Load development for point of aim

[awaveritt]

I have shot a thousand or so 158gr. lead boolits over 3.5 gr. of Bullseye thru my S&W 638 and have become used to the point of aim relative to my sighting hold. Now I'm casting some 125 gr. RN and am wondering what kind of change in point of aim I can expect. I plan to also use 3.5 grs of Bullseye to start out, but will this raise or lower the expected point of aim? I'm assuming an increased charge will be needed to return to my original point of impact with the 125 grain boolits. Anyone already invented this wheel?

[fredj338]

You will not be able to match the POI of the 158gr w/ a 125gr regardless of vel. The heavier bullet just kicks the bullet that much higher. Since most fixed sighted 38sp are sighted for 158gr loads, you will shoot lower w/ anything lighter.

[Wally]

Ligher bullets tend to shoot low as they have less barrel time....I shoot 110 grain and up to 160 grain bullets in my .38 Special and find this to be the case.

[mooman76]

That's something you will have to experiment with to find out. You may or may not be able to do what you are wanting. Make up a bunch of different loads and see where they shoot.

[cajun shooter]

All of the above post are correct. You will not have same POI with 158 and 125's. The law of physics gets in the way. You also don't want to get in the habit of changing your POA as that will lead to misses. The 158 is a good all around weight for the 38 class, that's why the wadcutters are as close as 148 and 150.

[fredj338]

Ligher bullets tend to shoot low as they have less barrel time....I shoot 110 grain and up to 160 grain bullets in my .38 Special and find this to be the case.

It may be semantics, but the barrel time thing is questionable. If you shoot 110gr bullet @ 800fps or a 158gr @ 800fps they both have the same bbl time. The 158gr does offer more recoil impulse though & that is what changes POI.

[Whitespider]

...If you shoot 110gr bullet @ 800fps or a 158gr @ 800fps they both have the same bbl time...
Ummm... no, that’s not quite correct.
They are both exiting the muzzle at the same velocity, but barrel time would not be the same.
The lighter bullet accelerates at a faster rate and would spend less time in the barrel.

In theory it would be possible to slow the rate of acceleration of the lighter bullet to a level that would equalize barrel time and still produce identical muzzle velocities, if you could use a slow enough powder and if you could cram enough of it in the case.

But, with the technology we have available, the only way to equalize barrel time between the 110-grain and 158-grain bullets would be to launch the lighter bullet at a somewhat slower muzzle velocity.

[fredj338]

Ummm... no, that’s not quite correct.
They are both exiting the muzzle at the same velocity, but barrel time would not be the same.
The lighter bullet accelerates at a faster rate and would spend less time in the barrel.

In theory it would be possible to slow the rate of acceleration of the lighter bullet to a level that would equalize barrel time and still produce identical muzzle velocities, if you could use a slow enough powder and if you could cram enough of it in the case.

But, with the technology we have available, the only way to equalize barrel time between the 110-grain and 158-grain bullets would be to launch the lighter bullet at a somewhat slower muzzle velocity.
The diff. of two diff. bullets @ the same muzzle vel. would be negligable. If they both leave at 800fps, & the lighter bullet gets there 0.00001 sec sooner, it is not changing the POI. Try it using a lighter bullet 30-40fps slower, it still shoots low. So I do not buy the barrel time theory, more recoil impulse by the heavier bullet vs lighter bullet.:drinks:

[geargnasher]

I've had pretty good luck with keeping the same poi in my .357 Magnum by using HS6 for the business loads with 170 gr. boolits and 2400 with the 150 gr. swcs.

Now my Vaquero .45 shoots dead-on with 340-gr boolits out to 50 yards and a full 8" low at 25 and 12" low at 50 with 255 grain boolits at about the same velocity and with the same powder. Tried working up loads with 2 other powders and the 255s, Very little change in poi at all from the first powders. Guess I'll either file the sight or stick with super-heavy boolits in that one.

Just my 2 c.

Gear

[Whitespider]

Well fredj, since we’re hashin’ this out over a friendly couple of :drinks: ; allow me to point out three major flaws in your above thinking.

The diff. of two diff. bullets @ the same muzzle vel. would be negligable. If they both leave at 800fps, & the lighter bullet gets there 0.00001 sec sooner, it is not changing the POI.....
The flaw here is that you’re thinking of time in the absolute rather than the relative. What if the heavier bullet only spends 0.00002 seconds in the barrel? See, that would mean that the lighter bullet would only have half the barrel time, and that’s certainly not “negligible” by any stretch. Now, neither of us knows the actual barrel time of either bullet, but we can use some basic physics formulas to come up with ratios. A quick, off-the-top-of-my-head figuring tells me that the 110-grain bullet will spend some 30% less time in the barrel. I’m thinkin’ 30% ain’t anywhere close to a “negligible” amount.

.....Try it using a lighter bullet 30-40fps slower, it still shoots low.....
Again you’re thinking in the absolute, not the relative. Although 40 fps sounds like a lot, when we’re talking 800 fps it’s only one-twentieth of one percent (0.05%), that would be “negligible”. The bullet would still be spending some 30% less time in the barrel. If you wanted to equalize barrel time you would have to fire the 110-grain bullet some 250-300 fps slower than the heavier bullet.

.....So I do not buy the barrel time theory, more recoil impulse by the heavier bullet vs lighter bullet.
Well actually it’s not a theory, it’s all simple physics. But rather than get into all the science I’ll just ask you a couple questions. Say I increase the powder charge behind the 158-grain bullet so it exits the muzzle at 1200 fps instead of 800 fps, I would increase the “recoil impulse”, correct? Why then, does the faster bullet (of the same weight with increased “recoil impulse”) have a lower POI than the slower bullet?

[Edubya]

Whitespider; Why do some of you people make these details so simple minded? Don't you want everyone to enjoy the progress of learning the hard way? Can't we just read what our betters say and accept that they know best? Geez, you're ruining the fun and development for all of lowly minions. And, who are you to interfere with our developmental process? GEEZ!
EW

[armyrat1970]

Just to throw my .02 in, I think the POI will be close to the same. Your using the same charge for a lighter boolit. Pushing the lighter boolit faster. I believe a lighter boolit at the same velocity as a heavier boolit will drop quicker. It doesn't have enough weight to overcome the resistance of the air it is passing through But using the same charge is giving more velocity with the lighter boolit and a little more force to overcome the resistance.
I have been wrong before.

[Echo]

Since the boolit leaves the muzzle while the gun is in recoil, and a heavy boolit causes more recoil (for the same MV) as a lighter boolit, rationalizing says that if we drive the lighter boolit hard enough, the recoil can be made the same as with the heavier boolit, and POI should be about the same. Therefore:

Load the light boolit with the heaviest load of a slower burning pistol powder, up to a maximum shown in competent reloading manuals, and see where the POI is.

I make no bets on this, as it is mostly rationalizing...

[Whitespider]

Oh man... OK... Hummmm...
Y’all need to forget about muzzle velocity, forget about the amount of total recoil... Those things have very little affect on POI at typical handgun ranges (25-50 yards).

Understand that even with the heaviest bullet, with the heaviest recoil, the muzzle has risen less than a sixteenth-of-an-inch (probably a whole lot less) by the time the bullet exits the gun. What matters is how far the muzzle rises during the time the bullet is inside the gun; recoil has zero affect after the bullet exits the muzzle. There is virtually zero difference in the amount of time it takes for the muzzle to raise that tiny distance (while the bullet is in the gun) between a heavy recoiling load and a light recoiling load, because.... The recoil is still overcoming the inertia of the guns weight, the gun has just barely begun to move, and it is still accelerating back towards the shooters hand.

Just to illustrate this a bit more... If the gun was under full recoil (full rearward velocity) when the bullet exited the muzzle it would be impossible to get any sort of accuracy from a repeating handgun, because... Every time you shoot the thing you remove weight from the gun (i.e. bullet, powder and even brass from a semi-auto), the recoil is different for every shot. The weight ratio and balance changes each time the trigger is pulled, making the total recoil affect different for each shot. But, the gun is not under full recoil when the bullet exits, the flesh of the shooters hand has barely even begun to compress.

Keep the bullet in the barrel just a split millisecond longer and the muzzle has risen a tiny bit more and the velocity of the guns rearward movement increases just a tiny bit more. This means that the barrel time and rise in POI isn’t linear, double the barrel time and you more than double the rise in POI.

There is only one way to raise the POI (if all else remains equal), and that is to increase barrel time.
And, there are two ways to increase barrel time...
1) - Increase bullet weight.
2) - Decrease bullet velocity.
...both of which lower the rate of bullet acceleration in the barrel.

A lighter bullet has a lower POI because it spends less time in the barrel.
A lighter bullet needs to be traveling at a significantly lower velocity than the heavier bullet to equalize barrel time.

[armyrat1970]

Ballistics are ballistics and they hold true for pistol or rifle. A light boolit leaving the muzzle at the same velocity as a heavier boolit will drop quicker if the same boolit design. Don't think it matters of how long before leaving the muzzle and it doesn't matter about recoil at all.

[thebigmac]

you'll have to use a bench rest to get the accuracy needed to justify your difference in poi. Also do a lot (whole lot ) of shooting so you know the difference whenever you you change boolits. Poi should be close enough that you may not know the difference until you become a very, very good shooter..