Physics and bullet velocities

[savage644]

So to start off, I have an actual background in ballistics, (I teach at Sonoran Desert Institute) both the 100 and 200 level courses. I've been flying both the 109G and the 109F4 and I'm noticing a massive discrepancy in the velocity/performance between the two rounds. Having both planes convergence set to 500m, the 13mm projectiles from the MG 131 in the G fall 2-3x faster (or at least feel 2-3x slower) than the 7.92mm in the MG 17. The stated FPS in the 13mm is 2500fps (750mps). The Stated FPS of the 7.92 is  2,800-2970 fps. The added mass of the 13mm projectiles means that they will actually retain their velocity better than a 7.92 projectile, meaning they will perform better at longer distances. Also, that extra mass will allow them to penetrate more effectively. It's my observation that the G's 13mm projectiles are going FAR slower than just the 3-400fps would indicate because the amount of lead needed to hit a target, especially moving ones, is 2-3x what I usually need to hit with the 7.92 projectiles, this is magnified at distance, which should not be the case.

I have heard that the 30mm is being dealt with, but I believe the 13mm needs some attention as well.

[Poker]

 

A pleasure reading through your brief analysis. CRS are always looking out for volunteers and I’m sure they’d welcome a discussion with another specialist in the field.

[DOC]

2 hours ago, savage1r said:

So to start off, I have an actual background in ballistics, (I teach at Sonoran Desert Institute) both the 100 and 200 level courses. I've been flying both the 109G and the 109F4 and I'm noticing a massive discrepancy in the velocity/performance between the two rounds. Having both planes convergence set to 500m, the 13mm projectiles from the MG 131 in the G fall 2-3x faster (or at least feel 2-3x slower) than the 7.92mm in the MG 17. The stated FPS in the 13mm is 2500fps (750mps). The Stated FPS of the 7.92 is  2,800-2970 fps. The added mass of the 13mm projectiles means that they will actually retain their velocity better than a 7.92 projectile, meaning they will perform better at longer distances. Also, that extra mass will allow them to penetrate more effectively. It's my observation that the G's 13mm projectiles are going FAR slower than just the 3-400fps would indicate because the amount of lead needed to hit a target, especially moving ones, is 2-3x what I usually need to hit with the 7.92 projectiles, this is magnified at distance, which should not be the case.

I have heard that the 30mm is being dealt with, but I believe the 13mm needs some attention as well.

I don't know what the ballistic coefficients are for the two bullets in question, but a lighter faster bullet retains less interia at ranges beyond it's maximum acceleration only if the BC is identical to, or less than the heavier bullet, is that correct ? I ask this because a .223 bullet travelling at 3000 FPS exhibits less drop at long range than a heavier .311 bullet travelling at 2600 FPS does, and when shooting at a moving target, seems to require less lead angle, principly because it's slower I had thought. This is out of a rifle of course, so I'm not taking advantage of a constant stream of bullets nor is there tracer to track the flight path of the bullet either. In this example the two bullets do have different BC's, where the .223 bullet is higher so it sheds kinetic energy less quickly. I understand that bullet weight is not the issue but I am merely noting that they differ.

If their BC was identical, would the heavier slower .311 bullet shoot flatter than the faster lighter .223 bullet ? I'd guess yes but I thought I'd ask an expert. In my experience bigger caliber heavier rifle type bullets seem to consistently have a lower BC than lighter caliber ones, but this experience is restricted to basically .223 (5.56 metric) and .311 (7.62 metric) spitzer pointed boat tails.

Edited April 19, 2020 by DOC

[madrebel]

boat tail didn't exist in ww2 that i'm aware of ... at least my lyman manual doesn't reference it for rounds i've looked for - exception here is 7.62x54R but afaik those came after ww2. fwiw the 7.62x54R is the longest serving military cartridge and it has evolved some post ww2. BCs were pretty bad too for ww2 rounds. i'm surprised a ballistics teacher doesn't understand that bullets hit the ground at the exact same rate, because, you know ... gravity is constant.

meaning, doesn't matter the FPS, two bullets from two guns leaving the barrel at the exact same time and they hit the ground at the exact same time down range, presuming everything is perfectly flat and you could time it such that the projectiles leave the barrel at the exact same time. all velocity does is get you down range further before hitting the ground in this context.

also, bigger heavier slower rounds do NOT penetrate better. in the ww2 context this can be referenced easily. compare the stug's cannon to the panther. better, compare the US or Brit AP rounds to HVAP. same gun fires both rounds yet the small dense ultra fast penetrator ... well .. it penetrates.

german 13mm is more or less similar ballistically to 20mm. german 8mm is not similar, it shoots 'flatter' meaning due to velocity advantage it goes where you're expecting out to a further distance before it goes to where you're not expecting.

[savage644]

So, ballistic coefficient relates to the efficiency at which the bullet cuts through the air. A bullet with a higher BC will be able to travel faster/farther than a bullet with a lower BC. However, mass also figures into the equation. For example, many people can use .223/5.56 and .308 BDC's nearly interchangeably because the trajectories are very similar, however, like stated above, 5.56 is a 55gr bullet heading out at 3000fps whereas a 147gr .308 typically comes out around 2650fps. When I was talking about penetration, I'm merely comparing like/like things. i.e. If you have a 8mm fmj compared to a 50 bmg (closest equivalent to 13mm) fmj, the 50bmj will naturally penetrate more than the 8mm because it has more mass behind it.

>  i'm surprised a ballistics teacher doesn't understand that bullets hit the ground at the exact same rate, because, you know ... gravity is constant.

I'm having a hard time finding the place where I said they wouldn't. Based on the fps/mass/bc, one will hit out further than the other one, however.

[madrebel]

50 cal ball will out penetrate 13mm ball not due to mass but due to velocity. Same for AP rounds. Look at tests of the 556’s m855 vs the 762x51 ball in combat. Despite having twice the mass the velocity and steel penetrator of the 556 round grossly out performs the 762. 
 

velocity is king with penetration. 
 

also 55 grain 556 is MUCH flatter than 308. 77 grain m262 round is very similar to 308 though and 77 grain is typically what is used when you’re shooting long distance for sport competition or in combat. 762x51/308 is superior For distance for many reasons but the m262 round really rounds out the options for the 556 platform. Also fwiw US military both army and marines have essentially fully shifted to the 62 grain ss109, m855 0/1, or m318 0/1. 62 grain is just much better all around. 

[savage644]

Velocity makes a large difference, but projectile design and materials are also very relevant.