Why does my PF go down?

[38SuperDub]

So I went out the other day and shot at the range - I ran 30 rounds through my Chrono - not super fast but at a stead pace - looking back at the results - it seems that the latter shots were slower than the first ones - any reason? Would the barrel heating up cause this?

3.3gr TG @ 1.145 XTreme Plated 147gr - CZ SP-01 Shadow

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 AVG ES SD PF

915 905 908 909 901 917 915 906 894 911 908 23 7 133

886 879 898 889 906 915 912 910 902 886 898 36 13 132

877 880 878 911 894 888 898 893 909 893 892 34 12 131

[bbbean]

So I went out the other day and shot at the range - I ran 30 rounds through my Chrono - not super fast but at a stead pace - looking back at the results - it seems that the latter shots were slower than the first ones - any reason? Would the barrel heating up cause this?

3.3gr TG @ 1.145 XTreme Plated 147gr - CZ SP-01 Shadow

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 AVG ES SD PF

915 905 908 909 901 917 915 906 894 911 908 23 7 133

886 879 898 889 906 915 912 910 902 886 898 36 13 132

877 880 878 911 894 888 898 893 909 893 892 34 12 131

I'd guess a simple random distribution that didn't "look random". There's no reason for velocity to change appreciably over the course of 30 rounds.

BB

[calxd]

So I went out the other day and shot at the range - I ran 30 rounds through my Chrono - not super fast but at a stead pace - looking back at the results - it seems that the latter shots were slower than the first ones - any reason? Would the barrel heating up cause this?

3.3gr TG @ 1.145 XTreme Plated 147gr - CZ SP-01 Shadow

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 AVG ES SD PF

915 905 908 909 901 917 915 906 894 911 908 23 7 133

886 879 898 889 906 915 912 910 902 886 898 36 13 132

877 880 878 911 894 888 898 893 909 893 892 34 12 131

unless your measuring out each individual load on a scale I wouldnt worry about it to much. I run the same components through a dillon . Random powder checks shows me it throws a little less every once in awhile, a tenth or so. Dont know if its me or the cases or something else.just keeps me from setting the powder to low and not make power at all.

[mda]

So I went out the other day and shot at the range - I ran 30 rounds through my Chrono - not super fast but at a stead pace - looking back at the results - it seems that the latter shots were slower than the first ones - any reason? Would the barrel heating up cause this?

Yes, barrel heating up could cause lower velocity due to expansion\dilation of the bore.

MDA

[Hi-Power Jack]

it seems that the latter shots were slower than the first ones ? Would the barrel heating up cause this?

Yes, barrel heating up could cause lower velocity due to expansion\dilation of the bore.

MDA

Not sure, but I would think the bore wouldn't expand with heat, but

the metal barrel might expand a bit, making the bore (hole) even a

little smaller?? Which would increase velocity???

But, I've noticed the same effect a few times - seems to me that I've

noticed my earlier shots being higher, I get all excited that I have

a nice load that meets PF, and then the next ten shots are lower, and

I have to bump up the powder charge.

Haven't done it enought to consider it significant, and I have never

mentioned it because I assume it is coincidental??

[Graham Smith]

In most rifles, clean bores shoot slightly different than dirty bores. Not sure if the same thing goes for pistols or not. May depend on the pistol and the load.

Try shooting a few "fouling shots" before you start recording results or just throw out the first few shots you record.

[superdude]

the other possibility is chronograph drift. the reading shifts slightly over time. i've seen this by using two chronographs.

[38SuperDub]

Hmm - interesting - I'd like to say random but its pretty consistently moving downward - maybe it is random - who knows

[co-exprs]

Yeah, tons of little variables. My 2011 in 40, shoots noticably faster when it's warmed up. Don't know if it's the powder warming up in the case, or the barrel changing in size as it heats up. Maybe the other bullets in the mag had more time to slam a red bull before heading down range.

[98sr20ve]

3.3gr TG @ 1.145 XTreme Plated 147gr - CZ SP-01 Shadow

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 AVG ES SD PF

915 905 908 909 901 917 915 906 894 911 908 23 7 133

886 879 898 889 906 915 912 910 902 886 898 36 13 132

877 880 878 911 894 888 898 893 909 893 892 34 12 131

That doesn't look random to me either. I think its safe to say that the hotter barrel runs a little slower in your gun. It's not a lot, but it's worth knowing.

[ksmirk]

Look at it this way, a chrono stage is normally 3 shots, your over PF either way! I used to really pay a mess of attention to my reloading and chrony results then I figured out I was spending so much time thinking about little things that I couldn't tell made a difference on paper or steel that I was changing more stuff around than shooting. Lot's of variables to put into account and I doubt you could put a finger on one thing, the barrel heat, the heat from the barrel causing the powder to warm in the case, possible setback of bullets during recoil, plating thickness, the planet Mars relationship to the Sun and Venus see where I'm going?

You meet power factor so your golden, just keep it simple and have fun. Later,

Kirk

[Braxton1]

it seems that the latter shots were slower than the first ones ? Would the barrel heating up cause this?

Yes, barrel heating up could cause lower velocity due to expansion\dilation of the bore.

MDA

Not sure, but I would think the bore wouldn't expand with heat, but

the metal barrel might expand a bit, making the bore (hole) even a

little smaller?? Which would increase velocity???

But, I've noticed the same effect a few times - seems to me that I've

noticed my earlier shots being higher, I get all excited that I have

a nice load that meets PF, and then the next ten shots are lower, and

I have to bump up the powder charge.

Haven't done it enought to consider it significant, and I have never

mentioned it because I assume it is coincidental??

Actually, when a tubular object heats up, the hole in the middle does get bigger. The expansion of the area occupied by each atom when it gets excited by heat occurs equally in all directions. A tubular object has more atoms stacked side-by-side than it does "on top of each other". More atoms expanding results in more expansion overall, so the diameter of the tube gets disproportionally larger than the metal thickens.

Bigger bore size equals lower velocities.

[gm iprod]

Braxton1 is spot on with heat expansion. Basic high school metalurgy / engineering. Heat up a ring and you can drop a larger pin in it. Ask any mechanic trying to install a bearing.

The heat expansion on a handgun is not likely the whole reason for the drop in velocity.

The list of variables is large and unfortunately how much each one affects your results is "variable", I can think of the following that will work against you.

Barrel Expansion. Not sure how much it does change as we don't know the start temp and finish temp and exact metallic composition of the barrel

Change of light angle on chronograph. Worse than many think.

Increase in humidity - only really noticeable if humidity goes from one extreme to the other.

Distance from chronograph. 2fps per foot of travel. If you are accidentally 3 feet different then you will loose gain 6fps across the range of the sample tested.

Increase in fouling of a firearm over a period. Long range shooters have long known that many firearms work better (not necessarily faster or slower) once fouled.

I tend to agree with the comment 'random sample that just looks unrandom'.

Sample 1- hi vel 917 SD 7, sample 2 hi vel 915, this is well within SD, even hi velocity of sample 3 911 is within SD. So all hi velocity of each sample is within the SD of the first sample and all subsequent samples.

Average speed across all samples is 899.5fps. Average speed of the thee averages is 899.333fps. Only the average speed of each sample is slightly different to the others, but again difference is within the sample range. Even the slowest from each group is within 1fps of the SD of the tightest sample from each other.

Taking the very highest and the very lowest of all 30 shots gets and average of 898.5. Take any one shot out of a group of samples and add it into any other group and you will not change the SD by much or the average by much. You will only possibly increase the Extreme Spread by a small amount.

The Average power factor change of less than 1% from one sample to the next is not enough of a change to cause me to panic. It is however interesting to keep the data. Do the test again in 2 weeks and repeat for twice again to see if a definitive pattern evolves. A larger set of ten shots over a longer period of time with temp readings of the barrel would then prove or disprove the theory of is it a dirty barrel / gun that affects velocity.

A proper statistician would demand a larger sample of samples and over a wider range of dates and weather conditions to confirm one way or the other. By the time you do all that testing the gun will be somewhat worn out and he will declare the test invalid as the firearm has changed significantly and data from later tests will be unreliable.

Edited June 21, 2012 by gm iprod

[Graham Smith]

So I went out the other day and shot at the range - I ran 30 rounds through my Chrono - not super fast but at a stead pace - looking back at the results - it seems that the latter shots were slower than the first ones - any reason?

FWIW, some of the best advice I have seen is to shoot 20 rounds at a measured rate. Throw out any clear flyers then take the average of the lowest 5 velocities and figure your PF for that. With that, you can be fairly confident that you will make PF.

[bbbean]

So I went out the other day and shot at the range - I ran 30 rounds through my Chrono - not super fast but at a stead pace - looking back at the results - it seems that the latter shots were slower than the first ones - any reason?

FWIW, some of the best advice I have seen is to shoot 20 rounds at a measured rate. Throw out any clear flyers then take the average of the lowest 5 velocities and figure your PF for that. With that, you can be fairly confident that you will make PF.

+1

I run strings of 20 and take the average of the fastest three of the slowest 7 (basically following match procedure with the slowest rounds). If those make PF and the overall average and SD are good, I'm pretty confident.Ideally, my slowest rounds make PF and I have an average of 170-172 with a low SD.

I also repeat this at least 2 or 3 times with my final load. I figure there are worse things than 40-60 rounds of focused slow fire for trigger practice, and I don't have to worry about a bad run on the chrono stage.

BB