/r/ballistics
Ballistics is the science of the mechanics of projectiles. It is concerned with three separate but related parts: Launching the projectile, that projectile's flight path and the affect the projectile has on what it hits - internal, external and terminal.
The ballistics subreddit is meant to be fairly technically based. Ballistics is focused on the physics involved in launching a projectile and its flight. It doesn't even need to be limited to guns; arrows, missiles and any other projectile can be relevant.
This can seem fairly broad, so I encourage you to think critically about the relevance of your content before you post it, but not feel too hesitant if you're sitting on the fence.
But most of all, enjoy! Ballistics is an exciting field of study not dependant on global location, culture or experience. We're all here to learn and interact, so be friendly and welcoming to those who are less knowledgeable on a certain aspect of ballistics to you.
Check out our wiki.
/r/ballistics
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Sectional Density Equation Seems Incorrect
I have been digging through all of the fundamental ballistic math and have found a few issues. The one I'm trying to understand presently is regarding sectional density. Why do white papers specific to ballistic account for the area of the bullet as a square as opposed to a circle?
What's consistently listed: SD = Mass / diameter²
Whereas in reality it should be: SD = Mass / pi * radius²
These two equations give different results. For example, if we use a 0.338 with 248gr and do some conversions to get pounds per square inch, we get:
0.03542858lbs / 0.338² = 0.310113266.. psi
0.03542858lbs / pi * ( 0.338 / 2 )² = 0.394848447.. psi
While these may seem relatively close, those decimal values matter. Is the diameter squared just short hand to simplify the process, or does it play a role in correcting further equations?
11:36 UTC
.30 cal. differences
What are the differences in diameter between .300 wm .300 blk 30.06 30.30 etc.?
12:30 UTC
What is most dangerous?
I have a .22 air PCP carbine, it shoots 1.3g metal pellets with 21.5 joules of energy
And I have too a .68, it shoots 1.20g rubber ball steel core with 16 joules of energy
My garden is about 10m long
If I have and accident with a animal, human or other livingthing which causes more damage, I usually shoot in my property but I would like to know that if I involuntarily have a wrong shot and it doesn't hit the target ¿What is the most dangerous shot? Thank.
15:30 UTC
Trump Graze Wound Ear - What are ballistics experts saying?
Have any ballistics experts weighed in on the wound yet? Can a bullet from an AR-15 at that distance cause that small of a wound? Wouldn't the spinning of the bullet cause more damage? There have been graze wound photos shared on X and they seem to cause significant damage and none look like anything an ear could survive.
13:47 UTC
Infuriated by this
Could anyone help me understand why half of the articles online define sectional density as mass over diameter squared, and the other half say mass over cross sectional area? Pi is not equal to 4!!! That’s a 27% difference, which is huge. Which quantity is usually used when quoting B.C.s? Thanks!
13:56 UTC
Trajectory question
My in-laed had a bullet come through their kitchen window o the 4th of July and end up hitting the refrigerator, going through the fridge and freezer portion to get stuck in the side. Based on the hole in the screen and the window, as well as the entery hole in the side of the fridge I figured the bullet was angled downward.
My wife is convinced that someone .Just have shot the bullet from right outside the window for it to go into the fridge like that but I think it was likely someone shooting a firearm into the air at an angle and it coming down.
Would a bullet be able to maintain enough velocity to be shot into the air and penetrative through a window and refrigerator like that if it had been shot into the air (say at a 45 degree angle)?
03:39 UTC
This hypothetical cartridge is research for a book i plan to write..
.30 carbine case but 2-5 mm shorter. Projectile is fmj .30 boat tail 165-180 grain. The shorter case and larger mass of the bullet makes the round subsonic ~ 1100 fps. The o/a length is the same as .30 carbine and will feed in its mags. Chamber/slide/ gas chamber would have to be modded but would this work? Be a viable cartridge out to 200+ yards. I know it would get loopy past 125-150 but the improved flight of a traditional rifle vs pistol bullet would help at longer ranges right? Thanks
01:55 UTC
Trajectory question… pertaining to scope zeroing.
I’m brand new to owning a rifle, and will be mounting and zeroing my first scope and rifle in a couple weeks.
I’ve sorta researched the topic, but like many things, everyone’s opinion is different and many have made convincing, if not conflicting, arguments as to why any particular zero is better than _____.
I have a AR-15 with a .223 Wylde chamber, a 16” barrel with a 1:8 twist rifling, and I plan to (or hope to) shoot heavier-grain projectiles loaded to 5.56 NATO spec. The mfg. recommended Hornady Black ammunition because in their testing it performed best. They didn’t exactly explain what “best” meant, but I took it to mean it cycled the rifle more reliably than other ammunition in all circumstances. That said, it seems to me, in my humble opinion, that the Hornady Black ammunition line was meant as a middle-of-the-road / do-it-all cartridge that performed flawlessly in terms of cycling but isn’t exactly a 1MOA precision cartridge.
I’m not expecting to win any PRS matches with the rifle, but I am hoping to squeeze out the finest accuracy I and it can do. My thought is, 62gr is a happy median between the lighter M193 stuff, and the heavy match-grade stuff. But, it’s almost all M855 green-tip and there’s less than a handful of range-friendly FMJ lead-core stuff, so it’s pretty restrictive on ammo selection.
Which leads me to believe that I’ll eventually settle on 62gr, 75gr, or 77gr, but begs the question… which zero is best on my given platform and for whichever cartridge and projectile my rifle performs “best” with?
Is there any way of knowing/predicting? My first thought is to look at the trajectory tables from the ammo mfg. (If available). However, it seems most of them use a 24” test barrel, so the trajectory would be significant different with my 8” shorter barrel, wouldn’t it? For example, if their 24” test barrel provided a trajectory that has the bullet coming up into the line of sight at about 50yrds, then back down into LOS at about 200yrds, that would suggest to start there and trial-error until I got as good a result as I can.
But I wonder if there’s a better way to calculate a particular barrel length and twist rate and projectile to get a good idea ahead if time what zero strategy to adopt.
02:51 UTC
Custom Cartridge Physics Simulator
Does anyone know of any good physics simulators that allow the creation of custom cartridges (projectile weight and caliber, case length, etc.)? I've been working on some worldbuilding for an alt-history universe and I don't want to use a lot of existing modern cartridges like 5.56, 7.62 anything, 5.7, 9x19, etc.
18:55 UTC
Are there any good ballistic dummies that aren't... you know... $2500
16:58 UTC
How does Uranium AP ammo work?
Does someone of u experts know how exactly this "self sharpening" works? I hear it often on thr internet but i cant find infos on under what circumstances it occurs or what alloy this exactly is! All i know is that uranium is soft and i wouldnt use it as AP ammo!
18:18 UTC
Grabovac hill fort Traste weird cupola damage and debris.
Does anyone know what might've caused this damage? Wiki says it was under small-caliber artillery fire and saw little damage, but the debris there says otherwise. Also this cupola damage looks weird.
(some other place)
Sources:
13:09 UTC
Tubular bullets - projectiles of the future in the 19th century?
15:07 UTC
What is the best materials for kevlar and best way to bind them together
What is the best material to lare with kevlar. I heard fiber glass is good and polyethylene fabric are good to lare it with and what would be the best way to bind them all together to make a helmet mold ??????
08:12 UTC
Whats the best type of bullet resistant glass
I want a bullet resistant glass for a helmet visor thats not insanly THICC and can be seen through ealisly
07:32 UTC
Musket Ball Heat
I’m asking this just cause I’m curious. So how hot do musket balls get, and upon contact, if it is hot enough, does it burn/melt the surrounding surfaces?
03:20 UTC
Is a saboted bullet in a straightwall cartridge just as fast as a bottleneck? If so/not, why?
I know there are quite a few setbacks to sabot cartridges, but I was wondering if velocity was one of them.
21:32 UTC
Question about speed of a bullet
Why do heavy, slow rounds pass through tissue more effectively than a light, fast round, even when that faster projectile will penetrate hard materials that the slow one can’t?
00:05 UTC
Need help calculating speed of a lighter projectile
Is there an equation to calculate the velocity a bullet might reach using the velocity a heavier bullet is capable of? I'm kind of new to this so sorry if it's a dumb question.
10:15 UTC
Just want your guys opinions on if this hypothetical idea would work.
This is kind of rocketry related but oh well. If I made my own rocket motor, like a tube, with a hole through the middle, basically just a normal looking model rocket motor (potassium nitrate + powdered sugar) and I compacted it into the casing with a dowel, what would happen if I slipped some cut up razor blades or small glass shards into the propellant. Could I in theory hold the motor sideways, set it off, and have the glass shards fly out of the tube like shrapnel at mach 2 or 3 or whatever the exhaust velocity of the rocket was?
23:11 UTC
Need help calculating the range of a rocket in a videogame with an equation
Good afternoon, I am unsure if this subreddit is still active but I would like someone to assist in calculating the maximum range of a rocket in a videogame I play called Brick Rigs. The rocket's launch parameters are as follows:
Weight: 93.8kg
Dimensions: 2.2m x 0.424m x 0.424m
Angle: 45deg
Motor burn time: 2.45s
Speed (after 2.45s): 353m/s
Altitude (after 2.45s): 394.4m
Vertical speed (after 2.45s): 221.7m/s
Atmospheric drag: None
Gravity: -9.81m/s
Conducting some tests, I found the rocket had a range of 8250m/s, impact speed of 218.156m/s, apogee of 2300m and flight time of around 45s. Using ChatGPT to find the solution, I found its answers to be off by around 2-3000m. Could anyone find a way to accurately match the listed range, as well as the pitch of the rocket after motor burnout, with calculations?
03:22 UTC
PSI Of A Musket Ball
I'm working on a custom musket intended to fire ball bearings instead of lead, lead being harder and harder to get, and more and more restricted on where/when/how you can shoot it in my area. Beyond that, lead is a neurotoxin, so less of it is always a good thing. I've seen a few videos of 12GA shotguns firing ball bearings and that piqued my interest in having a custom barrel made. Modern muskets made in Italy already have barrels far exceeding the old quality steel and techniques for producing barrels 150 years ago.
I brought this up to a local black powder barrel maker and he laughed saying 'oh no, that's going to jump the PSI way too high to be safe, I can't make that'. That comment got me thinking.... how exactly would it increase the PSI?
Steel is less dense than lead. The dimensions are the same, a .65" ball in a .69" barrel, so the windage is still there. Steel won't deform to form a gas seal like lead will; if anything, it should lower the PSI, since some wasted energy is going to go into that .04" windage, as musket balls are not tight-fitting like a modern bullet.
Thoughts?
19:20 UTC
FBI Test Protocol Specification
Hey, does anyone know where I can download a copy of the FBI test protocol (Standard) for hand gun ammunition? I have googled everywhere and cannot find it. I know what it is and how to score but need the official document, not just a good article. Thanks!
19:55 UTC
Caveats of the 50/200 yd Zero with Sub-20" Barrels: A Case Study in 5.56 Ballistics
Hey, y’all. I am a relatively new rifle shooter and 2A enthusiast trying to learn basic ballistics and optimize my AR-15 setup. I recently went through a lengthy but fun exercise in determining the optimal zero for my specific rifle and optic pairing. Field testing is pending for validation. Meanwhile, I hereby submit the math to y’all in hopes of receiving some feedback from more experienced shooters. I also hope it is informative to anyone else just getting into ballistics and/or AR platforms :-)
TL;DR: The 50/200 yd zero is derived from the performance of 5.56 NATO out of a 20" barrel like in the original M16. As your barrel gets shorter, you need to factor in the changing ballistics to achieve your best zero and holdovers. For me, I will be using a 36/230 yd zero with my 14.5" AR-15.
For the long version...this is my setup:
- Rifle: DDM4 V7 SLW
- Factory build: 14.5” barrel (pinned and welded flash hider to achieve 16” total length), 1:7 twist, mid-length gas system
- Ammo: PMC X-Tac 5.56 NATO, 55 gr, FMJ-BT, MV 3,120 fps, BC 0.243
- Optic: Vortex Strike Eagle 1-6x24 SFP LPVO, AR-BDC3 reticle
Let's look at the scope’s reticle. Looking at the Vortex manual, we can see that the BDC reticle in the Strike Eagle is built to match the trajectory of a specific bullet and velocity. It is tailored to a 60 gr bullet with a MV of 3,050 fps zeroed at 50/200 yd. I assume the choice of a 60 gr bullet is a compromise to keep the reticle faithful for both 55 gr M193 and 62 gr M855 cartridges. Standard atmosphere is assumed given we’re not told otherwise.
Now we get to what launched me on this project: the assumption of a 50/200 yd zero on this optic. Every AR-15 shooter "knows" that zeroing your 5.56 NATO rifle at 50 yd will give you a remarkably flat trajectory out to 200-225 yd, allowing for accurate medium range shooting without going through any mental gymnastics. It is a fantastic zero. But, if I understand correctly, it is also built on an important assumption: a 20” barrel.
We know that as we shorten an AR barrel, we lose muzzle velocity. Sonoran Desert Institute (our favorite Guntuber sponsor) provides the following estimates of MV for various AR barrel lengths:
These estimates are very close to field testing results by the guys at Classic Firearms on their video titled “How Much Does Barrel Length Affect 5.56 Bullet Velocity?” (https://www.youtube.com/watch?v=TbdNZuCvEh4). For the 14.5” barrel they specifically used a DD M4A1, very similar to my DDM4 V7 SLW. Pinned and welded flash hider, 1:7 twist. One difference that could be significant: carbine-length gas system. To preserve my sanity, I set this difference in gas system length aside and assumed “close enough” performance between the two rifles. Classic Firearms’ M4A1 clocked in at 2,763 fps. I can therefore trust Sonoran’s MV estimate of 2,750 fps as a pretty good generalization for 5.56 NATO in a 14.5” barrel. Let’s run with it.
First, I validated the reticle on my Strike Eagle by comparing the manufacturer's reported bullet drop (in the table above) to the ballistic trajectory of my PMC X-Tac 5.56 NATO at the MV of 3,120 fps that it would achieve if I was shooting out of a 20” barrel. We can see in the graph below (generated using Shooters Calculator; assuming no inclination, no wind, and standard atmosphere) that, with a typical 2.5” height over bore, we can get that nice 50/200ish yd (50/217 yd) zero that the reticle assumes. From there, our calculated bullet drop closely matches the bullet drop in the Vortex manual.
However, if I look at the same ammo shot at 2,750 fps out of my 14.5” barrel with a 50 yd zero, I can clearly see my far zero shifts significantly. It is now a 50/174 yd zero. This results in bullet drop becoming significantly different from what the scope's reticle is built for.
So, my initial conclusion/fear was that with my rifle, this LPVO was no better than a simple red dot and magnifier, since the BDC reticle lost accuracy quickly beyond 200 yd (leaving the center dot as the only useful graduation). But ballistics are neat. What if I moved my near zero from 50 yd to 36 yd? By doing so we actually achieve a 36/230 yd zero that closely matches the reticle’s bullet drop out to 400 yd, extending the usefulness of my optic.
Pretty good! But we can do even better through a little mental gymnastics. On further examining the numbers, it becomes evident that my actual bullet drop at any given distance past 450 yd matches the reticle’s bullet drop at the same distance plus 50 yd.
So, if I shoot known range at 450 yd, I can aim using the 500 yd graduation on my reticle and expect decent results. For shooting at 500 yd I go to the 550 yd graduation, for shooting 550 yd the 600 yd graduation, and for shooting 600 yd the 650 yd graduation. This makes me a happy camper knowing I can make full use of my optic to push the capabilities of my rifle. I am not shooting past 300 yd yet, but I hope to in the future.
One caveat before concluding: with the 14.5” barrel propelling the bullet at 2,750 fps at the muzzle, the bullet slows down to transonic speed (1,340 fps) at around 461 yd. This is the speed and distance where the bullet may start destabilizing because the center of pressure shifts forward as it approaches Mach 1 (aka, as it approaches the sound barrier or subsonic speed). Hopefully my barrel’s high twist ratio (1:7) and the boat-tail shape of the bullet can help it power through the transonic region a bit better until it hits the sound barrier (1116 fps) at 587 yd. Once subsonic, the bullet may start tumbling, losing accuracy (and, I would assume, penetration). I derive from this that my setup’s peak performance still resides within the 450 yd range (nice because I can use my reticle without mental gymnastics up to that range), but can still be hopeful almost out to the full 600 yd. By comparison, with our 20” barrel and 3,120 fps MV model the bullet goes transonic at 555 yd, then subsonic at 681 yd.
In conclusion, the 50/200 yd zero is most applicable to the original 20” barrel M16 platform pushing bullets at 3,120 fps. An 18” barrel likely works very well with the same zero, as the loss of muzzle velocity is not dramatic with the 2” barrel length reduction. But as you go down to 16,” 14.5,” 12.5,” or 10.5” barrels you have to do your homework. Acknowledge the loss in muzzle velocity and work out your own best battle zero and holdovers to achieve peak performance.
Ballistics are cool and I hope to learn as much as I can. If you find any error in my work please chime in! It will be to the benefit of everyone.
23:51 UTC
Is this a bullet ricochet mark?
Heard something really loud, could tell my car got hit. Happened right after passing an erratic driver on the highway. Reported the incident, cops looking into it but the guy that came out wasn’t sure. Other car’s front tire was about lined up with my back tire. Mark is on my passenger door. My car is taller than the other, the angle makes sense.
The circular mark is about 10-12 mm diameter, but I could measure it.
17:21 UTC
