is above 125, I'm good to go.

[Sarge]

I want every round out of 10 to be at least 7 over pf.

I like every one of 20 shots over the chrono to be over the necessary PF,

if you shoot 20 rounds and each one is 125.5 you call that load good?

I would, but it's never happened, unfortunately.

I usually get a range of pf 126 - pf135, or so, for 20 shots, and as long as that lowest number is above 125, I'm good to go.

Flirting with disaster Jack. Let's say out of those 20 rounds you get that one 126 PF load. It's too easy to say, "If I don't go lower than that I'm good".

Now let's say you go to a match and their chrono doesn't read the same as yours. Or it's hotter, colder, higher or lower (elevation), etc... Those 126 loads are now 124.9's.

This is where the importance of all the little things like sorting brass by headstamp, smooth operation of the press, etc come into play. With a nice low SD in the single digits or close to it, you will be able to target, let's say, 132 PF and there won't be anything even close to 126 coming out of your gun. With work you loads can chrono within 1-2 PF of each other quite easily. If I get all 130's across a chrono at a match I stand a much better chance of making minimum PF.

It's a very nice feeling to get Duplicates on a major match chrono.

Edited January 27, 2014 by Sarge

[GunBandit]

Keep in mind, it's not only the "average", but the variability of your load speeds,

I agree. I used average here because I was lazy with my typing. I fired 7 rounds that I had worked up and had a high-low spread of 20 fps. I had one round at 732 and several at 745-746 and one at 752. Needless to say I increased my charge by 0.1gr to make sure they were good and I need to check those to see what the increase is. It is about 13 fps higher on paper, but that is not rounds down range. I did go back and weigh some of the bullets and random selection I got was about 2 grains light. With the increased charge I should be ~PF+7 but we will see. I'm probably overthinking this by trying to build a lighter load that still makes PF. Like ZackJones said, I probably couldn't tell the diff between 165-175 PF loads , but I would like to think I could tell the difference between my loads and say WWB that is making 190+ according to their posted velocity and bullet weights for 45 Auto.

I appreciate everyone's input on this and I will try to keep it 10+ to be on the safe side.

[Hi-Power Jack]

I like every one of 20 shots over the chrono to be over the necessary PF,

if you shoot 20 rounds and each one is 125.5 you call that load good?

I usually get a range of pf 126 - pf135, or so, for 20 shots, and as long as that lowest number is above 125, I'm good to go.

Flirting with disaster Jack. Let's say out of those 20 rounds you get that one 126 PF load. It's too easy to say, "If I don't go lower than that I'm good".

Now let's say you go to a match and their chrono doesn't read the same as yours. Or it's hotter, colder, higher or lower (elevation), etc... Those 126 loads are now 124.9's.

This is where the importance of all the little things like sorting brass by headstamp, smooth operation of the press, etc come into play. With a nice low SD in the single digits or close to it, you will be able to target, let's say, 132 PF and there won't be anything even close to 126 coming out of your gun. With work you loads can chrono within 1-2 PF of each other quite easily. If I get all 130's across a chrono at a match I stand a much better chance of making minimum PF.

It's a very nice feeling to get Duplicates on a major match chrono.

Yup, for major matches I sort brass - my last major match with 9mm minor was PF 130 (Area 7).

And, my last Area 6 with 9mm major, two of my chrono's rounds were right at 165, but the

third was 174, so I was safe:)

Guess you're right, might be good to go another .2 grains, but so far, so good:)))

[snertley]

+1 on being aware o what your load will do under different temps. I had my ammo drop almost 10 PF due to the temps being around 90* out that day. It when from 174PF to 165.2!

[warpspeed]

So for major PF with a 230gr bullet, you need to have a velocity of 718 fps assuming the bullets weigh 230gr.

For me, I'd pull 10 rounds at random from my ammo and shoot them all over the chronograph. If they all are > 718, the load is good. If they are not, bump up 0.1 gr and repeat.

[chirpy]

Remember also, that changing lot #'s of powder can cause velocity to change!

Richard

[Nimitz]

For those of you who sort your brass, what head stamps are you looking for?

[ttolliver]

+1 on being aware o what your load will do under different temps. I had my ammo drop almost 10 PF due to the temps being around 90* out that day. It when from 174PF to 165.2!

Mind if I ask what powder you were using?

Let's play Devil's Advocate for a bit --

If we think about all of the factors that cause variation, the reloader really shouldn't be surprised by anything but minor differences in chronos. Everything else we either control or should anticipate, right? Per Sarge's comment, we may not have chosen to develop a particularly low SD load to begin with. Temperature we don't control, but we do know the extremes we'll shoot at and we make the powder selection.

There's nothing wrong with shooting a 231 load developed in Antarctica at a match in Death Valley and that was loaded into mixed headstamps with your 7 year old nephew running the press. Well, maybe a little. There's no excuse for not making your nephew sort headstamps

Say we allow 3 PF of padding for random acts of the cosmos, variations in chrono lighting conditions, etc. How much more to pad really should be based on the reloaders understanding of the load they're bringing to the chrono.