Weird trajectory issue

[ATH]

I have a 358 WSM 1.8". This is an odd cartridge created to maximize the performance allowed under Indiana's unique deer cartridge regs which dictate minimum .357 bullet diameter and maximum 1.8" case length. This particular gun is throwing the 225gr Accubond at 2770 fps. The barrel was chambered and installed on the action by a very reputable smith. I bedded the rifle and did all the other work myself.

The accuracy is good, but not spectacular, .5-.8 MOA on average. The Bushnell Tactical scope is set in Burris Tactical rings with the +/- 10 ring inserts to add MOA to the range (scope canted down at the front) on a flat Picatinny base.

The weird thing I am experiencing is that the gun is shooting essentially the same zero at both 100 and 200 yards. Then 2.5 MOA low at 300 yds, from the 200 yd zero.

In practical terms, this is a good thing as a round with relatively poor ballistics becomes a 300 yd point-blank gun. But the physics of it has me somewhat stymied. I've verified this numerous times, it is not an artifact.

 

[climb-101]

maybe the bullet is on the way up at 100 yds and on the way down at 200 yds and continuing to drop at 300 yds.

where is it hitting at 150 yds?

 

[davkrat]

maybe the bullet is on the way up at 100 yds and on the way down at 200 yds and continuing to drop at 300 yds.

where is it hitting at 150 yds?

Yep. It can be a major problem with pellet guns. You can be zeroed at 10 yards and 50 yards and shoot clear over a starling at 30. I'd bet you're high at 150.

 

[Schnyd112]

158's out of a 6" 357 at 1200 fps show very little drop from 0-100 yards. If you (I sure can't) can hold a pistol steady enough, 100 yards is doable. I don't have any experience with your rifle, but I shoot a marlin .357 lever gun with a 21" barrel. We hold about 6" low inside 50 yards then can hold on out to about 150 before the impact drops below the sights. Big heavy pistol bullets carry energy very well until a certain point then they almost fall out of the sky. Even .45 acp shoots very flat out of a pistol to about 90 yards. Not sure if that is the case here as I have never shot it 300 yards, and am working with 1400 fps max.

 

[davkrat]

It can be flat out to 100 yards it's the arc between 100 and 200 that would cause the issue. If the 100 is on the upslope the arc might peak at 150 and cross back over zero at 200.

 

[mountainman56]

Sounds like it will be a fine cartridge for your hunting requirements. I can't make those numbers work out exactly but I don't know your altitude, temperature or most other variables. If I was you I would play around with a ballistics calculator and come up with my best zero range versus drop at 300 yards versus peak trajectory. Then I would shoot it at 25 yard intervals from 100 to 300. JM2C gun)

 

[ATH]

The only logical conclusion is that due to something with the geometry of the scope above the bore, cant in the scope mounting, and trajectory, the bullet is on the way up at 100 and down at 200. Physics can be weird but laws are laws.

What I cannot figure out is how this could be so. There is nothing unconventional about the geometry of the scope mounting to cause such an issue. Unless there is something screwy with the angle of the barrel in the action.

I'd like to know what the deal is but it is hard to complain, it makes the cartridge easier to shoot not having to worry about drop until 300 yards.

And I have shot one deer with the rifle...those fat pills are impressive on impact.

 

[tbrice23]

I'm shooting a .358 Grant 1.8 (essentially the same thing ) with 180gr ttsx right at 3000fps. The Barnes has a lower BC but it looks like my 200yd zero crosses the line of sight with scope height at 1.9" at 60 yards.
And my 100yd zero is 1.4 MOA (or 3") low at 200.

Not really apples vs oranges but more like green vs red apples comparison.

 

[davkrat]

For some reason I thought the original post was referring to a subsonic round hence my assumption that the bullet was still climbing at 100 and arcing down at 200.

 

[ATH]

I'm shooting a .358 Grant 1.8 (essentially the same thing ) with 180gr ttsx right at 3000fps. The Barnes has a lower BC but it looks like my 200yd zero crosses the line of sight with scope height at 1.9" at 60 yards.
And my 100yd zero is 1.4 MOA (or 3") low at 200.

Not really apples vs oranges but more like green vs red apples comparison.

That sounds more reasonable. I've just never had a gun with a low mounted scope have un "upward" zero as far out as 100.

 

[tbrice23]

I wonder what powder you are using? And if you are experiencing this effect from winter temp to summer temps??

I'm using Hodgdon Benchmark it's an extreme powder.

 

[ATH]

I wonder what powder you are using? And if you are experiencing this effect from winter temp to summer temps??

I'm using Hodgdon Benchmark it's an extreme powder.

H322, and POI has been consistent 30F to 70F.

 

[tbrice23]

Ok.

If you aren't dead set with h322 try some Benchmark I'm getting great accuracy, 25" 12 twist Douglass. 200 yard group.

 

[bruce_ventura]

A zero at 100 and 200 yds is not ballistic for that round. Here is the drop table for a 225 gr AB @ 2770 fps @ sea level, showing good agreement with your drop at 200 and 300 yds, but definitely not at 100 yds. Impacts at 100 yds should be 1.7-2" below point of aim.

Range
(yards) Drop
(in) Drop
(moa) Wind.
(in) Wind.
(moa) Veloc.
(fps) Energy
(ft-lbs) Time
(sec)
0
↓
-1.62 0.00 0.00 0.00 2777 3853 0.00
25 -0.30 -1.15 -0.00 -0.01 2722 3702 0.03
50 0.72 1.38 -0.01 -0.03 2668 3555 0.06
75 1.44 1.84 -0.05 -0.06 2614 3413 0.08
100 1.85 1.76 -0.11 -0.10 2561 3275 0.11
125 1.92 1.47 -0.22 -0.17 2508 3142 0.14
150 1.64 1.05 -0.38 -0.24 2456 3014 0.17
175 1.01 0.55 -0.61 -0.33 2405 2889 0.20
200
⊕
-0.00 -0.00 -0.92 -0.44 2354 2768 0.23
225 -1.40 -0.59 -1.33 -0.56 2304 2652 0.27
247
↑
-3.05 -1.18 -1.80 -0.69 2260 2548 0.30
250 -3.21 -1.23 -1.85 -0.71 2254 2539 0.30
275 -5.45 -1.89 -2.49 -0.87 2206 2430 0.33
300 -8.14 -2.59 -3.27 -1.04 2157 2324 0.37