Advanced Marksmanship Shooting a rifle canted during stages, how do you guys do the math????

[matt2143]

I have now shot more than a few stages off of hard surfaces or flat on the ground requiring me to engage targets with the rifle canted at either 45 or 90 degrees. I know the basics of if canted left the bullet flies in this general direction etc. How are you guys doing the math to get accurate solutions??? I have a scope mounted 2.8" above the bore on a 20 MOA rail, sighted in at 100 yards pushing 185 berger Juggernauts @2700 fps.

Thanks,

Matt

 

[Rob01]

Go shoot it. I don't do math. I shoot it at 100 yards and see the hold off for both ejection port up and down and log it. Then I can add any elevation/windage needed on the reticle to make the hit just like a standard hold data from a 100 yard zero.

 

[Graham]

The math is simple: Elevation zero equals bullet drop from muzzle to zero range plus mechanical offset. When you cant 90 degrees, your elevation zero becomes your windage zero.

 

[Papa Zero Three]

I have now shot more than a few stages off of hard surfaces or flat on the ground requiring me to engage targets with the rifle canted at either 45 or 90 degrees. I know the basics of if canted left the bullet flies in this general direction etc. How are you guys doing the math to get accurate solutions??? I have a scope mounted 2.8" above the bore on a 20 MOA rail, sighted in at 100 yards pushing 185 berger Juggernauts @2700 fps.

Thanks,

Matt

Do the math before hand and it becomes your rule of thumb for that rifle. Ie the bullet drop in inches for 100yds plus the mechanical offset of the optic should come out to something like(depending on your specific set up) .75mils(drop) + .5 mils= 1.25mils

With the bolt/ejection port up the shot will be left 1.25mils of the POA and .75 low. So hold right 1.25 and .75 high for shots at 100yds. Do the math for your gun and distances and make a cheat sheet. Then, as mentioned, go shoot those to verify them and pretty soon you will be able to do it on the fly without really giving it much thought, as in no counting fingers and toes.

 

[Graham]

If you are making a rifle-canted-sideways shot at greater than your zero distance I suggest that you establish a parallel bore zero. Then you hold to the side by the amount of the sight height and hold elevation equivalent to the actual bullet drop at that distance.

 

[sled_mack]

You can also run JBM Ballistics with a cant value. It is normally set to 0 degrees. Get the numbers to match your known data with a 0 degree cant, then change the cant to 90 (read the help to get the +/- correct for right/left cant) and print the numbers. Finally, test those before the next match!

I keep this printed on the back of my regular data card - then I don't have to think about converting my regular data for these shots. It is more simple for me, but find what works for you.

 

[mouse07410]

Do the math before hand and it becomes your rule of thumb for that rifle. Ie the bullet drop in inches for 100yds plus the mechanical offset of the optic should come out to something like(depending on your specific set up) .75mils(drop) + .5 mils= 1.25mils

With the bolt/ejection port up the shot will be left 1.25mils of the POA and .75 low. So hold right 1.25 and .75 high for shots at 100yds. Do the math for your gun and distances and make a cheat sheet. Then, as mentioned, go shoot those to verify them and pretty soon you will be able to do it on the fly without really giving it much thought, as in no counting fingers and toes.

Sorry, I think my brain just locked up for no obvious reason.

How do you convert optics height (mechanical offset above the center of the bore, in inches) to mils?

 

[Bill Larson]

Go shoot it. I don't do math. I shoot it at 100 yards and see the hold off for both ejection port up and down and log it. Then I can add any elevation/windage needed on the reticle to make the hit just like a standard hold data from a 100 yard zero.

good advice.............

 

[Papa Zero Three]

Sorry, I think my brain just locked up for no obvious reason.

How do you convert optics height (mechanical offset above the center of the bore, in inches) to mils?

Assume that we're talking a 100 yard shot, and you have a 100 yard zero on your scope.

Your elevation zero is composed of two parts. One is compensation for the drop from the muzzle to 100 yards. With a .308, that drop is about 2.7 inches or so, about 2.6 MOA or 0.75 mils.

The other part is the mechanical offset of the line of sight from the line of the bore. Assume a sight height of 1.75 inches, or, at 100 yards, about 1.7 MOA or .5 mils.

Add those up, and you get about 4.3 MOA, or about 1.25 mils.

 

[mouse07410]

Assume that we're talking a 100 yard shot, and you have a 100 yard zero on your scope.

Your elevation zero is composed of two parts. One is compensation for the drop from the muzzle to 100 yards. With a .308, that drop is about 2.7 inches or so, about 2.6 MOA or 0.75 mils.

The other part is the mechanical offset of the line of sight from the line of the bore. Assume a sight height of 1.75 inches, or, at 100 yards, about 1.7 MOA or .5 mils.

Add those up, and you get about 4.3 MOA, or about 1.25 mils.

Thank you! Let's see if I understand. For example, I'm computing the "90-degree canted to the left" adjustment for 300m. "Normal" ballistic calculator that doesn't have support for cant, gives me correction: 2 mil up and 0.5 mil right for the wind. Assume the scope is 1.75in above the bore centerline, equal to 0.5 mil (at 100 yards = 91m).

I would need to dial (or hold) elevation using the windage dial (or former horizontal reticle line). I would dial 2 mil minus 0.5 mil correction for the scope height offset (which is no longer adding extra 1.75in above the bore) plus now-uncompensated 100-yards-zero drop of 0.75 mil = 2.25 mil to the right (which with a canted rifle is up).

I would need to hold windage using the former vertical reticle line. I would hold right (towards the former bottom) 0.5 mil wind correction plus 0.5 mil of scope offset (which would make bullet impact 0.5 mil left now) minus 0.75 mil that the scope is now adding towards the left (bullet drop at 100 yards that now is aiming to the left) = 0.25 mil down (which with the canted rifle is "right").

Did I get it right?

TNX!


Mouse
NRA Life Member Sent from my iPad using Tapatalk - now Free

 

[Papa Zero Three]

Sorry, but I've read that about 10 times now, my head hurts and I am no where close to understanding what you wrote or how you came to that conclusion. With your guns data and the formula posted above, is that the solution you obtained?

 

[mouse07410]

P03, let me try again, straightening up.

What I have​:

• Rifle zeroed at 100 yards.
• Scope centerline is 1.75in above the bore centerline, which equals to 0.5 mil at 100 yards.
• Bullet drop at 100 yards is 0.75 mil (according to ballistic calculator).
• Bullet drop at 300 yards would be 2 mil (according to ballistic calc).

What I want:

• Hit a target at 300 yards with rifle canted 90 degrees to the left (bolt side up).

What I need:

• Determine what elevation and windage to set on my scope to hit the target.

I understand that as the rifle is canted, the bullet impact would shift:

• to the left of POA because both the scope now is horizontally offset from the bore by 0.5 mil to the left and because of the zeroing effect (what was a 0.75 mil correction for elevation, now as the rifle is canted - makes POI 0.75 mil to the left of POA). One question is - the bullet impacts to the left by how many mils? It seems to me that both offsets - offset of the scope form the bore line and the offset for the bullet drop - move the actual POI offset at 100 yards to 1.25 mil to the left, as 0.75 + 0.5 = 1.25 mil
• down of POA because in this canted position the rifle is not zeroed, and the drop at 100 yards would be 0.75 mil.

So for 100 yards target I would set the scope at 1.25 mil to the right, and 0.75 mil up.

Now I try to figure what to set the scope to for 300 yards target. Since the rifle is canted, the true elevation is adjusted by the scope windage knob, and the true windage is adjusted by the scope elevation knob.

It appears to me that I would need to set the scope to: 1.25 mil to the right, and 2 mil up. Since the rifle is canted, it would translate to 1.25 mil down on the elevation knob, and 2 mil to the right on the windage knob.

Does the above make sense? One thing know I don't seem to understand is whether I "convert" scope offset from the bore centerline (1.75in) to mils at each given distance (so at 300 yards it would be much less than 0.5 mil), or use the number I got at 100 yards (0.5 mil)?

Or perhaps you could just run the math for a shot at 300 yards with a canted rifle that was zeroed at 100 yards?

 

[Papa Zero Three]

It looks/sounds like you are trying to use 100m data for a 300 yd shot. The example I gave you was for a 100 yard shot with a 100yd zero. If you go beyond 100yds the numbers/mils used in the formula are going to change respectively for the given range, so adjust it accordingly. Here is a chart generated by JBM with a 90 cant (bolt up). The only data you won't find on the chart is the sight over bore, which we know to be .5mils for 100yds so for 300yds it would be 1.5mils

Plug your updated data for 300 yds into the formula and then either hold or dial your solution. Elevation knob becomes your windage (left/right) and windage knob becomes your elevation (up/down)

 

[mouse07410]

It looks/sounds like you are trying to use 100m data for a 300 yd shot. The example I gave you was for a 100 yard shot with a 100yd zero. If you go beyond 100yds the numbers/mils used in the formula are going to change respectively for the given range, so adjust it accordingly. Here is a chart generated by JBM with a 90 cant (bolt up). The only data you won't find on the chart is the sight over bore, which we know to be .5mils for 100yds so for 300yds it would be 1.5mils

Thank you!! Perfect!

My only remaining confusion is how to determine the sight over bore offset at various distances. I.e., how (what's the logic & algorithm?) to convert my (for example) 1.75in over the bore to mils at 200yds, 300 rds, etc.

I don't understand how we determined that it is 0.5 mil at 100yds, and then why it would be 1.5 mils at 300m. I feel totally lost there.

 

[Papa Zero Three]

What is 1 inch at 100 yds in MOA, what is it at 200, etc? Convert inches to MOA, convert MOA to MILS. Once you have it converted for your scope height for 100yds in MOA or MILs you know how much to add per the yardage.

 

[mouse07410]

What is 1 inch at 100 yds in MOA, what is it at 200, etc? Convert inches to MOA, convert MOA to MILS. Once you have it converted for your scope height for 100yds in MOA or MILs you know how much to add per the yardage.

This is where I'm stuck.

1in at 100 yds is 1 MOA. Scope height is 1.75in, therefore it is 1.75 MOA at 100 yds. But that 1.75in at 200 yds would be 0.88 MOA, at 300 yds 0.58 MOA. I.e., the farther away - the smaller size ("less MOA") the 1.75in representing the scope height would be, as 1in at 1000 yds is only 0.1 MOA.

So when I add the correction for the scope height at 300 yds - should I use the MOA value obtained above, 0.58 MOA for 300 yds (approximately 0.17 mil)?

Did I get it now?

 

[Papa Zero Three]

OK, I went back and re-read my own post and I mispoke when I mentioned 1.5mils, so scratch that. What you posted about the diminishing height over bore numbers as the distance increases in your post above is correct.

Don't be surprised if the numbers you come up with are a bit off from the real world data when you shoot them, they should get you on steel but you might have to tweak them a bit to get your shots where you want them. I hope this helps you out but go shoot and confirm your data, doing it will make it more apparent to you than discussing it here.

 

[mouse07410]

Great! Now I got it.

Thanks!!


Mouse
NRA Life Member Sent from my iPad using Tapatalk - now Free

 

[Ring]

90^ hold @ 100y with my gun was 1.5mil right, and 1mill high

 

[Sterling Shooter]

I have difficulty with math, charts, and holding cant consistently. Plus, since the eyeball wants to naturally balance and center things I will abandon muscular relaxation in the sitting position which produces a cant and shoot with whatever exertion is necessary to keep the rifle squared up. This is an exception to the fundamentals; but, it appears the consequence of muscling the rifle is less than the consequence of not properly holding off. At this point I have developed enough muscle memory to make the tension to square the rifle consistently enough to be able to produce results indistinguishable from those when I have shot with cant, muscular relaxation, and a good zero for such a position.

 

[Erud]

I have difficulty with math, charts, and holding cant consistently. Plus, since the eyeball wants to naturally balance and center things I will abandon muscular relaxation in the sitting position which produces a cant and shoot with whatever exertion is necessary to keep the rifle squared up. This is an exception to the fundamentals; but, it appears the consequence of muscling the rifle is less than the consequence of not properly holding off. At this point I have developed enough muscle memory to make the tension to square the rifle consistently enough to be able to produce results indistinguishable from those when I have shot with cant, muscular relaxation, and a good zero for such a position.

Hmm, maybe this is a factor in why you haven't been able to earn a HM card in OTC as of yet?

 

[Sterling Shooter]

Hmm, maybe this is a factor in why you haven't been able to earn a HM card in OTC as of yet?

I wish that was the case. Glaucoma is what has suspended NRA HP for me now but my shortfall before glaucoma was never sitting, just standing, which put me at about 96 percent agg using the Service Rifle.

 

[Erud]

I wish that was the case. Glaucoma is what has suspended NRA HP for me now but my shortfall before glaucoma was never sitting, just standing, which put me at about 96 percent agg using the Service Rifle.

96% standing is very solid and puts you way above most HP shooters. Shooting a 192 leaves you with 16 points to drop over the rest of the match to stay in the HM class, which is a ton. You're obviously a pretty good prone shooter, so I wouldn't expect you'd drop many there. If you're also good at sitting, where were you losing the other 16+ points?

 

[sobrbiker883]

You can also run JBM Ballistics with a cant value. It is normally set to 0 degrees. Get the numbers to match your known data with a 0 degree cant, then change the cant to 90 (read the help to get the +/- correct for right/left cant) and print the numbers. Finally, test those before the next match!

I keep this printed on the back of my regular data card - then I don't have to think about converting my regular data for these shots. It is more simple for me, but find what works for you.

This for odd ranges, what Rob01 said for your 100yd zero.
Once you have what your rifle does at 100, tweak JBM's sight height to match that and then use it for projections.

 

[Sterling Shooter]

96% standing is very solid and puts you way above most HP shooters. Shooting a 192 leaves you with 16 points to drop over the rest of the match to stay in the HM class, which is a ton. You're obviously a pretty good prone shooter, so I wouldn't expect you'd drop many there. If you're also good at sitting, where were you losing the other 16+ points?

I did not say I shoot 96 standing, I said a 96 agg, or about 770 average with standing in the range of 185 to 192, from most recent events.

 

[Erud]

Ahh, that makes more sense. Standing is always what keeps people down.

 

[Graham]

Shooting a rifle canted during stages, how do you guys do the math????

But how do you know the rifle is squared-up? One either uses a level (Tubb), or levels the reticle (also Tubb). Either way, at known distances on square ranges you can discover and dial your cant or cant your scope. Problems arise when the horizon disappears and unknown winds affect your result.

 

[LastShot300]

ColdBore from Patagonia Ballistics, is the only package I'm aware of, that accounts for Canting, and most important it yields the correction, in two modes, POI or Fire Soluion, and for any angle, including compensation for any incline angle. I've been trying this feature in the field, and it's super fast. POI mode yields the actual POI, while Fire Solution gives the fully compensated solution ready to dial (in any units, MRAD, MOA, Turret, Clicks, IPHY). No need to do any math or offsets, just concentrate on the trigger. Note: both modes are available on the PC and Windows Phone versions, while the Mobile (PDA) only computes Fire Solution mode.

 

[Ring]

BTW..

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