Variable range scope trajectory

[Chipperman]

I'm sure this has been asked before, but I did a quick search and did not find it.
Anyway, if you have a scope with a variable range setting from 50- 300 yards (or whatever). In theory, you should be able to adjust the scope for said distance and hit the target. The caliber of your rifle is going to have a large impact on the REAL trajectory, however.

So my question is, what sort of trajectory do the scope manufacturers use to calibrate the markings on the scope itself? Is it an average 30.06 trajectory or something else?

I hope my question makes sense.
Thanks.

 

[Schmit]

Chipperman,

I think I know what your asking and will try and answer.

In regard to scopes that have either set markings on the reticle or set setting on the elivation turret for variable distance zero most are designed for a specific cartridge and projectial

Take the Leupold Mark 4 M-3 for example. The turret has a cam that can be changed for what you are shooting. The following cams are available

• .308 M 168gr at 2,650 ft/sec
• .300 Winchester Magnum 190gr at 2,900 ft/sec
• .30-06 Springfield 180gr at 2,700 ft/sec
• 5.56mm (.223 Rem.) 55gr at 3,200 ft/sec

The most likely projectial used for these would be a boat-tail match Hollow Point.

Same goes for scopes that have variable marking etched on the reticle. They are designed for a specific cartridge/projectial.

If you change the projectial, say from a BTHP Match to a Round Nose Soft Point the trajectory is going to change... however keep in mind that at close range the change is going to be small but increase as range increases. So at longer ranges the cam/markings will be off.

Also keep in mind that environmental conditions (Temp, Altitude, Humidity, etc) all have an effect on exterior ballistics. If you zero your rifle in say Death Valley in the heat of summer and then go shoot it for zero halfway up (Mt) Denali in AK in the dead of winter your zero is going to change.

Hope this helps

 

[stick]

Forgive me but got to ask...

Are you asking about the adjustments on the ring on the objective optic? If so, that's not a bullet drop compensator. It's for focus/parallax.

Otherwise, Schmit has it all wrapped up.

 

[Chipperman]

Sorry the post was not worded well...
I tried to quickly post it before I had to end my lunch break.
--
I'm pretty new to scopes in general. My query was instigated by my new scope. It's a Simmons 4-12x 40.
The front of the objective has markings for magnification, and the far end has (what I believe are) distance markings. I forget the measurements, but it goes from about 25- 600 yards.

If the projectile you are using delivers the indentical trajectory that they used to put the distance markings on the scope, then you should be on target when you adjust for whatever distance you are shooting. Weather, bullet shape etc will obviously impact that as well.

The instructions that came with the scope did not make any reference to the particulars they used for the calibration. My question was what caliber they probably used.

Suppose it was calibrated for a 30.06, for example, and I shoot a .223 with it. I'll be ok for the zeroed distance, but would then be low at longer distance, because .223 drops faster than 30.06. If I shot .308. I would probably be closer to the calibrated markings.

There are no cams to change, it's set for just one trajectory.

Does that make sense?

 

[Jim Watson]

If your Simmons 4-12 is at all like my Simmons 6.5-20, stick has it. The yardage markings at the objective bell - the end toward the target - the "object" are for parallax correction. That places the image of the target and the image of the reticle in the same plane so the crosshairs do not appear to move if you shift your eye position behind the ocular lens. Most mechanical adjustments for range are tied into the elevation adjustment in the turret by means of a cam or a graduated scale. Some, like the Leatherwood, use the power change to both estimate and adjust for distance.

A 4-12 x 40 Simmons is probably a Prohunter, right?
If so, the Simmons www site makes no mention of range and trajectory compensation. If you want to be able to handle range, you just have to adjust for it with the regular elevation. Then make a chart and tape it to your stock.

 

[Mikey]

Jim, being the true "operator" he is, would tape the drop chart to his stock - probably with OD Duct Tape (or 100 mph tape for true operators). Some operators even incorporate some velcro and lots of tape so the chart is weather resistant but easily accessible by lifting the homemade tape/velcro cover.

Us wannabes just put and Eagle or Blackhawk strap on cheekpiece with offside pouch and stick our drop charts in there.

Mike

 

[HankL]

You got the PRO answer from Schmit, Sorry, I should have said GUNNY Schmit. . Chipperman, what can be done with some variables is to range , is this what your are thinking of? Some scopes have yardage figures across from the power figures on the power ring. If you know what measurement your reticle substends then you have a basic rangefinder but it is up to you to know the trajectory and where to hold on the target.
You really need to prove a BDC scope to yourself. Zero it at 100 yds and then shoot 300, 600, 800 and see where thing are.
HTH

 

[Chipperman]

OK, I'm home now looking at the scope. It's a "Deerfield" model. The markings go from 20- 1000 yards. The instruction sheet is pretty much useless. It makes no mention of the marks or adjustments. So the markers are for parallax, NOT for changing the point of aim. That makes sense.

So if I'm aiming at a target, I want to adjust the objective to the approximate range to get rid of parallax problems, and then use the elevation knob to adjust for trajectory, right?

I thought that the objective changed the point of aim, which is why I was confused. The parallax does not care what caliber you are using.

Thanks guys.

As far as the chart, I'll prolly have a brass engraved chart mounted on the side of the stock

 

[Schmit]

HankL... I don't know about "Pro" answer but I gave him an answer.

Chipperman.

Yep you want to adjust the objective to the approximate range to get rid of parallax problems, and then use the elevation knob to adjust for trajectory.

However, to do so you need to know your range. Now, like HankL said you can use you reticle to range. Be it a veriable power or a fixed powder. It just takes some foreplay.

If you don't have a dedicated ranging reticle (i.e. MilDot) you need to head to the range. Most reticle are made up a various parts... thick edge stepping down to thin cross hairs. There is two measurements there in addition to the space the small cross hairs take btwn the thick cross hairs. At the range see what each of those subtend at 100 yards (if a variable scope you can either do this at one power setting or every power setting).

Then you can either do the math to see what the subtend at 200, 300, etc or run through the drill again at those ranges (this is a better initial drill). Yor range will probably only be XXX yards long so if you want to get the subtends for longer distance you will have to do the math.

Write all the information down as you take it. Later you can make up a chart or whatever that you can take to the field with you.