Posted: 10/17/2016 7:39:03 AM EDT
|
So I built a 450 bushmaster ar for a hunt this year. I've been trying to figure the ballistics out for the rifle at different temps. I live in S FL so I sighted in at 90F at 100 yds. I'm using the Strelok+ app to calculate drop at 300 based off of 2250 fps at 5ft from the muzzle .21 B.C. and 250gr bullet.
At 90F that gives me 28 inches of drop at 300yds. Which I can confirm by shooting down here. But when I put in the temp in the app to 20F my drop at 300 goes to 39in? Obviously I cannot replicate those conditions in SFL. But where my hunt is it will be that cold. Would that difference in air density really make the round drop that much further? |
|
Quoted:
Temperature has 2 effects: a) it changes the viscosity of air b) it changes the MV by heating the cartridge. The effect of b is bigger than the effect of a. So you have to have a chart of MV versus temp in order to compensate for temperature. Interesting. I wouldn't have thought b>a. Do you have any recommended studies on cartridge temps and MV? |
|
Quoted:
Interesting. I wouldn't have thought b>a. Do you have any recommended studies on cartridge temps and MV? Quoted:
Quoted:
Temperature has 2 effects: a) it changes the viscosity of air b) it changes the MV by heating the cartridge. The effect of b is bigger than the effect of a. So you have to have a chart of MV versus temp in order to compensate for temperature. Interesting. I wouldn't have thought b>a. Do you have any recommended studies on cartridge temps and MV? It is around 80-84 in the Sniper 101 video series. |
|
And that's where Rex introduces me. If you want the simple answer to your question then go to http://ballisticxlr.com and download BallisticXLR. Fill in your inputs (email me at ballisticxlr at gmail dawt com if you need assistance) and you'll get a nice DOPE chart with temp differences. Drop difference per 10deg F can vary pretty radically from one load spec to another due to BC and velocity differences. If you need custom charts I do offer a printing service. Check out my products tab. NOTE: BallisticXLR is not a business. I lose money on every paid deal. It's a service I provide for fellow shooters. |
|
Quoted:
Temperature has 2 effects: a) it changes the viscosity of air b) it changes the MV by heating the cartridge. The effect of b is bigger than the effect of a. So you have to have a chart of MV versus temp in order to compensate for temperature. B is definitely the bigger effect. Not to sharpshoot, but there's one more effect of temp: it changes the speed of sound as well, leading to slightly different Mach numbers over the course of the trajectory. |
|
I'm currently programming an iOS ballistic calculator, because, you know, there aren't enough already. (PM me with email if you want to get in on the Beta test). I used my algorithm to calculate some drops y'all might find interesting. I calculate the bullet drops given temperatures of 0 and 35 celsius (32 and 95 F or so), and a 200fps muzzle velocity difference. It's assuming 1000m, 77grain, 1000m muzzle velocity. (it's just a common bullet to test things, not necessarily realistic) Here's what I came up with:
200fps MV difference, all else equal: 
Less dense air, due to Temp diff, all else equal: 
Change in speed of sound, note that it benefits the cold day, all else equal: 
All factors combined: 
|
|
Quoted:
I just figured out that the temp factor was turned up in the app. I'm using lil'gun powder and I know its temp sensitive. Time to put a few rounds in the freezer and Chrono them. I would recommend THIS, get a large yeti cooler, couple bricks dry ice, place all your gear in there and go to the range, set up target, set up chrono, ect ect, put on gloves, pull out, assemble rifle and shoot 1 round, place back in cooler. see about changes to POI. dry ice is what -100F? that ought to get you close, and then divide the difference in half, and you ought to be DARN close to the half way point between 95F and -100F which would be 0 degrees. |
|
Quoted:
So I built a 450 bushmaster ar for a hunt this year. I've been trying to figure the ballistics out for the rifle at different temps. I live in S FL so I sighted in at 90F at 100 yds. I'm using the Strelok+ app to calculate drop at 300 based off of 2250 fps at 5ft from the muzzle .21 B.C. and 250gr bullet. At 90F that gives me 28 inches of drop at 300yds. Which I can confirm by shooting down here. But when I put in the temp in the app to 20F my drop at 300 goes to 39in? Obviously I cannot replicate those conditions in SFL. But where my hunt is it will be that cold. Would that difference in air density really make the round drop that much further? |
|
Chronographing the load in your particular rifle is the way to go.
When I lived out west, I took advantage of cold winter days and shot in 0 degree F weather over a chronograph to get a low temperature point for my preferred loads. in South Dakota 100 degree F summer temps were also not uncommon and were useful in getting another data point at 100 degrees F, along with testing in more normal 40 to 70 degree temps. What I found was a linear function with .88 fps change in velocity per degree temperature change when using IMR 4064 and a 168 gr SMK. Given that it tends to be a more or less linear function, extreme efforts are probably not needed. If you can shoot on a cold winter Florida morning at 40 degrees and a hot summer Florida afternoon at 90 degrees, the 50 degree spread will be good enough to extrapolate to colder or warmer temperatures. Take care however to use a large enough sample to get valid results. 10 rounds is an absolute minimum, and 30 rounds is much better, particularly if your Standard deviation in velocity is a little high (over 20 fps or so). I try to keep the SD in my long range loads to 15 fps or less. ----- Practically speaking my IMR-4064 load's velocity change of .88 fps per degree F is close enough to 1 that I just ball park it to an increase or decrease of 1 fps per degree F above or below 70 degrees F. I also generally use a Whiz Wheel rather than a phone app, so I'll set the density altitude and let it handle the atmospherics, while I extrapolate any temperature induced effects on the load when reading the elevation and windage corrections using the +/- 50 fps velocity scales. After all, it's possible to get a little too precise, and when shooting in real world conditions where wind and ranges are just estimates, it's more or less like measuring with a micrometer, marking with chalk and cutting with an axe. For example the velocity used to generate the data on the wheel is based on a velocity of 2665 fps at 70 degrees F. If it's only 30 degrees F, I'll get a velocity closer to 2625 fps. On a standard day (70 degrees, 29.92 in hg) I'll need 15.75 MOA of elevation at 600 yards. However when I adjust the density altitude for 30 degrees, the elevation increases to 16.5 MOA based on the normal 2665 fps velocity, while the - 50 fps velocity calls for just over 17.25 MOA. Consequently, I'll put 17.25 MOA on the scope rather than 15.75 or 16.5. |
|
Quoted:
Temperature has 2 effects: a) it changes the viscosity of air b) it changes the MV by heating the cartridge. The effect of b is bigger than the effect of a. So you have to have a chart of MV versus temp in order to compensate for temperature. I've seen good correlation using JBM ballistics software. Available free online. |