muzzle velocity question

[JMP]

I just bought a new tikka t3 lite in .300 WSM that I plan to take bivy hunting on the south island in new zealand, and then it'll be my backpacking rifle when I'm back here for good in a few years. I'm going to sight it in this weekend at 100 yards. I don't have access to a chronograph. I have a tactical scope on it. Right now I'll be shooting 180grn. factory Winchester Ballistic Silvertips for economical reasons mostly, and the published factory specs didn't seem to perform much worse than some of the more expensive rounds.

 

My question is this... can I sight the rifle in at 100 yards, and then shoot it at 200 yards, and again at 300 yards without adjusting my scope settings at all, and use the difference in impact points to calculate the muzzle velocity for use in the nightforce or other ballistic software programs?

 

Eventually I'm hoping to punch these rounds out to 500 or 600 yards at animals in the southern alps.

 

This is how I calculated the speed of my bow for use in archers advantage to print sight tapes, I imagine it would work, but don't know positively.

 

Thanks in advance.

 

Jay

[308Nut]

The short answer is no. While it can in some cases work there is more to making a 600 yard shot than knowing the velocity. Youre on the right track but you need to be responsible here. Published BC's are rarely spot on. Even if you know the velocity yet not the true BC then software is pointless. Once you have ALL the factors figured out, THEN and only then will your software help. Another issue is that there are still many other factors such as whether or not the scope is A: level with the rifle and B: the rifle is level when fired. This is just a couple of considerations when contemplating a 600 yard shot on game. The best policy is that you never shoot at game at ranges you have NEVER fired in the real world.

 

If all youre going to shoot is 500-600 yards then one thing you can do is sight in where you want your zero and test it at longer ranges in 100 yard increments and document the adjustments made to hit the mark at those ranges. Make a chart, follow it and be happy.

 

If you want to shoot farther than that then you need to use a chrono, find your true BC, factor in air density using the right tools and use the right software and shoot often.

[JMP]

Thanks 308Nut. Awesome goat hunt by the way. I really enjoyed reading about it, and reckon goat hunters are some of the toughest hunters out there. I hope to do that one day.

 

I'll probably just go about sighting my rifle to 600 yards as you suggest.

 

Just a question about chronos though, how do they account for differences in calibration. It's rare that you'll find two chronos that will read an arrow speed the same. are there similar differences with reading bullet speeds in different chronographs? and if so, how does it affect finding the true BC?

 

thanks again.

[pwrguy]

If all youre going to shoot is 500-600 yards then one thing you can do is sight in where you want your zero and test it at longer ranges in 100 yard increments and document the adjustments made to hit the mark at those ranges. Make a chart, follow it and be happy.

 

 

This is all I did for years, I didn't know my actuall velocity until a couple of years ago when I bought a chronograph. Worked great for me, at least out to 500 yards.

[308Nut]

Thanks 308Nut. Awesome goat hunt by the way. I really enjoyed reading about it, and reckon goat hunters are some of the toughest hunters out there. I hope to do that one day.

 

I'll probably just go about sighting my rifle to 600 yards as you suggest.

 

Just a question about chronos though, how do they account for differences in calibration. It's rare that you'll find two chronos that will read an arrow speed the same. are there similar differences with reading bullet speeds in different chronographs? and if so, how does it affect finding the true BC?

 

thanks again.

 

It is rare to find two chronos the same. I have 2 from the same manufacturer and the are 32 FPS apart (at least at the velocities I am working with). I dont know which one is the closest to reality but I use the one I feel is the better as a baseline. Then I fire over one set at 15' and at the same time at 915'. I deduct 32 FPS from the 2nd one, run the results through software and establish the BC. I did that very thing twice this year and made 1st round hits in the X ring at 700 yards with 2 bullets I had never fired prior to the tests. Neither were at their published BC's. If youre chrono is not 100% accurate, your percived BC will be slightly different than the true BC however it will be very close and they will both be close enough that one will absorb the other. For example, if your chrono reads high when the bullet is really slower then you will see a percieved BC lower than the real BC. You think the bullet is dropping faster than it should because the drops tell you one thing yet your chrono tells you another, you adjust your BC to match your calculator and even though the numbers inputed are not perfect they will output scary close results. The only way to test a BC and get it near 100% accurate is doppler radar. This is highly expensive and impractical. Finding an army base that will let you use theirs is impossible.

 

Another technique we use is the drop test. Know the velocity, fire at the top of a very tall target and measure the real drop (NOT ADJUSTED WITH THE SCOPE!!) and do the math that way. In any case, when youre testing the BC, you MUST factor in the temprature, barometric pressure AT LOCATION and humidity. Do not pay any attention to the altidue as youre using raw pressure at location. You would input 0' in any software for the calculations.

 

Hope that helps.