How Much Can 9mm Bullet Weight Affect POI (S&W 3913)

L

lwestatbus

New member
I have a new to me LE turn in S&W 3913. I am shooting consistently about 4" low at 20' and twice that at 50'. I've had multiple range sessions with this pistol, all using factory 115 gr ball and all with the same results. My gunsmith confirmed using a straight edge that my front sight is higher than the rear.

I've been doing a LOT of online research on this problem and have discovered two possible issues. One is that a batch of these pistols had a manufacturing defect in the grooves in the barrel. But I've also seen multiple posts, including this one in the Firing Line forums, that seem to indicate that increasing the bullet weight fixes the problem.

A dead on duplicate of my problem in a S&W forum reported a fix by just increasing the bullet weight from 115 gr to 124 gr. In the FL post linked above the OP reported the same problem but a responder reported no problems but he was using 147 gr.

So, could this pistol have been designed for a heavier bullet? Does anyone else have experience with this problem in this pistol? One piece of related information have been a couple of reports that S&W did make shorter front sights for this pistol, though I haven't been able to find one for sale. If true, this would be consistent with a design for a heavier bullet but an option (shorter front sight) for a standard weight (115 gr) bullet.

Thanks in advance for any insights.

Larry
 
Heavier, slower, bullets will always impact higher. They generate more recoil and spend more time in the barrel as it moves through the recoil arc.
 
There's very little recoil arc in a locked breech semi-auto while the bullet is in the bore.

The slide/barrel recoil virtually straight backwards until the unlocking process starts. At that point the recoil arc really begins but the bullet is already gone by then.

You can see how different revolvers and semi-autos are by comparing the bore vs sightline in both types of guns. The difference is very obvious.

In a revolver, the barrel is fixed to the frame and the shooter is resisting the recoil from well below the bore. So virtually the instant that recoil begins the muzzle begins to arc upwards. Since the bullet is still in the bore the muzzle rise has a significant effect on point of impact and differing bullet weights will alter the point of impact.

In a semi-auto, the barrel & slide are locked together and can recoil backwards virtually unimpeded. The recoil spring does provide some resistance but it is minimal. The muzzle doesn't really begin to rise significantly until the barrel unlocks and, by design, that takes place after the bullet leaves the bore. As such, there's not as much effect on point of impact when changing bullet weights.

Here are some figures I created from measuring some pistols.
attachment.php


Note that in the revolver, the boreline angles noticeably downward compared to the sightline. That is because by the time the bullet exits, muzzle rise has brought the boreline up to the point that it corresponds to the sightline when the hammer fell.

However, note that in the semi-autos, the boreline is parallel, or even angles slightly upwards compared to the sightline. That is because there's very little muzzle rise while the bullet is in the bore. During that timeframe the barrel/slide are recoiling nearly straight backwards. So there's no need for a significant disparity between the sightline and boreline as there is in the revolver.
 
Four inches at 20 feet is .06-.08" on the sights.
That is a huge change, half the height of most front sights.

I would ask Dawson Precision, they make a lot of sights.
 
"However, note that in the semi-autos, the boreline is parallel, .."

Well, not in some pistols. The bore of, say, a Model 1911 pistol actually points down in relation to the sights when the gun is ready to fire. As the bullet is moving up the barrel, the slide is moving backward, puling the rear of the barrel barrel back and down so that when the bullet exits, the barrel is pointing more in line with the sights, but actually a bit high so the pistol will be "zeroed" at roughly 25 feet (with fixed sights).

Jim
 
Well, not in some pistols. The bore of, say, a Model 1911 pistol actually points down in relation to the sights when the gun is ready to fire.
Click to expand...
I didn't test a 1911. I should have said: "However, note that in the semi-autos in the figure, the boreline is parallel, ..."

The way the link works in the 1911 changes things slightly since it begins dropping the barrel immediately when the slide/barrel combo starts moving.

It's also worth pointing out, I suppose, that there are various other locked breech semi-auto pistols which operate differently from a typical floating barrel, locked breech semi-auto pistol. They would have to be evaluated on a case-by-case basis to determine whether or not they are going to show a significant recoil arc while the bullet is in the barrel.
 
So Iwestatbus the thing you should absolutely, positively do ASAP is get a box of 124 grain AND a box of 147 grain 9mm and check this out. We ALL need the practice and besides it's fun. This way you'll know for sure what YOUR gun does in YOUR hands.

Just for kicks if there's someone at the range you trust and is a good shooter, let them fire off a few rounds and see how they do. Might give you some more info to consider.
 
Another vote for trying heavier bullets before altering the pistol. Quite some time ago a police officer friend (since retired) told me that [in his neck o' the woods] a 147-grain bullet was the preferred load for 9mm.
 


I test each of my guns with as many different brands of ammo and bullet weights as I can find. As my chart shows, some guns have very little change in POA/POI with different weight bullets and/or velocities. Other guns have a major change. Semi-auto vs revolver does not seem to make a big difference as long as both are sighted in properly.

In the case of the S&W 3913 (6th from the bottom), mine has a 1" low to perfect elevation with all of the rounds that I have used. All tests are done at a distance of 50 feet unless otherwise noted.

The first number represents elevation and the second number represents windage (1L/2L = 1" low and 2" left of point of aim)
Mark
 
Many Thanks

Many thanks for the extensive responses so far. DaleA's suggestion is definitely on the agenda. In fact, am about to schlep about 45 min each way to Bass Pro Shop which has 124 gr FMJ in stock and am going to try a specialty shop on the way for 147 gr.

I have had two other shooters try the pistol. Both are range safety officers at my favorite range (Police Hall of Fame range in Titusville, FL). One put five rounds right through the ten ring and the other duplicated my results. Go figure.

One exacerbating problem I have is that I don't have a convenient place to shoot the pistol from a rest. I'd definitely like to take the shooter's headspace and timing out of the equation. For what it's worth I am shooting reasonable groups with this pistol and my other pistols shoot right where I want them to. I was also getting the same results with the original factory grips and also after switching out to Hogue grips that fattened up the grip slightly. I also have a S&W 6906 and it shoots right where I want it to (though the 6906 is challenging for me as my hand is a bit too small for double-stack 9mm grips--had the same issue with my issue 9mm in the Army).

Thanks again for the advice, especially the extensive info some of you have provided.
 
ShootistPRS said:
Heavier, slower, bullets will always impact higher. They generate more recoil and spend more time in the barrel as it moves through the recoil arc.
Click to expand...

What you say is true for a revolver or a fixed-barrel semi-auto, but not true for a typical semi-auto using the Browning system. If a heavier, slower bullet hits higher when fired from a Browning design semi-auto, it's because the gun's sights made that happen.

With a revolver or fixed barrel semi-auto, recoil-induced upward tilt of the barrel starts as soon as the bullet's powder is activated by the primer. The barrel is immediately pushed back as the bullet moves forward because the barrel is directly connected to the frame in the shooter's hand. Its like pushing on a hammer's face as you hold the hammer's handle -- the hammer's face goes back, but the hammer head begins to tilt at an angle -- just as a revolver's barrel tilts as the bullet moves down the barrel. The barrel begins to tilt before the bullet leaves the barrel.

With the Browning locked-breech short-recoil semi-auto, commonly used in most center-fire semi-autos, the bullet leaves the barrel before the slide has move more than a fraction of an inch (about 1/10th of an inch with a .45), and all movement and forces at play are HORIZONTAL until the barrel begins to unlock from the slide. There are no other force at play until the slide and barrel unlock and those parts slam (directly or indirectly) against frame stops, and the weight of the moving parts shift to the rear changing the gun's balance in the shooter's hand. But the bullet is long gone before any of that happens.

Here's a MythBuster's video from You Tube that demonstrates this very clearly -- it's very high speed -- and see a clear example at about the 12 second point: https://youtu.be/7y9apnbI6GA?t=1 Bullet weight is not a factor because all forces are still JUST horizontal until after the bullet has left the barrel.

After the bullet leaves the barrel, gravity is the constant. A heavy bullet drops at the same rate as a light bullet and only velocity, bullet shape (aerodynamics?), and barrel angle affect (vary) the point of impact. With a revolver (or fixed-barrel semi), a heavier, slower bullet can hit higher (depending on target placement), because the barrel is rising as the bullet travels down the barrel. A heavier bullet (of the same design) moving at the same velocity as a lighter bullet might won't necessarily hit higher at the same distance.

With a Browning short-recoil locked-breech gun, recoil force typically doesn't cause barrel rise until AFTER after the bullet is gone when the barrel has unlocked from the slide and can no longer move horizontally to the rear. A heavier, slower bullet makes almost no difference in barrel rise with a locked-breech gun, but could make a big difference in where the bullet hits down range -- especially if the sights aren't adjusted appropriately.

I understood the theory, but was skeptical about its real-world application until I did my own test using 115 gr, 124 gr. and 147 gr. 9mm factory ammo on three small, side-by-side targets at 30 feet. I loaded the magazine so that the first round fired on target was 115 gr, the next was 124 gr, and the next was 147 gr., and repeated this sequence until I had 15 rounds loaded. I manually chambered a 16th round, and fired it off target so that all subsequent rounds were chambered by gun action.

The first target was hit by a 115 gr. round, the second by a 124 gr. round, and the third was hit by a 147 gr. round. I repeated that sequence until all of the rounds were fired. I posted the results here some time back. The groups shot (i.e. my shooting) were not impressive, but they all were similarly sized and almost impossible to tell apart. All were fired from a rest. There were no obvious differences between the points of impact based on different bullet weights. If the recoil was different between lighter/faster bullets and heavier/slower bullets, that should have been apparent in this simple test.​

I later found several You Tube videos, including one no longer easily found on that site that was videoed at a much higher frame rate than the 73,000 frames per second of the Myth Buster video. That faster video had with calibration markings on a 1911 frame and slide. These videos clearly show that the Browning designed short-recoil locked-breech design doesn't have barrel rise until AFTER the bullet is gone. I'm now convinced.

.
 
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The same recoil that moves the barrel back also pushes against your hand. It does so through the recoil spring but it does push the gun back. Now, if you are a strong person with wrists that can be held rigid then you might not notice the reaction of the recoil on your hand. If you are not that strong then you will notice the change in the point of impact from a light to a heavier bullet.
Physics tells us there is an equal and opposite reaction to the action of the bullets acceleration. Even though it is transmitted through the various parts of the gun it still acts upon the device that is holding the gun. It begins as soon as the bullet starts to move and the hand and wrist are absorbing that recoil and the gun is changing the vertical angle the entire time. The longer the bullet travels in the barrel the more that gun will move.
Another effect is that as your grip tightens the bullet hits lower. I see this a lot on the falling plates, the Texas Star and the dueling tree. After the first shot the shooter begins to grip harder to keep the gun on target and the bullets impact lower. It causes most of the misses on the reaction targets in competition. You can demonstrate this to yourself. Shoot three rounds at a 25 yard target putting the gun down between shots. Then pick the gun up and hold it tighter to "control the recoil " for faster follow up shots. Still put the gun down between shots just change the grip to better control recoil. You will end up with two distinct groups. You are using the same sight picture so the only thing that is changing is the amount the gun is rotating in your hand. Follow those two grips with a "loose" or "soft" grip and the group you shoot will be higher than either of the other two.
 
Physics tells us there is an equal and opposite reaction to the action of the bullets acceleration. Even though it is transmitted through the various parts of the gun it still acts upon the device that is holding the gun. It begins as soon as the bullet starts to move and the hand and wrist are absorbing that recoil and the gun is changing the vertical angle the entire time.
Click to expand...
All correct. But one only needs to look at the figure I posted (or do their own measurements) to demonstrate that the change in muzzle rise while the bullet is still in the bore is obviously very small in a floating barrel semi-auto. Given that the muzzle rise is obviously very small, the effect of different weight bullets can't be large, since altering something that's very small to something else that's also very small doesn't have a large effect.
The same recoil that moves the barrel back also pushes against your hand. It does so through the recoil spring but it does push the gun back.
Click to expand...
Right. There is some small amount of muzzle rise due to the weak coupling of the slide/barrel to the rest of the gun via the recoil spring but it's obviously very small. Much smaller than would be expected in a fixed barrel gun such as a revolver. The figure I posted above is proof that there's virtually no muzzle rise while the bullet is in the bore in the semiautos examined compared to the revolver measured. Otherwise their borelines would show the same relationship to the sightline as was obvious in the revolver.
If you are not that strong then you will notice the change in the point of impact from a light to a heavier bullet.
Click to expand...
This all happens VERY fast and the coupling between the slide/barrel and the rest of the frame has a very small effect in the timeframe that the bullet is in the bore. A tighter/stronger grip will have a theoretical effect, but measuring that effect could be very difficult or perhaps even impossible.

It's worth reiterating that not all guns are the same. As noted in my last post and James' post, even some floating barrel semi-autos operate differently enough that they might show somewhat different results.
 
the change in muzzle rise while the bullet is still in the bore is obviously very small in a floating barrel semi-auto
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Small but not zero.
My Colt OACP .45 will shoot 200 and 230 grain bullets into the same group, but 185s strike lower.
 
The OP just needs to try some different loads to see what happens.
All the theorizing doesn't really solve anything.

If changing loads doesn't give the desired results, or the OP just prefers the 115 gr loads, then it's time to break out the files and lower the front sight.
 
Agree.
If a change in loads does not cure it, better to look for a taller rear. If he cuts the front down by that much, there might not be anything left to aim the gun with.
 
My Colt OACP .45 will shoot 200 and 230 grain bullets into the same group, but 185s strike lower.
Click to expand...
Right. I mentioned this in an earlier post.

Because of the way the link affects the barrel, the 1911 design is going to operate slightly differently from the linkless designs. The link starts tilting the barrel almost the instant that the slide starts moving so longer time in the barrel will change the point of impact due to the tilt being different at the time of bullet exit.

For the designs where the barrel doesn't begin the unlocking process until after the barrel/slide have travelled together for a small distance there's going to be little to no change in poi as the result of bullet weight.
 
ShootistPRS said:
The same recoil that moves the barrel back also pushes against your hand...
Click to expand...

I've had only minimal exposure to the study of physics, so I may get some of these terms and concepts wrong... (Being self-taught on some of these complex and sophisticated subjects can mean that both the teacher and the student are ignorant. :( )

It moves back, to be sure, but it is a delayed process, and the bullet is out of the barrel very quickly. I don't doubt that there can be a very subtle but immediate movement of the gun components (barrel, slide, and maybe the frame) to the rear offset by a much less subtle movement of the bullet forward down the barrel. There may also be a trivial amount of barrel rise. I say "trivial" because any number of high speed videos don't show any vertical movement regardless of the bullet caliber being fired.

Equal but opposite reactions are certainly there, but the difference in the masses of the major components 1) barrel, slide, and less directly, the frame and 2) the bullet being pushed against friction out of the barrel and the force required to move them are hardly equal. That's why the bullet goes very quickly and everything else goes much more slowly.

When the bullet starts down the barrel, the inertia of the gun's barrel and slide which are locked together act as a temporary base against which the gases propelling the bullet can push. Once the barrel and slide begin to move -- which doesn't happen until the bullet is almost out of the barrel, the barrel and slide begin to move more forcefully and separate and they create a vector force by slamming into their respective frame abutments. The bullet is long gone by then. Even when the recoil spring is being compressed, the movement remains horizontal and BELOW the barrel. Not much pushing UP, yet.

The high-speed video I posted earlier (which was a .45, probably shooting a 230 gr. round) showed no apparent barrel rise until AFTER the bullet was gone. If, as you claim, the barrel does begin to rise as soon as the bullet is fired, that movement should be visible in the various high-speed videos available on the 'net regardless of bullet weight. It is visible in fixed barrel guns like revolvers, but not clearly visible in a Browning short-recoil locked-breech gun.

Here's another high speed video of what appears to be a SIG P320 semi-auto shot at 70,000 frames per second. Again, no barrel rise can be seen until AFTER the bullet is gone. Very little slide movement is visible before the bullet exits. https://www.youtube.com/watch?v=sINLJA60r08

This high speed video is of a SIG P2022 being fired, but videoed at only 25,000 frames per second. It looks the same, but it's shown from a lot of different angles. None of them show barrel rise until AFTER the bullet is gone. https://www.youtube.com/watch?v=HElQk2wEY5w If there is bullet rise, it OUGHT to be visible in some of these videos.

If you can find videos that show something different, please share them with us. Or do your own practical tests when you next make it to the range. Use two or three different weight bullets, all fired from the same magazine and gun, with a first shot fired to chamber the first test shot. Set the target at a distance of 25' - 30' feet and shoot from a rest. Then share the results with us. I'm sure your groups will be better than mine.

Even after seeing the test results I believed as you did, until I saw a bunch of high-speed videos... I tried to post photos of my targets from my test, but can't get my photobucket images to work with this software this evening. It has worked in the past!

Maybe you're right and I'm wrong... But I don't think that arguments based on theory alone will not work with this discussion, as the variables are too subtle to properly assess, measure, or quantify.

.
 
Wow

Wow, this thread has really taken off.

I'm moving forward on the problem. Picked up a box each of 124 gr and 147 gr standard velocity (not +P) cartridges today and heading to the range tomorrow.

Stopped by my local gun shop to buy the ammo and was talking with the owner about my problem. He whips out a 3913 from under his shirt and proceeds to tell me they are his favorite carry pistol and he picks out every one that comes into the shop and keeps it if it is in good condition. He then trotted back to the office and came out with two boxes of parts including front sights in multiple heights. So I've got a source if the ammo change doesn't help.

For what it's worth, I just don't think limp wrists or something similar are the issue here. I regularly shoot a 4" Kimber 1911 in .45 cal and Kimber's new Micro 9 with no difficulty. I'm open to trigger issues or something else but I don't think I'm having any difficulty controlling this pistol.

I'll report back here with the range results, hopefully tomorrow.

Thanks for all of the input.
 
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