I’m a machinegunner in the USMC, and I asked one of my junior guys a general knowledge question based on our job, and he got it wrong. The thing is, I tell my guys that if they can back up, or prove, or show me in some textbook why they answered in such a way, I’ll let it slide.
I asked him what the cyclic (full auto) rate for the M240 Bravo was. The correct answer is 650-750 rpm (rounds per minute) , but where you are taught can differ slightly here and there.
Well I asked him, and he answered wrong. (950-1050) So I was about to give him shit for it, but he hit me with this immediately after saying it: Well yeah, but I thought that at higher altitudes all fire rates go up?
Now, I’m no scientist, but I do understand that both fire and oxygen act differently at higher altitudes, and both are crucial components of weapons, and have to be perfect for weapons to operate properly.
So my question is:
Does altitude affect a weapons ability to function properly, and if so, at what altitude does it begin to take effect, and what effects might those be?
Weapons would absolutely fire in space - the propellant contains it’s own oxidizer -
I doubt very much that firing an automatic weapon in lower atmospheric pressure would change the cyclic rate significantly. Muzzle velocity would be very, very slightly faster - chamber pressures are in the tens of thousands of PSI, whereas atmospheric pressure at sea level is 15 PSI, so a lower pressure in front of the bullet is practically meaningless. And all the other factors involved in cyclic rate - like mass of the bolt & carrier, spring strength, etc. are completely unchanged.
The propellant in the round includes its own oxidant; it’s not dependent on atmospheric oxygen, so the combustion taking place behindthe bullet doesn’t care what the altitude is.
At higher altitude I expect muzzle velocity of the bullet will be higher, since there is less air mass in the barrel to be shoved out of the way.
Higher in-the-barrel average velocity means the bullet spends less time in the barrel at high altitude.
Since the bullet spends less time in the barrel, that’s less time for the propellant to be shoving the charger/slide back before barrel pressure is relieved.
So I would expect lower charger velocity at higher altitude.
Which means it doesn’t move back as far before reversing direction and returning to a ready-to-fire position. Which should, in theory, result in shorter cyclic time (higher cyclic rate) at higher altitude.
Unless I’m missing something obvious.
When the charger gets shoved back, does it typically hit a hard stop, or does it reverse direction entirely due to spring force? If the former, it seems like that would cause much more severe recoil, so I’m guessing not. But I’m not a firearms expert.
Based on the old Ma Duece, my father (brown show Air Force) always claimed the rate was about the same ground or high altitude. High altitude did give more jams though. I could see that with the various recoil and gas operated systems but I wouldn’t expect it with the more modern electrical systems.
Based on my grandfather’s comments (B17 top turret gunner in 1943) , the jamming at high altitude was almost certainly due to cold temperatures, not ambient pressure.
Welcome to the Straight Dope Message Boards, spartans117117.
Not at all a firearms expert, but my WAG is that the propellent is a combination of fuel and oxidizer, so whether or not it’s fired in a high, low, or even in a vacuum or not wouldn’t make a difference. Where altitude might make a difference would be if the M240 (not saying “gun”, know better than that!) was operated pneumatically, it might operate a little faster because of lower atmospheric pressure, but not as much as your junior guy asserts, because the mechanics of the gun is probably designed to damp out higher rates of speed.
I’d also argue that the exit velocity of the bullet would increase as well as the range because the decreasing drag.
I have a question for the board: does a sniper have to take altitude in account when shooting?
Altitude will have negligible effect on the rate of fire. But the lower air pressure will increase the range the bullet can travel.
Weapons in space are tricky. The cartridge should function just fine, but the weapon itself can run into difficulties. First, most firearms require some internal lubrication - this will evaporate/freeze/thicken in space. Second, heat will dissipate much slower. In extreme case, it is possible for the bolt to friction-weld itself to the receiver.
I’ll await someone with detailed info on this, but …
It really sounds like this difference should be trivially small. Even at sea level, the resistance the bullet encounters as it moves along the barrel is largely due to friction with the barrel and its rifling - only a tiny percentage would be attributable to air resistance. If you reduce the mass of the air (at 18,000’, it would be about half that of sea level) you thus should see only a tiny increase in muzzle velocity.
Machine Elf: From my experience, which includes domestic weapons, and many foreign weapons, if a weapon has a hard stop at the back end of a stroke, bad shit is happening. For example, the barrett 50 cal has the largest buffer (recoil spring) of any small arms weapon that I know. If you were to fire that weapon without**** the buffer spring and unsupported, you could potentially tear your arm out of its socket.
User_Hostile: Thank you! Now I’m no sniper, but I have a few friends that went that route, and I do know that snipers have to take altitude into account. From what I hear, thinner air allows rounds to fly straighter, further.
I would have thought most of the resistance was due to accelerating the mass of the bullet. Gross estimate here, for a .22, assuming a gas pressure of 30,000 psi, that’s over a thousand pounds of force behind the bullet. How much drag is caused by friction on the barrel? 10 pounds? 50 pounds? (it appears too that actual barrel pressures are much higher than that.)
Having said that, a .22 bullet weighs about 4 grams, whereas the air in the full length of the barrel (at sea level) weighs about 0.03 grams. So you’re right, the difference in the mass being accelerated at sea level versus at altitude is negligibly small and would probably not noticeably affect the operation of the gun.
How far is the sniper shooting? Using a web-based ballistics calculator, I looked at the trajectories for the known current 7.62 mm NATO U.S. long range round, the Mk 316 Mod 0. It uses a 175 grain Sierra boattail bullet, and will have anywhere from 2550 to 2650 feet per second muzzle velocity, depending on the length of the barrel. I looked at the situation for a level shot at STP at sea level, and at 10,000 feet. Zeroing the rifle for 300 yards, a bullet fired at sea level will drop 37.6 inches at 500 yards, and 369.9 at a 1000 yards. The same bullet fired at 10,000 feet will drop 32.8 inches at 500 yds, and 285.7 at 1000. Windage and lead show similar effects.
So it matters a quite a bit, depending on how far you’d like to shoot. IMHO, getting the relative wind right is going to matter more—even with the shorter time of flight at 10,000 feet, a 10 mph crosswind is going to move the impact 11 inches at 500 yards, and 50 inches at a 1000—but getting the altitude, temperature, humidity, and actual air pressure is going to matter too. Of course, that’s what their spotter is for.
Well, the weapon in question is the M240B. With a cyclic rate of fire of 650-750 rounds per minute, and a suggested barrel change every minute at said rate, the rounds per minute shouldn’t be an issue in one minute.
I could see continuous fire for 2-3 minutes being a serious issue, but that’s not really what’s at question.
M240B - Maximum rate of fire: 100 rounds/minute (sustained), 200 rounds/minute (rapid), 650-950 rounds/minute (cyclic)
The cyclic rate of auto-loading firearms can be altered to suit the purpose for which the weapon is intended. Generally-speaking, adding or subtracting weight from the bolt can increase/decrease the cycle-rate (lighter means a faster cycle-rate).
Increasing/decreasing rebound spring pressure would also alter cyclic rate (less would mean a faster cycle but could result in more damage to the bold and receiver).
For gas-pressure operated systems, increasing/decreasing the size of the gas pressure bleed-off hole would alter cyclic rate and increasing the size or weight of the gas piston would also affect firing rate. Of course, all of these alterations should only be done by qualified experts.
Meanwhile, back to your question about altitude, higher altitude would mean less barometric pressure but barometric pressure alone wouldn’t result in a jump from 650 to 1050 rounds per minuite.
Higher altitudes usually mean colder temps and colder temps can alter the size of the metal parts, decrease the viscosity of any lubricant, and slow the rate of fire. I’ve spoken to several Alaskan hunters who chose to remove all lubricant from their bolt action firearms to prevent jamming during the coldest months.
Colder temps will also reduce muzzle velocity. The difference between the exact same rounds being fired at 0 deg F could be 200 fps slower than if they had been fired at 100 deg F.
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