Intro/ short barrel velocity loss

[jakhamr]

Hey everyone,
I have a pretty limited knowledge of long range shooting. Most of my career has been spent doing fast reactive cqb style shooting. I've only recently gotten into long range precision shooting.

I really joined this forum after I was directed here seeking an answer to my question.

I am shooting m118 ammo and I would like to know if the muzzle velocity lost by cutting my barrel down to 18 inches will result in my hold over/ under changing? Will each mil dot still equal 100 yards for me?

I appreciate any input I can get,

[Cleck]

First off, welcome. We are glad to have you here.

Second off, we have a lot to talk about.

Originally Posted by jakhamr

...I would like to know if the muzzle velocity lost by cutting my barrel down to 18 inches will result in my hold over/ under changing?

Most likely. I say most likely because I do not know the set up of your rifle now. If you have a 20" rifle and are cutting to 18", you may not notice much of a difference at all with your ammo, and may notice a large difference with other ammo.

As a general rule, yes the barrel being shorter gives you a little less velocity and therefore may require additional elevation in order to hit targets (especially at further distances). However, with 118LR, I hit our 920 steel at 34 moa when the school's rental rifles hit it at 32 moa. A difference? Yes. A large difference? No. I actually see a larger difference between brands of ammo for me rather than barrel length. For example, changing to Fed. Gold Match 168 requires me to use 39.5 moa in order to hit the same 920 target.

As far as "hold over/under" goes:

Although using a holdover may be helpful in certain circumstances, we really preach knowing your elevation data on your scope for certain distances. We are expecting 1 moa or better accuracy at all distances. It is hard enough to hold over and do that, it is even harder when you are trying to hold 3 mils over and 1.5 mils right for wind. Doing so has your target floating off of the reticle which means you are not going to be able to be precise nor focus on your reticle while you shoot like you should.

Holding under? If you zero your .308 at 100 yds, there is not a distance for which you must hold under. At 25yds, you should be about 4 moa up in order to hit your point of aim, and at 500yds you should be around 12 moa up in order to hit your point of aim.

Originally Posted by jakhamr

...Will each mil dot still equal 100 yards for me?...

This one confuses me. I hope you are not saying that you intend to use a mil dot hold over for every 100 yds of distance you are shooting. First off, that does not work, and second off, even if it did, what would you do when you had to shoot past 500 yards (assuming you only have 5 mil dots below the reticle)?

I hope you use the search function on this forum to learn a lot of this information. However, risking this information not being consolidated elsewhere, I will give you a crash course.

A mil is an angular measurement. It is short for "Milliradian". Mil=metric for 1/1000th (as in millimeter is 1/1000th of a meter) Radian=Mathematical term for the angle made in a circle when the length of the radius is measured out along the circumference about 57.29 degrees).

Picture a piece of pizza where the side is as long as the crust. The side would be the radius (measurement from center to edge of the original pizza pie), and if the crust was the same length, the angle the piece represents would be 1 radian. Now, make 1000 little tiny equal marks along the crust. The angle made from the tip of the piece of pizza to two adjacent marks on the crust is 1 mil (milliradian, or commonly mil-radian).

By imagining this set up, you should be able to see that one 'mil' is a 1/1000th of any distance you are looking at. The distance is the edge of the piece of pizza (also the crust length) and 1/1000th of that workes out to being a 'mil' at that distance.

So, 1 mil is...
1 yd at 1000 yds
1 meter at 1000 meters
1 inch at 1000 inches
and so on...

It is amazing how many people will swear a mil is only 1 meter at 1000 meters since that is what they are told and don't realize that a meter is not part of the definition of a 'mil' but rather a 'mil' as the angle from the top to the bottom of 1 (something) at 1000 (somethings) away.

Without getting into tons more, your bullet does not drop 1 mil for every 100 yards. Your bullet should drop about 2 moa at 200 yds, 12 moa at 500 yds, and 20 moa at 700yds. MOA = Minute of angle - appx. 1" at 100 yds. If I were to round the numbers, 1 moa = appx 3.5 mils. So, this means your data for 200 is 0.57 mil, your data for 500 is 3.4 mils, and your data for 700 should be about 5.7 mils. Definetly not 1 mil per 100 yds.

So, long answer to your question. No, your mil dot will not equal 100 yds after cutting to 18" and it does not equal 100 yds now. If you thought it did.. be happy, you already learned something here.

[hubbell]

Cleck - You have a career as a high school math teacher if your shooting skills ever dry up. Nice summary of a complex topic.

[hubbell]

jakhamr - I shoot the same ammo with my Rem700 18" barrel and actually have a higher muzzle velocity than out of my 24" barrel on my Kimber 8400 Tactical. Longer barrel does not equal higher velocity.

Once the powder has burnt and the projectile continues down the barrel faster than the gases behind it are expanding, the frictional forces of the projectile against the wall of the barrel outweigh acceleration forces of the expanding gases, thereby reducing the velocity of the projectile.

Consider the case of an 18" (inches) barrel versus an 18' (feet) barrel. If the longer the barrel the higher the velocity assumption held true, the projectile leaving the 18' barrel would be greater. And if this were true, a 180' barrel would produce yet higher velocities. I intentionally exaggerate to make a point. At some distance along the barrel, the opposing forces,
1. The friction from the contact of the barrel and the projectile, and
2. The expanding gases
are equal.
The trick is to have this occur a fraction of a second prior to the projectile moving from interior ballistics to exterior ballistics, which is when it leaves the barrel. So if the gases behind your projectile are still expanding when the projectile leaves the barrel, making the barrel shorter will decrease muzzle velocity. However, if the gases behind the projectile are not expanding and thereby no longer accelerating the projectile, making the barrel shorter will increase muzzle velocity.
And in reality, none of this has anything to do with your ability to hit a target. Collect good DOPE with your ammo and rifle and use that to adjust to a target at a known distance, maintain good form and trigger control and you’ll be hitting targets all day long.

[jakhamr]

Thanks for every ones info. I do agree, and I do dial every chance I get, however I have found that when covering a large area with multiple targets at different distances, it is more practical to set my dope for the most likely distance and to hold over/ under for targets that appear at different distances. Yes, sometimes I do find myself aiming with nothing when I add wind and using an imaginary grid I have found it to be effective. It is not my preferred method, and I would not use it for a one shot/ one kill scenario, but in my job that is pretty rare.

I may not understand the math, but I know it be true that if I set my elevation for 300 yards and I need to take a shot at 400 I can hold one mil dot down and I will hit, at five hundred I hold two, six hundred I hold three, at two hundred I hold one high. The formula is the same no matter what yard dope I set on my scope, one mil dot down for every one hundred yards further, and one dot high for every one hundred yards closer. I also know that with different ammo the hold is not exactly one dot, so I am assuming that a change in muzzle velocity will also change this.

Based on the responses I've received I am assuming that this not a very popular technique, but again it is one I was taught and I have found it to work with my 20 inch barrel and ammo combination and to be useful in certain circumstances.

[Cleck]

Originally Posted by jakhamr

...Based on the responses I've received I am assuming that this not a very popular technique, but again it is one I was taught and I have found it to work with my 20 inch barrel and ammo combination and to be useful in certain circumstances.

Understood. A bit of a toolbox method approach. It may not be preferred, but if you find a use for it, pull the technique out and use it. I do keep my holdovers for different distances for such a situation. Handy when targets are shooting at you. I do not use full mils per 100 yds though, I break mine down into smaller chunks.

It also has a lot to do with your target size. With the military E-Type targets, you have a little over a 3.5 moa (1 mil) spread on the surface of the target. So, even if you are a a couple of minutes off of your dead center dope, you may still hit the target. An E-Type at 500 yds is a little over 7 moa (that's 2 mils tall!) So, I am not surprised that you could be up to 3.5 moa (1 mil) off of your actual true data in either direction for that distance and still get a "hit".

With your explanation, I understand how your technique works for you. Holding mil dots for different distances and floating off the grid for wind, may get hits on a certain target, but that is not within the 1 moa standard we usually desire. I am happy to hear that you dial the correct dope every chance you get and only use this technique when you in fact are just looking for a quick hit on a target.

You really got me thinking about your technique though. I could see it working for a assaulter/rifleman that has access to mil dots within his scope. It isn't the most accurate, but if a soldier could get a hit quickly on a target, it may have some merits.

[jakhamr]

Your right. I never claimed to be a sniper, I'm not sure if I have the patients, perhaps a rifleman with a scope and an overpriced gun would be a more accurate description. But like I said I am trying to get as much data for my gun as I can and use it whenever I can. This is just a technique I have learned to deal with a target that exposes itself at a different distance for a limited amount of time.
Two different mindsets and each one has its place
Thanks

[Cleck]

Originally Posted by jakhamr

...This is just a technique I have learned to deal with a target that exposes itself at a different distance for a limited amount of time...

Often how it works.

When walking along, and a target pops up at 300 shooting at you, it is handy to send a couple rounds downrange (especially with a semi-auto) while you are seeking cover. And with hold overs, you are at least getting the rounds close if not making hits.