IMR-4759 temperature/velocity?
Dec 11, 2010 11:04:03 GMT -5
Post by esshup on Dec 11, 2010 11:04:03 GMT -5
Does IMR-4759 slow down when it's colder out? I don't have time today to break out the chronograph, and the weather is supposed to turn for the worse for the next 4 days.
Reason for asking:
Stainless ML-II, accurigger, .50 cal. I did a OCW test this past October in 65°F temps and determined that 43.0g of IMR-4759 behind a wonderwad/MMP short sabot/250g Shockwave ignited by a CCI 209M primer was the most accurate, but the velocity was only 2175 fps. (Oehler Model 35 w/proof screen). 2 different 3 shot groups both were <1/2 MOA at 100 Yds, so I shot it at 200 to verify the drop and found that when on 10x the bullet would impact right at the top of the lower duplex post. Easy aiming reference, right?
Now I'm hunting in 30°F to 5°F temps and I'm seeing a lower bullet impact at both 100 and 200 Yds. At 100 Yds, the bullet hits about 2" lower and at 200 Yds, the bullet hits 6" lower than it did in October using the same aiming point. I'm shooting off of a lead sled type rest for accuracy tests.
The other thing is that I saw an 8" bullet drop at 105 Yds from previous point of impact for one shot, when I shot a doe. While it could have been me (shooting off of safari type shooting sticks at a deer), I'm usually pretty good. I did get the deer, but it was pure luck. At 105 yds I aimed for a high shoulder shot and the bullet struck the deer in both front legs, passing thru the lower part of the brisket, but not into the body cavity. She snowplowed/ran about 80 yds, and laid down. It was past legal shooting light when I followed up on the shot, and she was bedded down, but her head was still up. I left her overnight and she died in that bed. Both front legs were broke, and I think she bled out from having both veins in the front legs cut.
That is what caused me to check the gun at 100 and again at 200 the next day. It was shooting 2" low at 100 and 6" lower at 200. At 200, if I turned the scope to 6x, the bullet would hit right at the top of the lower duplex post again - good aiming point.
The load that I shot the doe with was in the gun for about a week, and the sabots are 4 years old. Same can of powder that I tested with in October.
Fast forward to the next day. I shoot another deer at 205 Yds, using the top of the duplex post, this time with the scope set at 6x, and the bullet hits right where I wanted. The shot was a slight quartering to shot, bullet hit a tad far back (too much allowance for wind), still clipping the near side lung, BUT it never exited the deer. The deer reared up on it's hind legs at the shot, and walked away in to the woods and laid down. I could see that it was breathing thru it's mouth. It laid it's head down and I decided to give it 10 minutes before following up. I ranged the deer and reloaded. At the 9 minute mark the deer got up and started walking away. I quickly estimated the drop and sent another one. The deer took 4-5 more steps and disappeared behind a tree. 20 minutes later I walked up on it, and it was dead. The last shot was at 265 Yds, and the bullet entered the right ham, clipping the hip and exiting the deer right in front of the hindquarters thru the lower abdominal wall.
Fired sabots look normal.
I don't know why the bullets are hitting lower than before, my guess is slower velocity, but until I can get the chrono out I won't know for sure. Is IMR-4759 known for going slower in colder weather or should I be looking at something else? Leupy VX-III 3.5-10 x 50 scope.
Reason for asking:
Stainless ML-II, accurigger, .50 cal. I did a OCW test this past October in 65°F temps and determined that 43.0g of IMR-4759 behind a wonderwad/MMP short sabot/250g Shockwave ignited by a CCI 209M primer was the most accurate, but the velocity was only 2175 fps. (Oehler Model 35 w/proof screen). 2 different 3 shot groups both were <1/2 MOA at 100 Yds, so I shot it at 200 to verify the drop and found that when on 10x the bullet would impact right at the top of the lower duplex post. Easy aiming reference, right?
Now I'm hunting in 30°F to 5°F temps and I'm seeing a lower bullet impact at both 100 and 200 Yds. At 100 Yds, the bullet hits about 2" lower and at 200 Yds, the bullet hits 6" lower than it did in October using the same aiming point. I'm shooting off of a lead sled type rest for accuracy tests.
The other thing is that I saw an 8" bullet drop at 105 Yds from previous point of impact for one shot, when I shot a doe. While it could have been me (shooting off of safari type shooting sticks at a deer), I'm usually pretty good. I did get the deer, but it was pure luck. At 105 yds I aimed for a high shoulder shot and the bullet struck the deer in both front legs, passing thru the lower part of the brisket, but not into the body cavity. She snowplowed/ran about 80 yds, and laid down. It was past legal shooting light when I followed up on the shot, and she was bedded down, but her head was still up. I left her overnight and she died in that bed. Both front legs were broke, and I think she bled out from having both veins in the front legs cut.
That is what caused me to check the gun at 100 and again at 200 the next day. It was shooting 2" low at 100 and 6" lower at 200. At 200, if I turned the scope to 6x, the bullet would hit right at the top of the lower duplex post again - good aiming point.
The load that I shot the doe with was in the gun for about a week, and the sabots are 4 years old. Same can of powder that I tested with in October.
Fast forward to the next day. I shoot another deer at 205 Yds, using the top of the duplex post, this time with the scope set at 6x, and the bullet hits right where I wanted. The shot was a slight quartering to shot, bullet hit a tad far back (too much allowance for wind), still clipping the near side lung, BUT it never exited the deer. The deer reared up on it's hind legs at the shot, and walked away in to the woods and laid down. I could see that it was breathing thru it's mouth. It laid it's head down and I decided to give it 10 minutes before following up. I ranged the deer and reloaded. At the 9 minute mark the deer got up and started walking away. I quickly estimated the drop and sent another one. The deer took 4-5 more steps and disappeared behind a tree. 20 minutes later I walked up on it, and it was dead. The last shot was at 265 Yds, and the bullet entered the right ham, clipping the hip and exiting the deer right in front of the hindquarters thru the lower abdominal wall.
Fired sabots look normal.
I don't know why the bullets are hitting lower than before, my guess is slower velocity, but until I can get the chrono out I won't know for sure. Is IMR-4759 known for going slower in colder weather or should I be looking at something else? Leupy VX-III 3.5-10 x 50 scope.
