Will light & heavy bullets at same velocity have the same POI ?

[jmbaccolyte]

Sorry if this has been asked before, but will a 155 grain bullet at say 1,100 feet per second impact the target at the same place as say a 200 grain bullet at the same velocity? I'm only concerned with short distances, say 50 yards and less, so hopefully aerodynamic drag won't be a huge factor. The reason that I am asking is that I'd like to have tritium nights sights and they tend to be fixed sights, but I'd also like to have the option of practicing with a light load and hunting with a more powerful load.

[Jeanjacket]

They very well should impact exactly the same. Especially moreso if they are factory rounds and not reloads that might be off here or there. (Not that reloads can't be consistent, just throwing that out there.)

That's really just physics. Now you may personally be able to shoot one better than the other, so you might have better groupings with one over the other, but all things being equal, say if you benchrest a pistol in place, two bullets of different weight (even different velocities unless extremely underpowered) at relatively "close range" should show no difference on target.

[Yardbird]

The heavy bullets will hit the target higher than light bullets.

[Jeanjacket]

The heavy bullets will hit the target higher than light bullets.

Explanation?

[Merlin Orr]

Stays in the gun longer and the barrel has a chance to rise some with recoil prior to leaving the muzzle... Or so I have heard it told.....

[Doc Hunter]

No - the heavier bullet will hit higher.

[reptoid]

NO

[Jeanjacket]

I'm trying to wrap my head around that. Even if they leave at the same velocity, how does one hit higher?

[Punkin Chunker]

Stays in the gun longer and the barrel has a chance to rise some with recoil prior to leaving the muzzle... Or so I have heard it told.....

I would also speculate that getting the heavier bullet up to the same velocity as the lighter one would generate a stronger recoil impulse, thereby causing more/faster rise in the gun during the time the bullet is in the barrel.

Edited April 12, 2012 by Punkin Chunker

[Hi-Power Jack]

All great theory

Anyone ever try it?

[Joe4d]

the question was heavier at same velocity, Heavier at slower velocity tend to hit a bit higher inside 25 yards. Shoot the pistol, the gun as recoiled up slightly more before the slower bullet leaves the barrel. Thats in general though. SO many other variables involved. To get bullets that far apart to have the same velocity your gonna have to do some drastic changes to powder type or charge density. Which could completely change the group sizes. SInce you mentioned 200gr at 1100 I am gonna guess you meant 10mm. And 50 yards is a long long ways.

I can give you some first hand testing results i have done. That has worked for quite a few different powders in 40 S&W loaded to 1.150-1.180 oal, with WST tightgroup, American select,

Develop your major PF load with a 180 gr bullet. Do nothing else but switch to a 155 gr bullet and you'll come up with a good 140pf minor load that will hit same point of impact, and still cycle reliably without respringing.

[jmbaccolyte]

Thanks, I was thinking about 170pf for the light load and 220pf or so for the hunting load. And I was thinking about using way different powders and expecting some load development on each. But after reading your comments, I think I may be in over my head on this.

My Tanfo .40 Match allows 1.250" OAL cartridges (.40 loaded LONG, i.e. to 10mm length) without coming close to the rifling and the K10 mags feed fine at that OAL. So I thought the vast spread in power factors was doable, but it sounds like this project should wait for a more experienced reloader to break the trail. Thanks for the advice.

[Youngeyes]

I shot this target using a S&W 29 44mag 6inch at 50 ft last weekend. Aim point was the same or as near as I could tell. The top group was 240GR XTP with 23 GR H110 which should have gotten me~ 1250 FPS. The bottom group was with 180 GR XTP using 10.5 GR unique which S/B ~1300 FPS. I unfortunately did not chrono the loads. I was surprised at the difference in group location. Does that prove the heavier, higher grouping theory? I think it worthy of aurgument over a beer. Just sayin.

[TriggerJockey]

The recoil impulse is a mute point I believe, when I watch high speed footage the projectile has cleared the barrel before it starts rising. Maybe if you had a really bad spring setup it would sling the slide back fast enough to pull the muzzle up before the projectile exited, but in my experience I don't believe the recoil impulse is a factor. We are talking milli seconds here, so I understand why some would think that recoil is a factor on single shot (non follow up or double tap for lack of a better term) type exercises. Check this out, helps the understanding of recoil impulse and it's interaction with single shot point of impact. (I don't disagree that bullet weight makes a considerable difference in recoil management and follow up shots, generally heavier = flatter impulse while lighter = more flip) I would post a youtube clip but my post count isn't high enough! Check youtube for high speed footage of recoil impulses.

[jmbaccolyte]

Joe4d, I calculate 903fps for the 155 grain bullet (at your stated 140pf) and 916fps for the major power 180 grain load (assuming 165 power factor). Those velocities are a lot closer than I would have expected with that much change in bullet weight and the same powder charge.

STI-Edge, if we can ignore recoil impulse then is Jeanjacket right, that point-of-impact elevation changes on the target are due solely due to changes in bullet velocity?

What the rest of you guys are saying makes sense. I vote that Hi-Power Jack has to actually do the experiment, after all; it was his idea to test it.

[HoMiE]

Lighter bullets tend to have a smaller ballistic coefficient and will loose speed quicker making it take longer to get to same distance, thus it will drop slighly more than a heavier bullet at same speed. Is it enough to make up for probably the best that I could shoot a group at 50 yards. Probably not.

[Jeanjacket]

Lighter bullets tend to have a smaller ballistic coefficient and will loose speed quicker making it take longer to get to same distance, thus it will drop slighly more than a heavier bullet at same speed. Is it enough to make up for probably the best that I could shoot a group at 50 yards. Probably not.

Yeah probably not pertinent, and especially not at "close" range.

I guess it depends on whether or not the bullet clears the barrel before the recoil has enough time to cause physical motion to the point that affects the aim. Of course physical motion occurs when a bullet ignites due simply to physics, but how much, if any, "rise" does it cause prior to the bullet leaving the barrel. Honestly it's either negligible, or sights are already engineered to account for rise due to recoil impulse. Guns and aiming just wouldn't make sense otherwise...

Edited April 17, 2012 by Jeanjacket

[Amerflyer48]

We need to call someone who can do trig in their head and figure how little you have to move the angle of departure to change the impact point 2-3" on a 50 yd. target or how many thousandths of an inch on a Bo-Mar rear sight on a limited gun to move that distance.

Only a minimal amount of recoil needed to do that as recoil is started at the moment the gasses start to expand Newton made a LAW not an idea (action-reaction).. all bullets have a "time in barrel" factor (internal Ballistics) usually slower rounds need more follow through (.45ACP vs. .38Super) either velocity or inertia or both control this time.

The question you are posing with same velocity a different effect (Newton again)... Inertia causes the heavier bullet to get up to speed only a fraction slower than the lighter projectile and in that fraction the pistol is recoiling and due the bore being above center line starts to recoil up due to this inertia lag the muzzle travels upwards a few perhaps ten thousandths more and shows as 2-3" higher on target.

STI-Edge I agree the videos show the recoil after the bullet leaves but look what happens just before

at 1:25 you see things start to move, a "puff" of air then the bullet leaves the gun moves before the bullet leaves ever so slightly and that is where you are getting the vertical difference in the shot groups.

as far as time in barrel effecting outcome.. ask someone who shoots 10M air events. nothing is negligible we can only compensate most guns will shoot different from a rest to some extent due to using sights to compensate for something the shooter does during offhand

John

Try this fun experiment next time at your practice session , go shoot 2 groups one at 25 and one at 50 yds. find center of group and then bench rest shoot 2 groups one at 25 and one at 50 yds. target and compare group centers not sizes, centers might see something interesting