POI shift between major & minor loads

[d3ydx3]

I have seen various posts on the forum that indicate it is completely normal to see heavier bullets shoot higher than lighter bullets. Some other posts say that POI should be the same between major and minor 40 S&W loads.

 

While testing some loads recently I noticed my minor loads shooting 2 inches lower than my major loads at 15 yards. I am keeping the powder charge the same and just dropping a lighter bullet into the round (180 vs 140gr). Both loads group well so I’m not worried about accuracy, sights or technique but the 2 inch difference in elevation is consistent. PF is around 175 for major and 140 for minor. 

 

For those of you that keep your charge the same and just use a lighter bullet for your minor loads have you seen this? 

Edited January 18, 2021 by pealandco

[Guy Neill]

We normally think of a heavy bullet as going slower than a lighter bullet, meaning it is in the bore longer during recoil.  With the same powder charge, the lighter bullet should be going faster, resulting in the lower point of impact.  Here's an example..

 

If you know the muzzle velocity, we can calculate the time in bore for a given length barrel.  For instance, a 5" barrel with the heavier bullet going 950 fps (171 PF) at the muzzle will have a bore time of 0.000789 sec.  A 140gr bullet going 1000 fps (140 PF) will have a bore time of 0.00075 sec.  Thereby we can see that the heavier bullet leaves the muzzle later than the lighter bullet, resulting in a higher point of impact on the target by the heavier bullet.

Edited January 19, 2021 by Guy Neill

[d3ydx3]

10 minutes ago, Guy Neill said:

We normally think of a heavy bullet as going slower than a lighter bullet, meaning it is in the bore longer during recoil.  With the same powder charge, the lighter bullet should be going faster, resulting in the lower point of impact.  Here's an example..

 

If you know the muzzle velocity, we can calculate the time in bore for a given length barrel.  For instance, a 5" barrel with the heavier bullet going 950 fps (171 PF) at the muzzle will have a bore time of 0.000789 sec.  A 140gr bullet going 1000 fps (140 PF) will have a bore time of 0.00075 sec.  Thereby we can see that the heavier bullet leaves teh muzzle later than the lighter bullet, resulting in a lower point of impact on the target.

 

Yup, I chrono'd the loads. The 140gr minor load had an average velocity of 982fps and the 180gr major load had an average velocity of 940fps. Based on my math, that means the major load was in my 5" barrel for 0.00044s and the minor load was in the barrel for 0.00042. Is there a special formula for bore time other than (barrel length in inches)/(bullet velocity fps * 12)? Seems our numbers are a bit different and your calculation is closer to a 9" barrel than a 5". 

 

Edit: Actually, I'm sure there is another formula than just a simple distance/velocity calculation since I'm assuming you have to take into account the ramp of velocity and the powder burn. I'm sure the barrel twist rate and type of bullet also changes the values, too.

Edited January 18, 2021 by pealandco

[Guy Neill]

I thought I was using the distance divided by the velocity - in a spreadsheet from some time back.  Looking again, it multiplies by 2 for some reason I cannot presently determine.  Therefore, your numbers look good to me (until or unless I can determine why the old spreadsheet multiplied by 2).  My first thought was that it dealt with v = sqrt(2as), but there is no reason to bring the acceleration into this.

 

Still, the principle is the same, and does show the lighter bullet exiting the barrel sooner.

[Guy Neill]

I believe I have worked it out.  Multiplying by 2 seems to be correct.

 

So, t = (2s/v) for the barrel time.

 

I won't go into the derivation unless someone really wants to know.

 

 

[zzt]

If you want major and minor to shoot to the same POI without changing anything on the gun, use the same bullet for both and vary powder charge.

 

For example, my major load was 3.7gr e3 under a 180 for 171 PF.  Minor load was 2.8gr under the same 180 for 139 PF.  Both cycled with 100% reliability with the same recoil spring, and both shot to the same POI.  I preferred the feel of 3.0gr under a 165 for 139 PF, but that load did not shoot to the exact same POI.  So I stayed with the 180s.

[Joe4d]

36 minutes ago, zzt said:

If you want major and minor to shoot to the same POI without changing anything on the gun, use the same bullet for both and vary powder charge.

 

For example, my major load was 3.7gr e3 under a 180 for 171 PF.  Minor load was 2.8gr under the same 180 for 139 PF.  Both cycled with 100% reliability with the same recoil spring, and both shot to the same POI.  I preferred the feel of 3.0gr under a 165 for 139 PF, but that load did not shoot to the exact same POI.  So I stayed with the 180s.

Disagree,, at least as far as 40 is concerned,  been doing this a coonage. vary the powder you slow down the same weight bullet... Ever thrown a softball or football ? on what planet does a slower ball have same arc ?
Buy 180's and 155's from same company, same profile.... develop your 180 major load. Do NOTHING to your press, powder measure etc,,,,,,,, just switch to 155 at same profile. same brand.
You will get a 140 pf load, with same POI, that works same same springs and has a bit lighter recoil than the major loads.

SAME arc , lighter recoil due to less mass, but same impact

 

Edited January 18, 2021 by Joe4d

[zzt]

8 minutes ago, Joe4d said:

Disagree,,

 

Fine.  I know that the two loads I mentioned went into the same hole at 15 yards.  I tried the 155 approach.  Bad accuracy and different POI.  YMMV.

[d3ydx3]

14 minutes ago, Joe4d said:

Disagree,, at least as far as 40 is concerned,  been doing this a coonage. vary the powder you slow down the same weight bullet... Ever thrown a softball or football ? on what planet does a slower ball have same arc ?
Buy 180's and 155's from same company, same profile.... develop your 180 major load. Do NOTHING to your press, powder measure etc,,,,,,,, just switch to 155 at same profile. same brand.
You will get a 140 pf load, with same POI, that works same same springs and has a bit lighter recoil than the major loads.

SAME arc , lighter recoil due to less mass, but same impact

 

 

I am having a 2" shift with that approach between the lighter and heavier bullets. I went and tested loads today with the same charge - the only difference was the bullet, which was 140gr vs 180gr. Velocities were more or less equal.

Edited January 18, 2021 by pealandco

[Joe4d]

15 yards?,,, unhuh

also could be a recoil management issue,,,
Do the math,.,,,   Swapping a 155 load for a 180 has been working a long time. Zero ur gun go play do what works.
All the stuff we say here is just guide lines to give a start point. That is what is cool about reloads. You do wat works best for you.... 

ride your own ride

Edited January 18, 2021 by Joe4d

[d3ydx3]

Great, so now we have 3 opinions on this thread.

 

1) 2" shift is expected between heavier and lighter bullets based on barrel time

2) POI should be the same if same bullet weight, but different charge

3) POI should be the same if different bullet weight, and same charge 

[Joe4d]

buy a chrono and play,,, life is good

 

[d3ydx3]

Just now, Joe4d said:

buy a chrono and play,,, life is good

 

 

If you read the thread, you'd see I already have a chrono. Not helpful?

Edited January 18, 2021 by pealandco

[superdude]

In some cases using the same charge with different bullet weights results in the lighter bullets going slower.  The link below shows data for a 45 Auto. See Table 2.

 

https://www.shootingtimes.com/editorial/power-factor-recoil-bullet-weight-gives-edge/99399

[Joe4d]

 

hummm, typical handgun velocity, shoot same velocity,,,  you'll get same arc. same POI

Use SAME everything other than a 180 or 155 you get SAME arc, and velocity... only change is recoil.. ( go read Newton)

OR you can argue, do a gazzillion experiments, and end up with up nothing..... \

If you have a 40,,, and shoot lots of games develop a 180 gr  major,,

Then do NOTHING but change to a 155... you will have a minor load that runs your gun, has less recoil, and has same POI

[AHI]

First the gun Glock 24  Major load is a 180 gr coated bullet over 4.3 gr of Bullseye. Minor load is a 140 gr coated over  same change ofBullseye.

Same point of aim and impact to 25 yards. Maybe I got lucky. This also works in my 35. So my personal experience two bullets

Two different weights can hit same impact.

Edited January 18, 2021 by AHI

[zzt]

2 hours ago, pealandco said:

Great, so now we have 3 opinions on this thread.

 

1) 2" shift is expected between heavier and lighter bullets based on barrel time

2) POI should be the same if same bullet weight, but different charge

3) POI should be the same if different bullet weight, and same charge 

 

Won't discuss #1, because if I did not misread his comments, he is mistaken.

 

#2:  That is precisely what I found and used those loads for 2.5 years

 

#3:  I found exactly the opposite in 40sw.  However, I will say that my Limited gun despised 155s and 135s, so that could have something to do with it.

 

What I did find surprising was that my two 180gr loads going 950 and 775 fps respectively DID hit the same POI at 15 yards.  I did find something similar in 9mm.  My 171 PF major loads using a 115gr bullet and my 124gr minor Steel Challenge loads @ 140 PF both hit the same spot at 25 yards.  My 170 PF 45ACP major loads hit the same POI as my 155 PF bullseye loads.  My PCC shoots 147gr @ 890 fps to the same POI as the 115gr bullet @ 890 fps.

 

I think hitting the same POI at 15 or 25 is more a matter of the quality of the gun, the shooter and the load rather than anything else.

Edited January 18, 2021 by zzt

[Guy Neill]

zzt - I'd be interested in your explaining #1.

 

Thanks.

 

[zzt]

On 1/17/2021 at 9:20 PM, Guy Neill said:

If you know the muzzle velocity, we can calculate the time in bore for a given length barrel.  For instance, a 5" barrel with the heavier bullet going 950 fps (171 PF) at the muzzle will have a bore time of 0.000789 sec.  A 140gr bullet going 1000 fps (140 PF) will have a bore time of 0.00075 sec.  Thereby we can see that the heavier bullet leaves teh muzzle later than the lighter bullet, resulting in a lower point of impact on the target.

 

I don't understand how you generated those numbers.  Bullets accelerate down the barrel.  However, it is the last sentence I question.  The recoil impulse of the heavier bullet and the fact it is going slower will raise the POI, not lower it.  Even using your numbers, the slower heavy bullet takes more time to exit the barrel, so the muzzle has more time to rise before exiting.

[Guy Neill]

As I read it, he said the lighter bullets were hitting lower than the heavier bullets.

 

The heavier, slower bullet was in the bore longer, and would be expected to hit higher than the lighter, faster bullet.

 

Oh, okay, I see your confusion.  I simply misstated.  It should have read that the heavier bullet strikes higher.  The math should confirm.  The longer time translates to higher impact.

 

Now if I could type what I had in mind and was trying to say.

 

Thanks for the information.