Was it me or a short carbine barrel?

G

Goldendog1

Inactive
I was at the range this week-end sighting in a Remington 742 Carbine in 30-06 with iron sights at 50 yards. Was shooting a variety of off the shelf ammo: Hornady 165 gr BTSP, Remington Premier Accutip 165 gr BT, Federal Premium 150 gr Nosler BT and a 3 rounds of Remington 200 gr round nosed soft points. My question stems from my surprise that the 200 gr bullets' points of impact were actually higher in elevation on the target than the lighter bullets. I don't know the powder brands or loads that the manufacturers used to drive the assortment of bullets, -- everything was just off-the-shelf ammunition -- No reloads, custom loads, etc. I fully expected the heavier bullets to have a lower trajectory and point of impact than the lighter ones. My first thought was that it was probably me, shooter error, not holding on the same aim point as the others; but since the three shot grouping was under a half inch, I don't think I changed my aiming fundamentals. I shot the 3 200 gr rounds last!

Could the fact that the 742 carbine's barrel length at only 18" versus the "normal" 30-06 hunting rifle having a barrel length of from 22"-24" have anything to do causing velocity variances which could create such an outcome? Could the smaller volume of powder that SAAMI approves for the 200 gr 30-06 burn more efficiently in the shorter barrel than the larger volume powder load approved for the lighter bullets, thereby creating a flatter trajectory at this distance of 50 yards?

Or did I merely hold "high" on all three shots of the 200 grain bullets?

I would appreciate your insight on this. Thanks.
 
Hopefully one of the experts will weight in on stuff like barrel harmonics, head space, exit velocity, etc. Remember that the force of gravity is constant; therefore, a 60 pound cannonball "falls" at the same rate as a 200 grain bullet.
 
Physics

:) I would guess that barrel harmonics would be the culprit . Simple physics would dictate that the heavier bullet launched at a lower velocity would drop faster than a lighter bullet at higher velocity . That's the only explaination I can come up with .
 
At 50 yds,the nature of trajectory/ballistics is not really in play much.
To understand what is happening,realize the rifle is not static in firing.As soon as the striker falls,the rifle is in motion.Two things will be noticeable at 50 yds.The rifle is moving in recoil motion.Muzzle jump is part of it.The heavier bullet will often lift the muzzle a little more before the bullet exits.This is very noticable in something like a BPCR like a Rolling block,with a lot of stock drop and bullet time in the barrel.This idea is also useful sighting in fixed sight handguns.You can move POI with bullet weight and burn rates.
Another factor is barrel harmonics.Barrels flex,and firing is like plucking a string.The bbl may be whipping in a different direction as the heavier bullet clears the crown.
 
Heavier bullets often shoot higher at close to medium range than lighter bullets. Although it is more commonly seen in handguns. The heavier bullets generate more recoil causing more muzzle rise while the bullet is still in the barrel. At longer range the lighter, faster bullets will usually drop less.
 
Ok, I will take a stab. At short ranges, the angle of the barrel at the "Magnus Moment," when the bullet leaves the barrel is crucial. Heavier bullets accelerate more slowly in the barrel than lighter bullets and the recoil "stroke" begins at firing. Hence, the barrel angle has risen more when the heavier bullet leaves the barrel. Higher angle means higher strike at short range.
 
I think you fellas nailed it with the added recoil factor. Since I was using a shooting rest, I chose not to use my left hand to support the rifle on the forestock reducing jump; but rather to rest it under the buttstock. Therefore I was getting major vertical recoil with all ammunition fired, and the heavier 200 grain bullet jumped more than the rest.

Thanks for all your responses.
 
Most carbines

have a barrel that is both short (18", in your case), AND thin.

The thin barrel heats up faster, so I suspect that by the time you got to shooting the heavy bullets the barrel heat caused vertical stringing.

Maybe!?

Try shooting the heavy bullets thru a cold barrel, followed by shooting light bullets, thru the hot barrel.:cool:
 
it happens in handguns often...

The combination of short range, and the recoil/velocity of heavy bullets print them higher on the target than lighter bullets.

Basically here's the way it works. All guns begin to recoil as soon as the bullet starts moving (possibly before), but in any event, the gun is actually recoiling as the bullet moves down the bore. Heavier bullets move slower, and so, spend more time in the recoiling barrel (moving upwards, due to gun stock design), so they print higher on the target at short ranges.

This is much more pronounced with handguns, due to the bore being higher over the grip than with rifles (proportionally greater barrel rise), but the same principle is at work.

IF you move your target further out, you will see the slower heavier bullets striking lower than the lighter faster ones.

Each weight & speed combination has its own arc of trajectory. One is higher than the other at any given point, and they will cross at a certain point, then the higher one becomes the lower one, and usually stays that way until the bullet hits the ground.
 
It probably has more to do with recoil characteristics than anything else is my guess. In addition to all of the rifle/barrel harmonics that have been mentioned above, remember that the recoil of the rifle interacts with various parts of YOU when the gun goes off. When someone else shoots my rifles, there POI is invariably different from mine, even with the exact same loads.

Even when just myself is involved, I notice very large POI shifts moving between different weight bullets. Sometimes the shift is only vertical, but usually it has a horizontal component as well. Even changing things on the rifle can affect POI. I have played around with those big, heavy lead wedges that you put in an A2 buttstock, and my POI changes.
 
+1

44Amp is on target. Up close, heavier slugs will print higher.

Farther out, the heavy slug will slow and drop more quickly, and then print lower.
 
I agree with 44amp as well. It has always been explained to me that the recoil impulse is longer with heavier bullets. They will usually print higher at 100 yards as well not just 50 yards.
 
Goldendog1:

Bullets are moving down the bore while the gun is in recoil. The bore time for a light bullet is less. In other words the light bullets exit the muzzle lower on the target than the heavy, slower bullet. This is only one reason from a multitude factors including barrell vibrations.

Semper Fi.

Gunnery sergeant
Clifford L. Hughes
USMC Retired
 
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