Mind-Bothering Questions

[Dance&Sting]

Hi. I just have some simple questions that irritates me if I can't find an answer especially when I'm going to purchase a rifle. Thanks again.

BOLT ACTION VS AUTO LOADERS: VELOCITY

In semi-auto, the gas of the cartridge in action is used to push the bolt back releasing the spent cartridge and bringing up the next one in line. That obviously decreases velocity but how much does it affect the bullet's trajectory? Does that matter if the "gas holes" are further down the stock near the end of the forearm? Is the original velocity loss greatly when compared to a counter part of its mechanism like a bolt action rifle. Do you recommend semi-auto for accurate, long range shots?

BARREL LENGTHS: LONG OR SHORT

Long barrels have their advantages and disadvantages. Long barrels extend the bullet's rate of twist, makes a rifle look more acceptable, and the extra 2-4" gives you more umfph for more velocity. Shorter barrels might not be able to grant you what longer barrels can, but what I've heard is that the stiffer the barrel the more accurate your groups are. Short barrels are obviously stiffer, unless if someone has something to argue against this claim, which is supposed to generate less vibration when the trigger is pulled and after the bullet exits the muzzle. Is this even true? I mean a longer barrel makes the centerfire rifle look more aggresive and attractive, of course, but can stiffness and accuracy be said when compared to a barrel that may be 2-4" shorter? I would like to know

WINCHESTER SHORT MAGNUM: .270 / .300

Pick one

 

[Zak Smith]

In semi-auto, the gas of the cartridge in action is used to push the bolt back releasing the spent cartridge and bringing up the next one in line. That obviously decreases velocity but how much does it affect the bullet's trajectory? Does that matter if the "gas holes" are further down the stock near the end of the forearm? Is the original velocity loss greatly when compared to a counter part of its mechanism like a bolt action rifle. Do you recommend semi-auto for accurate, long range shots?

Purpose built rifles for accuracy or long range are generally bolt action because they have less parts (easier to return to same state each time). Magnum calibers are generally prohibitive to build into reasonable weight autoloaders.

BARREL LENGTHS: LONG OR SHORT

Long barrels have their advantages and disadvantages. Long barrels extend the bullet's rate of twist,

This makes no sense. The rate of twist is totally independent of the barrel length.

supposed to generate less vibration when the trigger is pulled and after the bullet exits the muzzle. Is this even true? I mean a longer barrel makes the centerfire rifle look more aggresive and attractive, of course, but can stiffness and accuracy be said when compared to a barrel that may be 2-4" shorter? I would like to know

The quality of barrel and the chambering job are very important. A shorter barrel is stiffer, but if that is the only difference between two barrels, it will take a benchrest competition skilled shooter to expose it.

For long range, practical accuracy is aided by muzzle velocity because it reduces wind drift.


As for the caliber choice, I would suggest 308 to learn to shoot with.

 

[USSR]

D&S,

I answered your question on Sniper's Hide, so I won't repeat it here.

Don

 

[MTMilitiaman]

Hi. I just have some simple questions that irritates me if I can't find an answer especially when I'm going to purchase a rifle. Thanks again.

BOLT ACTION VS AUTO LOADERS: VELOCITY

In semi-auto, the gas of the cartridge in action is used to push the bolt back releasing the spent cartridge and bringing up the next one in line. That obviously decreases velocity but how much does it affect the bullet's trajectory? Does that matter if the "gas holes" are further down the stock near the end of the forearm? Is the original velocity loss greatly when compared to a counter part of its mechanism like a bolt action rifle. Do you recommend semi-auto for accurate, long range shots?

I have heard that it is generally accepted gas ports should be located as far down the barrel as feasible to allow the gas to cool a little bit before cycling the action but I have no idea how much of a concern this is or how valid it is as a concern. I have never chronographed a bolt action next to a semi-auto of the same cartridge and barrel length but I would imagine the velocity loss would be nearly inconsequential to trajectory. Even realitively large differences in velocity of several hundred feet often only amount to relatively minor changes in trajectory at most ranges. For example Federal loads the 165 gr Nosler Solid Base BT in the .30-06 Springfield, .300 Win, and .300 Remington Ultra Magnum to 2710, 3140, and 3350 fps, respectively. With all cartridges using 200 yard zeros, bullet drops at 500 yards are 47.3", 36", and 31.1", respectively. This means that although the 06 and the RUM are seperated by over 600 fps, the actual differences in trajectory 300 yards beyond the sight in range is about 16 inches. That is a lot by external ballistics standards. When comparing the same loads in the same cartridge in two rifles with equal barrel lengths, one being a semi and the other being a bolt action, I would think the actual loss of velocity with the semi would be less than 50 fps in most cases. The main concern would be accuracy because while it may be difficult to make a semi-auto as accurate as a bolt action, or nearly so, it is much more difficult and expensive to do.

BARREL LENGTHS: LONG OR SHORT

Long barrels have their advantages and disadvantages. Long barrels extend the bullet's rate of twist, makes a rifle look more acceptable, and the extra 2-4" gives you more umfph for more velocity. Shorter barrels might not be able to grant you what longer barrels can, but what I've heard is that the stiffer the barrel the more accurate your groups are. Short barrels are obviously stiffer, unless if someone has something to argue against this claim, which is supposed to generate less vibration when the trigger is pulled and after the bullet exits the muzzle. Is this even true? I mean a longer barrel makes the centerfire rifle look more aggresive and attractive, of course, but can stiffness and accuracy be said when compared to a barrel that may be 2-4" shorter? I would like to know

WINCHESTER SHORT MAGNUM: .270 / .300

Pick one

Whether a short barrel is stiffer than a longer barrel depends on things like contour and fluting. Given two barrels of the same contour and fluting options, the shorter one will be stiffer and in theory more accurate. But something tells me this would depend on a lot of other factors as well, not the least of which is the shooter and the ammunition. And as mentioned before, barrel length is independent of barrel twist rate. Barrel length might have a small effect on bullet spin and stabalization to the degree that it affects velocity, but that is it. In other words, two barrels can have the same twist rate, say .308 caliber 1:10 twist, but the bullet will exit the longer barrel spinning slightly faster because it has a slightly higher velocity.

As for your cartridge selection, depends what I am going to use it for. If shots are going to be a little bit longer and you think the slightly flatter trajectory of the .270 WSM is worth giving up bullet selection and the option of using bigger bullets on bigger critters, go for it. Game animals probably won't care near as much about your choice as you do and the meat will probably taste the same regardless.

 

[MachIVshooter]

Gas ports in autoloaders are typically located ~2/3 of the way down the bore. The majority of the velocity is developed prior to this point and the pressure bleed-off is minimal, so velocity difference from auto vs. bolt gun is negligable and using it to argue or make a decision is acedemic and trivial at best.

Accuracy-wise, bolt guns generally have the advantage. There are some autoloaders that will compete (AR-10(T), M1A NM, HK91, etc.), but tend to be significantly more expensive than an equally accurate bolt gun due to added machining costs for such precision.

Barrel length vs. accuracy: There is a happy medium here. Long barrels tend to "whip" more than shorter ones, but will help velocity and stability. The stability arguement tends to go out the window when talking heavy barrels, but they still need enough length to develop adequate velocity. Bottom line, the shortest TOF is the least affected by atmospheric conditions and gravity. IOW, the faster the bullet is moving, the less time there is for external variables to act on it. If you are shooting 1000 meters, you want a heavy and aerodynamic bullet traveling at fairly high velocity so that it will have a flat trajectory (speed) that it can maintain (aerodynamics) and be affected less by wind (weight). This is why people shooting such matches use cartridges like the .338 Lapua magnum, pushing 250 grain rebated boattails with S-12 or even S-14 secant ogives at over 3,000 FPS.

 

[Zak Smith]

So why did you quit shooting the LTR?

 

[Father Knows Best]

And as mentioned before, barrel length is independent of barrel twist rate. Barrel length might have a small effect on bullet spin and stabalization to the degree that it affects velocity, but that is it. In other words, two barrels can have the same twist rate, say .308 caliber 1:10 twist, but the bullet will exit the longer barrel spinning slightly faster because it has a slightly higher velocity.

True, but the difference should not affect accuracy. Bullets are either spin-stabilized, or they aren't. A bullet becomes stable once it reaches a sufficient rpm. Increasing the spin rate beyond the rate where the bullet reaches stability does not increase the accuracy potential.

 

[Father Knows Best]

Bottom line, the shortest TOF is the least affected by atmospheric conditions and gravity. IOW, the faster the bullet is moving, the less time there is for external variables to act on it. If you are shooting 1000 meters, you want a heavy and aerodynamic bullet traveling at fairly high velocity so that it will have a flat trajectory (speed) that it can maintain (aerodynamics) and be affected less by wind (weight).

The only rejoinder to this argument comes from my experience with small bore silhouette shooting. Competitors in that discipline use short barrels, which produce better accuracy (though they have barrel extenders that are used to maintain a long sight radius). The reason that short barrels provide better accuracy is that they are shooting relatively low velocity cartridges from unstable positions, e.g., standing. In that case, there is a lot of potential for the gun to move around between the time the shooter depresses the trigger and the time the bullet leaves the barrel. Ideally, when you get the sight picture you want, you would press the trigger and the bullet would instantaneously be on its way. In reality, there are delays caused by the lock time of the rifle (the time it takes for the firing mechanism to actually respond to the trigger and fire the cartridge) and the time it takes for the bullet to accelerate out of the chamber, through the barrel and out the muzzle. While all that is going on, any movement of the rifle can pull the shot off target, because the shot will go wherever the muzzle is pointing at the moment the bullet leaves it. Rifles with very short lock times and very short barrels have less opportunity for the rifle to be moved off target simply because the bullet gets out of the gun faster.

This effect is lessened greatly as you move into higher velocity cartridges and shooting disciplines with more stable positions. It can be almost entirely disregarded when shooting a centerfire cartridge from a rest or prone.

 

[30Cal]

Shorter barrels might not be able to grant you what longer barrels can, but what I've heard is that the stiffer the barrel the more accurate your groups are. Short barrels are obviously stiffer, unless if someone has something to argue against this claim, which is supposed to generate less vibration when the trigger is pulled and after the bullet exits the muzzle.

Sort of true in my understanding. Shorter barrels are on paper more accurate because the end of the barrel should have less deflection under vibration, but in real life, every barrel exhibits some degree of vibration and the bottom line becomes a matter of which is more consistant, which is more a function of construction and ammunition choice.

Heavier profiles tend to be more accurate because they are more forgiving of manufacturing variables and are more consistant over a wider range of temerpatures due to greater thermal mass and surface area for heat dissipation.

Ty

 

[Dance&Sting]

Thanks for sharing guys. Gave me some thoughts too

 

[g1726]

Just a little something to share here in regard to velocity loss in semi vs. bolt. A few weeks ago, I chronographed a few loads out of a Yugo SKS, first with the gas port open and functioning as a semi-auto, next with the port closed functioning as a straight-pull.

Wolf 122 gr. FMJ, newer polymer coated case

Port open Avg 2412 fps 104 fps e.s. 29 st.dev.
Port closed 2430 fps 57 fps e.s. 18 st.dev.

Wolf 122 gr. FMJ, about 4 years old, laquer coated case

Open Avg 2443 fps 80 fps e.s. 29 st.dev.
Closed 2448 fps 68 fps e.s. 20 st.dev.

Realistically, velocity loss is insignificant in the SKS. In the first case, there is a .75% difference and only .24% in the second. The standard deviation is greater than the velocity difference.

This is just one case, but the facts in this case are such. I'll be borrowing an M1A sometime in the future and repeating the test in .308.

 

[Jim K]

Test after test has shown that bullet velocity is not significantly different between a manually operated rifle and a semi-auto rifle, if all other factors are the same. As g1726 shows, the velocity difference is within the velocity variation among the individual rounds in the same box of factory ammunition.

Jim