how to deal with height over bore offset at close range?

[Robley]

This last weekends Inland Empire 3 gun match showed me a whole list of things I needed to change or do differently. One of the things I was having trouble with was point of impact vs point of aim at short ranges due to scope height over bore. Close range upper panel hits required a guestimate hold over that I never felt very confident about. What is the best way to deal with that 1.5" or more offset? A couple guys said something about different zero's where the bullet trajectory was 0 (on the way up) at either 25 or 50 yards and back on again at about 220 (on the way down), but that still leaves you dealing with the scope offset over bore at close range and I dont see how any zero could change that. So what is the answer, an angle offset set of sights like the Jp's?

Thanks in advance,

Robley

[BPiatt]

I know this isn't what you want to hear but the best way to deal with barrel off set is to do you homework!!

I don't mean that sarcastically, it's true.

Depending on your gun setup, you have to go out and try to see what works for you. If you're running one optic on your gun and don't have a dedicated "close up sighting system" like a second optic or BUIS, there two ways to deal with it;

1. Know your holdover point. With a standard AR/scope set up, a 200 yard zero is dead on at 50 yards. At 20 yards and in, it will hit 1 1/2 to 2 1/2 inches low. If you're trying for a USPSA head shot, you have to aim at the top of the head for a center hit.

2. If your scope has a graduated reticle for yardages, you have to go out and see which hash mark the gun hits at up close. CAUTION - using this method with standard variable power scopes will have different results at different power settings. This is an error waiting to happen.

I would suggest you learn the first method for two reasons. First of all you will eliminate any chance of an error of having using the wrong power setting. Secondly, with the first method, no matter what gun you pick up, red dot/scope/iron sights (standard mounting height), the hold over is the same.

I run a Burris 1.5 x 6 XTR scope mounted with the Burris AR-P.E.P.R.™ Mount. It's zeroed at 100 yards and I use the first method of just knowing how much to hold over. Ammo choice really doesn't matter.... your zero prefernce is negligable up close.... The only caveat I put on this post is if you have sight mounted higher than normal, i.e. scope mounted on top of an AR A2 style carry handle or a red dot on top of an ACOG. Using higher scopes will increase your hold overs. Once again... it's time to do your homework.

[mikegot38]

My rifle is sighted in at 50 yards. From 15 yards in I aim 2 inches high.

Edited September 28, 2010 by mikegot38

[gerritm]

My rifle is sighted in at 50 yards. From 15 yards in I aim 2 inches high.

My rifle is sighted in @ 25 yards for a 200 yard zero and it shoot 2" high also. I just use a 2" hold over at close range.

[Bryan 45]

But where does it hit at 75 yards?

42 yards?

129 yards?

158 yards?

Bruce's point is you have to know your hold overs and hold unders. It would be impractical to make a range cards with 5 yard increments, but you should have one with 20 yard increments. It's easy to do, and won't take more than about 60 rounds for an accurate card.

[mhearn]

My card is filled out for 25 50 75 100 125 150 175 200. After that, I don't have anything much more than the ballistic computer's estimates backed up with the previously mentioned numbers. It works out. Nothing to it but two hours at the range, a perfect weather day at the range (low to no wind), about 100 rounds, bench rest, spotting scope, and a lot of patience. Do I need to know all that. Seldom. But I like knowing it.

MHearn

[kellyn]

I was just going to say "HOLD HIGH" but Bruce's post is better.

[Corey]

practice and range time is the only way to truly know.

[gerritm]

But where does it hit at 75 yards?

42 yards?

129 yards?

158 yards?

Bruce's point is you have to know your hold overs and hold unders. It would be impractical to make a range cards with 5 yard increments, but you should have one with 20 yard increments. It's easy to do, and won't take more than about 60 rounds for an accurate card.

I also have all of this info @ even number yards. 25-50-75-100-150-200-300. I don't use a range card, but probably should. Get to know you rifle thru range time.

Edited September 28, 2010 by gerritm

[Stlhead]

Robley,

Those head only targets at 12 feet where meant to be a "mechanical offset" problem to solve. In that case the thing to do was to aim at the the very top edge of the paper (or as required for your rifle). As others have said one of the most important things to know before shooting a match with a rifle is exactly where your point of aim should be at the distances you expect to see targets. The other thing that gave people problems on that stage was the amount of shotgun reloading that had to be done before you even started shooting your rifle. With the heavy penalties for misses on paper, it was worth it to slow down and make every head shot. Another thing that can also help you at matches like the IE3G is that the R.O.'s are allowed to offer up some help, if you had asked Mr. Smith how to shoot the stage he would have given you some pointers that would certainly have included where to hold on the head targets. And I would have given you a couple of things to consider on stage 5 as well, but very few asked for pointers, even though the match director clearly stated in the shooters meeting both days that coaching was allowed and even encouraged for those that wanted it. It is good to see you on the forum, there is so much good info floating around here, I am sure that you will find some that will help you. And you can put your new hard learned knowledge of "Mechanical Offset" into use at the rifle pistol match at the other club next weekend! See you around the range.

[AlamoShooter]

my " Home Work" consist of many things = I will draw out a picture of what my bullet is doing . that helps me get my head wrapped around what I will have to do.

I use a Meopta scope and as I plan out a stage I will decide what part of the target I will look at and what part of my Dot. for the close targets I look -under - the dot - to place I want my hits to go.

If you draw out a side diagram of what your bullet is doing it will help you plan your stage attack

many of the ballistic programs will let you set it for 5 or 10 yard increments , it is surprising just how much the bullet drops in the last 20 yards to the target

my bullet starts 2.75 " inches under my line of sight at 47 yards it crosses my line of sight and continues up until it arcs out at about 135 yard and starts back down to sight in at 200 yards

Drawing things out can help to cement the concept in your head some times

[BPiatt]

But where does it hit at 75 yards?

42 yards?

129 yards?

158 yards?

Bruce's point is you have to know your hold overs and hold unders. It would be impractical to make a range cards with 5 yard increments, but you should have one with 20 yard increments. It's easy to do, and won't take more than about 60 rounds for an accurate card.

I'f we're talking 3-gunning, you're splitting hairs here.

With my Burris 1.5 x 6 XTR and a 100 yard zero, I use my 100 yard dot from muzzle distance to about 150 - 175. For the targets we shoot, hold the 100 yard dot on and shoot. Once you get to 15 yards and in, then you have to hold over.

[Robley]

Thanks guys. I suspected that the answer would be something like BPiatt gave (though I appreciate all of your comments). The other rifle thing I saw I needed to work on is field shooting / position shooting, and I know for sure the answer is the same.

Thanks again,

Robley

[bunsen27]

A slightly more complex solution is to utilize a two ammo system. I use 55gr FMJ 0-100yds and 69gr for 100+yds. The 55gr FMJ is ~3000FPS. The scope is zeroed for 200yds with my 69gr Nosler HPBT @ ~2750fps. My BDC dots are 250yds, 350yds, 425yds, and 500yds. By mainly dumb luck and a little bit of design the 55gr loads are POI = POA to +1.75 inches from 25-75yds so I can hold on or use a 6 o'clock hold on a vast majority of targets. Ultimately though as stated you do need to do some range work to determine what your rifle does with your ammo at varying distances.

A good online ballistics calculator I've found is http://www.vortexoptics.com/content/lrbc

I hope that helps more than it confuses.

[Alaskapopo]

This last weekends Inland Empire 3 gun match showed me a whole list of things I needed to change or do differently. One of the things I was having trouble with was point of impact vs point of aim at short ranges due to scope height over bore. Close range upper panel hits required a guestimate hold over that I never felt very confident about. What is the best way to deal with that 1.5" or more offset? A couple guys said something about different zero's where the bullet trajectory was 0 (on the way up) at either 25 or 50 yards and back on again at about 220 (on the way down), but that still leaves you dealing with the scope offset over bore at close range and I dont see how any zero could change that. So what is the answer, an angle offset set of sights like the Jp's?

Thanks in advance,

Robley

What every you do don't zero for short range. I know that should be obvious but I know of an agencys SWAT team who zeroed their rifles for 7 yards due to the off set issue. The obvious problem is you will be radically off target past that range. The best solution is to simply aim higher. In our rifle qualificaiton we have to do a drill where we shoot 2 rounds to the chest area and one to the head at 5 yards in 1.5 seconds. I just pull the trigger as soon as I see the top of the paper. What zero you use is up to you and your needs and it will vary depending on what you are shooting at a match. I prefer the 50/200 yard zero for general use. It keeps my bullet with in 2.5 inches of my point of aim from 0 to 250 yards or so depending on the ammo I am using.

pat

[gose]

What every you do don't zero for short range. I know that should be obvious but I know of an agencys SWAT team who zeroed their rifles for 7 yards due to the off set issue. The obvious problem is you will be radically off target past that range. The best solution is to simply aim higher. In our rifle qualificaiton we have to do a drill where we shoot 2 rounds to the chest area and one to the head at 5 yards in 1.5 seconds. I just pull the trigger as soon as I see the top of the paper. What zero you use is up to you and your needs and it will vary depending on what you are shooting at a match. I prefer the 50/200 yard zero for general use. It keeps my bullet with in 2.5 inches of my point of aim from 0 to 250 yards or so depending on the ammo I am using.

pat

Maybe they were snipers, but didnt have access to a longer range? (7y zero is roughly the same as an 820y zero)

[Vlad]

On a side note I love "Ballistic" for the iPhone, you can throw in it multiple loads, chrono data, weather data, bullet BCs, scope hight offset, all sorts of cool stuff and it will generate range data for all sort of ranges, wind condition, humidity etc, hold overs in MOA, mil's, etc.. Pretty neat and its output matches experimental data as well. 

[Kevin Holman]

On a side note I love "Ballistic" for the iPhone, you can throw in it multiple loads, chrono data, weather data, bullet BCs, scope hight offset, all sorts of cool stuff and it will generate range data for all sort of ranges, wind condition, humidity etc, hold overs in MOA, mil's, etc.. Pretty neat and its output matches experimental data as well. 

I love that app. It is an argument ender.

[Kyle O]

Something I ran into recently that deals with this is targets at an angle.

I was shooting out of an RV camper down at some head shots at about a 45 degree angle, and I was aiming at the top edge of the head as always, but 3 of my hits were low under the chin of the target, and counted as misses!

When angled that 2 inches becomes more like 4. Hard way to learn that lesson, but it was the best way, as I'll never forget it!

To answer your questions simply....Aim high.

[Kevin Holman]

Something I ran into recently that deals with this is targets at an angle.

I was shooting out of an RV camper down at some head shots at about a 45 degree angle, and I was aiming at the top edge of the head as always, but 3 of my hits were low under the chin of the target, and counted as misses!

When angled that 2 inches becomes more like 4. Hard way to learn that lesson, but it was the best way, as I'll never forget it!

To answer your questions simply....Aim high.

Never thought about that at close range - but the Pythagorean Theorem would apply - A squared plus B squared = C squared. At 45 degrees - a 2" height over bore would be exactly 2.82" low from POA.

[JimmyK]

Try this http://www.handloads.com/calc/

[Outsydlooknin75]

I do this all the time to figure POA POI. Set a target out at 25 yards, put an aimining point around the middle of the target, shoot a 3 shot "group" at the target, then move up 3 yards, and shoot another 3 shots, then again move up 3 yards, do it all the way to the target. You will see just where your POA POI is ... and you just have to know it. But that is some of the best training that you can do, instead of guessing, why not know?

[KentG]

For anything less than 5 yards or so I just point. Most matches I shoot are not USPSA rules so 2 on paper neutralize. Weather or not its a inch high or low in the A zone anyway isnt a big deal. If its tighter shots there is still usually plenty of exposed target to have a couple of inches easy to hit. For 3 gun "accuracy" is not really relevant unless its small targets like clays until the range gets close to the 100 yard range and depending on the size like small steel plates. Its all in your perspective I guess.