jwhittin

Forget about adding PF points, focus on the minimum average velocity needed to make PF and your STD. Its important to understand that there is a huge difference in measuring your velocity using 8 or 10 rounds at the range versus only 3 rounds used in the official PF measurement. You cannot directly compare the two as they are apples and oranges. That's the whole point of my article. Using the table of z values, you can now relate your measured velocity at the range to the official PF test but only via a confidence level. For more of an explanation see this post and the example on 25Feb at about 9AM. http://www.brianenos...opic=229005&hl= http://www.brianenos...opic=229005&hl=

Your data looks good. The resulting PF doesn't matter. You just need to know your avg velocity and SD. Your worst case SD is about 10 so as long as you keep your AV in the 870 to 880 fps range you are set for this load. If you get better accuracy/reliability or just like the feel, go with 890. Final step would be to check AV and SD at hot and cold temp and adjust to ensure both are at least 2SD.

Np. Post your AVG and SD for a few 8 shot strings. I would like to see them.

Yes but use 8 shots and measure SD 3 or 4 different times to see if it is consistent. Use the highest SD.

TGO's way is far too complicated. You shouldn't throw out data. As I said, velocity is random and the best way to understand and mange it is using the STD. Levi - how did you measure your STD? You should shoot 8 rounds and record the STD. Repeat the same process a few times to see if the results are consistent. It will change as the gun heats up. As long as your 8 round average velocity is above 863 (assuming your STD is correct) you should be good. But with a small STD, some unknowns and your concern, just use 3*STD and don't sweat it. Doubt you can feel the increase and you will have piece of mind.

Levi - Well it seems you didn't get what I was saying. Forget the PF, what is your bullet weight and STD. Reread the link in my earlier post.

I agree. If you see bullet variances that cause this, its time to switch to a different bullet maker.

Fishsticks, your measured STD does includes the bullet weight variance. If you are concerned, you can measure the variance of your bullet weight and calculate what the impact would be on PF. Weigh bullets from various boxes and use the worst case variance. It shouldn't have a big impact. If the variance in bullet weight is abnormally high you could switch bullets. I would think most bullets should be within +/- 1 grain may be +/- 1.5 grains. If you want more margin, you can simply bump up your load to say 2.25, 2.5, or even 3*STD.

Here we go again! How close to the min PF should you get? The answer is no less than a minimum of 2 times your standard deviation (STD) (measured using 8 rounds). In other words add at least 2*STD to the minimum PF for your bullet weight. Use 2.25, 2.5 or 3 times your STD depending on the unknowns. It is a waste of time and money to use 10 or 20 rounds as suggested. LeviSS see this post for clarification and examples. http://www.brianenos.com/forums/index.php?showtopic=229005&hl=

Fine tuning a trigger for a limited build. Looking for 2 to 2.25 lb pull. The attached photo shows the results thus far. Hopefully you can see the different colored lines. The sear and hammer in the picture are placed on pins in the frame so their relationship is as installed. I'm seeing conflicting posts on the best sear-hammer surface angle for a competition trigger. I'm at +2 degrees and the hammer hook contacts the sear about in the middle of the sear face. Thoughts, comments, recommendations. Sear Primary angle (green-white) = 81 degrees Sear Secondary angle (green-black) = 45 degrees Sear to Hammer surface angle (red-green) = 2 degrees

I had a chance to run some numbers. The graph below plots the Normal distribution and the T-distribution. The graph is the cumulative distribution function so the Y axis is the probability (p) that a random variable X is less than or equal to a specific value x written as P(X ≤ x) = p. So it’s easy to compare the two now. As you can see, using the Normal dist for x=-2, P(X ≤ -2) = 2.74 % but using the T-dist it is 14.8%. In our case, we want to know P(X ≥ x) = q which is simply 1- p. So using the Normal dist you get P(X ≥ 2) = 1 - .0274 = 97.3% compared to 1 - 0.14758 = 85.2% using the t-dist. This means, that until now, there was no way to relate the measurements we take at the range with the results from the 3 samples used for the official PF determination. Using the values of Z I provided in the table at the top of this post and using 8 samples when you chronograph your ammo allows us to now relate the two.

Sure, once you get closer to 10 points over, you are probably fine. Better to be high than not high enough.

Mark, the confidence levels I calculated are only based on the first 3 rounds. My point is that you can't use the Normal distribution at all. You have to integrate the area under the t-dist for the statistic under evaluation. You can clearly see that the tails of the t-dist are much larger than the Normal dist. I'll run some numbers when I have time to show you the difference. Trying to add a plot I generated.

Motosapiens, In your case, using PF=132 is equivalent to Z=3.2 which provides a high confidence level. As I mentioned above, you can’t use the Normal distribution in this case. It is not accurate for small sample sizes. Take a look at the T-distribution and you will see the tails are much higher. How did you insert the png file? I can show you the T-dist.

"Lies, large lies, and statistics. :devil:" LOL. ChuckS is exactly right! Are you feeling the power of statistics yet!

Great question rev1911! I’m glad you asked. We are using a few samples to estimate parameters such as average velocity and variance. We use these sample estimates to make inferences (judgements) about a much larger population; in this case, the population is all our reloads of the same bullet profile, weight, powder, OAL, primer, etc. It has long been established that for most applications that follow the Normal distribution (such as velocity) only about 8 samples are needed to provide good results when using the sampled estimates to make inferences about the population. Bad results are produced as the sample size goes below about 8 samples and in fact the Normal distribution can no longer be used. Going above 8 samples is not necessary and thus a waste of time and material. The chance of failure given in the table above corresponds to 8 samples.

LOL. This is the good STD.

Very cool. How much would you charge to ...

I still see a lot of people wondering why they fail to meet PF at a match. If you just add a few extra power factor points, now you know what can happen. The bottom line is that velocity is random in nature and the BEST way to understand and manage it is to use statistics. You don’t need to have any special math skills. This post provides the simple steps to follow. For the why and more details, see Frontsight Magazine, Jan/Feb 2015 Edition, Pg. 70. The standard deviation (STD) measures the amount of variation (uncertainty) around the average velocity and it should always be used when reloading. The table below maps the chance of failing an official USPSA PF check to the variable Z. Use Z for reloading by simply multiplying the value of Z by your STD (measured using 8 rounds with your chronograph) and add the result to the required velocity to make major or minor PF for your bullet weight. The result is your desired load velocity. Values of Z are shown from 0 to 3 to illustrate the tradeoff between Z and the chance of failure. My guidance is to use values of Z in the 2 to 2.5 range. Note that if you use the STD (i.e., Z=1), which I’m sure some people do, the chance of failing PF is a whopping 44%. Z Chance of failing PF (per USPSA rules) 3.0 3% 2.5 5% 2.0 10% 1.5 21% 1.0 44% Assume a shooter’s standard deviation for a load with a 155 gain bullet is 15 ft/second (chronographed using 8 rounds). An average velocity of at least 1065 ft/sec is needed to make major power factor. To limit the risk of failure to no more than about 10%, use Z = 2.0 and simply add 30 ft/sec (Z*STD=2*15) to the required velocity to meet PF. Thus the desired load velocity would be 1065+30 =1095 ft/sec. It’s that easy! In our example above, if we “just add a few PF points” and loaded to say 168 PF, this is equivalent to Z=1.26 and the result is a 1 out of 3 or 33% chance of failure. If instead we use the measured extreme spread (which can typically run 60 fps or higher) and load to 1065+60 =1125 fps, it is equivalent to 4*STD or a 186 PF and we needlessly incur excessive recoil and higher cost. If you always load to say a PF=170 (1097 fps), your chance of passing is okay so long as your STD remains below 16 fps (1097-1065)/2. However, if your STD is actually 23 fps, Z drops from just above 2 down to 1.4 (i.e., 32/23) and your chance of failure more than doubles going from below 10% up to 26%. This is why you should measure and use the STD. Shoot with any comments or questions!

As Kevin c pointed out, there are a lot of factors that affect velocity and they do so far more than chrono accuracy. And ALL of them are random in nature. Thus, the best way to manage them is to know and use your STD to ensure you meet PF.

For an explanation of why you shouldn't reload based on adding a few PF points, see page 70 of Jan/Feb 2015 edition of Frontsight magazine. As I explain, you should use the standard deviation (STD) instead. Add at least 2 times your STD to the minimum velocity to make PF. Use 2.5 or even 3 times depending on the unknowns. I think you will fing the example I give helpful. Shoot with any questions.

Interesting. I didn't know about this. I'll take a look and give you my thoughts.

You should definitely think about getting a BUG if you want to shoot a lot. You never know. Could easily take you out for a month or so in the middle of the season. That would suck! Used DW or Baer for $1500 or so would serve you well. I agree, Baer customer service is lacking. I've run both and DW is a great value. Bear is my BUG now. I built up my own gun on a Nighthawk lower and liked it so much its now my primary. Can't go wrong with a Wilson (have 2) or NH either. I don't have any experience with STI or SVI yet.

Flexibility to shoot major or minor, or shoot SS or move to limited all with the same caliber is a plus. Been shooting SS 45 and now getting into limited so got a start reloading 40 cal. in addition to 45. Wish I had a SS in 40 cal now but can't justify the cost ( and then would need primary and backup guns). So many projects, so little money.

I don't recommend the 47Ds. He will probably want to add a magwell later so a larger pad like the Tripps or ETMs work better. I would say run whatever ammo he can get that is reliable and economical. I run 45 and know a lot of people that do. Any weight and profile between 185 and 230 grns is fine as long as it feeds reliably. Get the ammo and then adjust the spring to ensure reliable feeding (will probably be 15 to 17 lbs.). Can always start with SS and get the 10 rounders later if he wants to shoot L10.