I worked up some 450 grain loads, using TSX and North Fork solids, to 2250 fps in my .458 Winchester Model 70 with a 22" barrel. With these I took a hippo, buffalo, and bull elephant.

Returning home, I decided to work up a lion load. I was able to get 2700 fps with 350 grain TSXs with no pressure signs.

But...the 350 grain TSXs shot 7" LOWER THAN the 450 grain bullets at 100 yards. Some 350 grain TSXs loaded to 2600 fps shot 4.5" lower than the 450s.

How to explain this? I always thought lighter bullets, given reasonably maximum charges of powder, would shoot higher. Posters on another forum suggested that the 350 grain bullets shot lower because they spent less time in the barrel while the barrel was flipping up. Well, maybe, but a guy I know who has shot .458s much more trhan I have, and who has even written a book on the .458, says that his rifle always shoots the lighter bullets, from 350 to 500 grains, higher.

So I'm confused. Maybe there's something wacky about my rifle. Some weird phenomenon that changes the zero depending on how it's held. I remember my first shot at the hippo (with 450s) hit about a foot lower than I aimed, shooting from sticks instead of offhand, so disturbing me that the first thing I did when getting home was to re-verify the zero. It showed no change, so I attrributed the low shot to a flinch.

This rifle shoots 1.5" groups.

So I'm confused.

Second problem first ... I have noted that my .458 AR must be firmly held when shooting from either sticks or rest in order to get groups and the same impact point. Seems that the rifle will begin to move before the bullet leaves the barrel if the rifle is not firmly held.

First problem: The rifle is moving up more during recoil with the heavier bullet ... thus it is impacting higher. There will be a difference in the recoil generated by 450 gr bullets and 350 gr bullets.

The reason the extremely fast, lighter bullet shoots lower is indeed that the barrel time is so much less.
The bullet exits the muzzle before the muzzle rises as much as with the slower moving bullet, even if muzzle rise and recoil are similar in amplitude and speed with both loads.

Just like a double rifle.
When finding the load that your rifle was regulated for, it will start off shooting high and wide (diverged) with the slower load.

The two barrels will shoot lower and converge their groups as the velocity is upped, with the same bullet.
Too fast for the regulation and you will be shooting low and crossed.
But this adds an X axis variation due to the offset-from-center-of-gun-axis barrels of the double, so the recoil is upward and left with the left barrel and upward and right with the right barrel.

Back to single barrel like your .458, for simplicity:
454 Casull Revolvers do have a lot of barrel rise too.
Have you heard of the unfortunates who bury the hammer spurs of Casulls in their foreheads?
Has it happened with a 500 S&W yet?

At 1600 fps with a 300 grainer my 454 Casull is spot on at 50 yards.
A 260-grainer at 2000 fps will shoot lower at 50 yards: Less barrel time. Bullet leaves the barrel before the muzzle rises as much as it had at the time the 300 grainer exited.

Of course sights can be zeroed for the lighter, faster bullet, and then it will be shooting higher at all points down range past the zeroed-in range, than the heavier, slower bullet does with its proper zero.

Not all rifles will behave like that.
Yes, Magna-Porting will reduce muzzle rise,
but the recoil involved happens almost entirely before the gas gets to the muzzle, and cannot do much about the effect. The muzzle porting gas venting begins to happen as the bullet exits the muzzle, already pointed upward.
For some rifles with little muzzle rise, less recoil, straight-stocked and properly pitched, recoil motion may recoil more straight back with little muzzle rise.
Then the faster, lighter bullet may shoot higher than the slower, heavier bullet, with same sight settings. This seems to make sense, intuitive, and correct in such a case.
However, everything is relative, it depends, there are no absolutes, Viking Law, etc. ...



Shooting lower off sticks than offhand?

Either you did a funny jerk there or you are holding the rifle down harder on the sticks than in the free recoil mode of offhand.
Work on your technique.

Strive for a neutral downward force on the rifle forearm at all times.
If I hold a rifle down hard on the bags at the bench, it will shoot lower than when I shoot it offhand.

I do not hold down on the rifle in the sticks.

I just use the sticks to rest the back of my hand, the hand that grips the rifle forearm.
A thin leather shooting glove on this offhand is a good idea, for sticks.

I suspect that it's due to the shorter "barrel time" of the faster, lighter bullet, as you already guessed.

You didn't ask about solutions, but one option is having your rifle Magna-Ported.
Not to reduce your recoil (you aren't really complaining about that), but to have a portion of the gases vented directly upwards, decreasing the muzzle rise.

Talk to the guys at MagnaPort about this, and they can tell you what they can do (again, this is specifically for muzzle rise, not recoil).
The firearms that I've seen them work on for this issue (especially shotguns and pistols) don't seem to be any louder than they were before having the gas vents cut.

I had a similar problem with a 45 ACP Colt Officers Model...muzzle flip prevented me from getting back on target for a rapid second shot.

I had a custom compensator cut into the barrel, venting gases directly upwards (through the slide), and making follow-up shots a breeze. Didn't change the recoil at all, just the torque (rotational force).

If you have a banded front sight, the cuts can be hidden there, and won't be easily seen.
Really works.

Garrett

For a lion bullet try a 450 North Fork Soft.

Getting 2700 fps out of a .458 Winchester Magnum case is extreme.

Thanks, everyone, for your well-informed comments. I guess it just is what it is, though my friend does not have this problem with his CZ550 in .458. Maybe the stock is different or straighter.

I do not want to have the rifle magnaported.

450 No. 2: That's a good idea about using the 450 grain North Fork softs. Besides, the North Fork solids were slightly more accurate than any other bullets I have tried in my rifle.

Buliwyf: 2700 fps with a 350 grain TSX is NOT extreme. Here's how you do it.

The Model 70 has a large amount of freebore and the bullet has three possible grooves for crimping. So, in effect, you have a larger case. I can crimp on the middle groove and still get the rounds in the magazine. 2700 is easily possible with H4198, a Hodgdon Extreme powder that does not increase pressure with increasing temperature. My load is compressed but if you don't like this, you can drop it three grains and get 2600.

The CZ550 has an even longer magazine and lets you crimp on the last groove, I am told by my friend who has one.

In my Model 70 I run out of volume before I get into any pressure problem. AA2015 might allow higher velocities but I have not tried it.

..I was wondering about your velocity also .but my Ruger and Mark X only have 3.340 mag boxes ,,,,I get 2550 fps with 84 gr Accurate 2230 .. I have the same elevation problems with my 458s .. I like the 300 gr bullets going 2700 fps ....

Here are my loads...CAUTION, WORK UP TO THESE, THEY MAY BE OVER MAXIMUM IN YOUR RIFLE:

78 grains H4198 with 350 TSX, 2700 fps

75 grains H4198 with 350 TSX, 2600 fps

72 grains AA2230 with 450 (TSX, N.F., or
Barnes monolithic solid, 2250 fps

All used F215 primers in Winchester .458 cases. The 350 TSXs were crimped into the middle of three grooves. Velocities with Shooting Chrony at 15 feet, corrected to the muzzle.

AA2230 is a bit denser than H4198 or H4895. I used the slightly faster powder with the 350 grain bullets because I thought I could not get enough AA2230 in the case. AA2015 is a little faster still, and might be better with lighter bullets.

As noted, while I did not measure the exact freebore in my Model 70, it is A LOT. A rifle with less freebore might exhibit higher pressures.