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175g uld rbbt testing at 800 yards

 
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  #22  
Old 01-17-2007, 11:39 PM
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Re: 175g uld rbbt testing at 800 yards

I can not add much to what has already been mentioned as far as ballistic information and such.

I would however like to make a comment or state my opinion on zeroing at 100 yards and then working off that zero for long range shooting.

Personally, I see to much error in this to be a real accurate way to set your long range rifle up. Its about the same thing as shooting groups at 100 yards to give you an idea of your load is accurate and consistant or not.

What I like to do is develope my load, get the velocity where I would like it if possible and then with the estimated BC I will be using I will develope a drop chart on the ballistic programs.

I will take that print out to the range and set up.

Most of you know I like to use the reticle reference points for hold over at long range in my big game rifles.

So what I will do is for example, and this is simply an example, say my drop chart says my 200 gr ULD out of my 7mm AM should be dead on at 600 yards using the second mil reference line down in the TMR reticle.

What I will do is set up a targer at this 600 yards and zero that second mil line dead on at this 600 yard range so that point on the drop chart meets as perfectly as possible to the actual bullet flight.

Once that is done, I will then set up and shoot groups at 300 and say 900 yards as well using the appropriate reference point that my drop chart tells me I should be using for those ranges.

For example, 300 yards may be dead on the main cross hair and 900 yards may be -4.5 mils down for the called for hold.

I will shoot groups at these ranges and record the data. On the first outings of developing a bullets BC more often then not even though you have the 600 yard zero dead on, the other two ranges will not be.

Say for example at 300 yards I was hitting +3/4 moa and at 900 yards I was grouping around 1 moa low.

I would then take this group impact position data and start tweaking my ballistic program using the BC mainly. In this case I would start to decrease my estimated BC until the projected bullet flight path came as close as I could get it to what I was seeing at 300, 600 and 900 yard impacts.

Then I would print out a new revised drop chart using the corrected BC and head back out to the range and test the three ranges again repeating the process until my ballistic model was matching my actual bullet flight out to the ranges I wanted to shoot out to.

Point is, a zero at longer range is much more accurate then a zero at 100 yards. Having a group of 1/2 moa at 100 yards could cover a pretty wide range of actual zeros at extended range.

I like to start out there a ways and work my way back in. A slight error in zero at 100 yards can often make a huge difference at long range. But, a long range zero will produce more accurate results at all ranges.

Just a thought.

Also, when I read your post the first thing I thought was I wondered what the actual travel in your scope was for each click. As has already been mentioned, do not rely on the advertised click value in your scope. They are seldom exactly what they claim to be and at times can be off a dramatic amount, especially at long range.

I am not saying your BC values are not close for your conditions. I have not used that specific bullet to be able to offer any ballistic information. I can however say that I have used the 169.5 gr ULD RBBT ALOT and the two bullets are very similiar in design. At my altitude and average pressure I use a BC in the .72 range for my 270 AMs to get the ballistic model to match up with actual bullet flight.

That said, we are alot higher then sea level and the air is drier then a popcorn fart most of the year which makes a big difference as far as how easily the bullet will slip through the air.

Good Shooting!!

Kirby Allen(50)
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  #23  
Old 01-19-2007, 10:29 AM
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Join Date: Jun 2003
Location: Hermiston, Oregon
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Re: 175g uld rbbt testing at 800 yards

Well guys, I think I've found the problem as to why the BC wasn't what I thought. I totally forgot that when talking to Richard, I specified that I wanted to ensure my bullet would open up at long range so therefor he opened up the point and meplat was a tad different. I'm sorry for all the fuss..just reporting what I had found. I knew there had to be a better explanation. Thanks for helping bring that to my attention. Thanks to everyone for the help and suggestions as well.
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  #24  
Old 01-19-2007, 04:11 PM
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Re: 175g uld rbbt testing at 800 yards

thanks for posting what you found out rem.
good deal, glad you figured it out.
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