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Muzzle velocity variation

 
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  #1  
Old 05-02-2004, 10:34 AM
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Join Date: Apr 2004
Location: Maricopa, Arizona
Posts: 70
Muzzle velocity variation

I have developed a couple of loads for my .308 that produce awesome groups at 100 yards. I havenít had time to test them at longer ranges yet, but I did collect a ton of chronograph data from each load. When I calculate bullet drop at 1000 yards it becomes very apparent that a 60 fps variation in muzzle velocity can easily mean the difference between a kill and a miss at great distances. The question I have is what amount of variation do we expect to have to live with?

My 168gr SMK load has a std. deviation of 9 fps. My 190gr SMK load has a std. deviation of 16 fps. Does anybody here know what muzzle velocity variation the 1000 yard pro shooters get?

Iíve also read some debate about match primers and their effect on variation. One article said that the Fed. Match primers were on the bottom of the match primer scale, and said that RWS primers were the best. I would love to know for sure, but I really donít want to burn up a barrel with a thousand dollars worth of ammo to prove which primers are the best. Has anybody seen any definitive data to support any theories?
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  #2  
Old 05-02-2004, 01:56 PM
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Join Date: Jun 2001
Location: Palmer, Alaska
Posts: 2,539
Re: Muzzle velocity variation

I use the 210M and 215M for most all my loads, sometimes the BR-2. It's the last thing I play with, right or wrong.

I also shoot for a 10 fps ES, but realistically and if I can only get 15-20 fps in the big cases I'm not burning ammo to tighten it by 5-10 any longer. If the groups are suffering in the least, I think it's more important to focus on the charge weight, powder choice, or shooting technique to keep the vertical at LR minimized.

This load here, I've got a few more groups to shoot with it to see what it'll produce, then I'll try the 215M as well. 20 rounds gives a guy a good idea if he'd headed the right dirrection or the wrong, in groups and ES at the same time... gotta have both working.

I think it takes far less than 60 fps to miss, although 60 fps can do it by itself. If errors stack up from changes in air density, MV variations from temp changes, BC variations, ballistic program errors etc., you'll want the ES to be low as you can possibly get it for LR accuracy.


Here's a load I just tested in the 30-338 Lapua Imp a couple of days ago.

210 JLK
88.0 gr RL-25
GM-210M
3.823" OAL (3.117" ogive w/ Stoney Point tool)
.005" into lands


Data from the Oehler 35P on the 100 yard groups.
Data from the screens with 2' spacing on left, 4' is on right.

3154 - 01 - 3144
3167 - 02 - 3151
3164 - 03 - 3156
3165 - 04 - 3158
3168 - 05 - 3149
3160 - 06 - 3152
3154 - 07 - 3153
3169 - 08 - 3168
3160 - 09 - 3158
3162 - 10 - 3159
3164 - 11 - 3163
3153 - 12 - 3151
3167 - 13 - 3163
3174 - 14 - 3168
3159 - 15 - 3157
3169 - 16 - 3169
3149 - 17 - 3149
3155 - 18 - 3155

3157 AVE - 10' from muzzle

ES is 20 - if you throw out the first fouler shot.
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  #3  
Old 05-02-2004, 02:28 PM
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Join Date: Apr 2004
Location: Maricopa, Arizona
Posts: 70
Re: Muzzle velocity variation

Not bad! There is definitely an advantage to using heavier bullets for long range work. From a rough calculation, that load should produce only a 4Ē variation at 1000 yards (everything else held consistent).

I agree that there are more factors to cause a miss than just the ammo. My weakness is in reading the mirage Ė but Iím getting better. I just figure that when you stack all the other variables together on top of my lousy shooting I really miss big. I intend to be able to say with pride that Iíve built as perfect a rifle/ammo combo as there ever was, and that I missed because I need to improve me.
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Old 05-02-2004, 03:19 PM
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Join Date: Jun 2001
Location: Palmer, Alaska
Posts: 2,539
Re: Muzzle velocity variation

I hear ya. Maybe I'll get to see if that load will hold 4" at 1000 today. Wind kicks me in the ***** more than anything. Mirage is something I'm learning more and more to read. It doesn't seem to be to awful bad up here compared to what many guys report in other areas of the country, mostly enough to just see what the wind is doing downrange.

Getting already to fire from 700 yards the other night, late when wind had calmed down to 1-2 mph at most I figured, I was now getting ate by the bugs and in a hurry to shoot the last ones with no windage dialed when I looked through the tube and saw wind flowing from 10-11 o'clock from 200-700 yards. I shot with zero dialed anyway just to see how much it related to at 700 anyway, knew I wouldn't be off by much. I was 2" and 3" off with the two shots, with a 1/2" vertical between them. The part I hate most is not being able to identify each shot's placement on the target from my shooting location. I think I'll work on getting four 12" wide by 20" high, 3/8" thick AR400 steel plates to hang side by side touching on my 1/2" pipe frame, for a 20x48 inch impact area. That would set me up for a long time to come. Easy to see impacts when painted white, as the paint usually comes off in a 1 - 1.5 inch circle.
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  #5  
Old 05-02-2004, 05:18 PM
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Join Date: May 2001
Posts: 5,750
Re: Muzzle velocity variation

Brent

I've been using a 24" x 48" steel plate for some time now. It really works well for giving you feedback as to which shot goes where. I can easily see the splats at 1,000 yards, while prone, through my rifle scope.

I now have three, 5 inch spray-painted squares located on the plate. (Only 2 used for this shooting session) The squares have sides that are about 1/2 inch thick and the squares are hollow (tan background color) in the middle. I leave a can of black and one of tan spray paint down at my target. Also a template that enables me to re-paint the 5 inch squares after I splat them.

Last week I had one of my employees out to do some work at my cabin. He's a nice young man and a hunter. I thought I ought to show off my new "Chris Matthews" rifle to him and show him what a good shot I am. [img]images/icons/smile.gif[/img]

So I got down prone and fired at this target at 888 yards. Well, my three shots were pretty good and I was feeling pretty smug! So I got him down into a prone position and explained to him how to hold the sand sock, how to dry-fire a few times for practice, etc.

Then he took two shots and I made him quit before his third shot! He was about to show me up. [img]images/icons/frown.gif[/img] Can't have that! This is Lance with his group on the left. You can see why I made him stop.

And as you can also see, Chris Matthews knows his gunsmithing stuff. More about this new rifle later.

[ 05-02-2004: Message edited by: Len Backus ]
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  #6  
Old 05-02-2004, 08:34 PM
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Join Date: Sep 2002
Location: Illinois
Posts: 836
Re: Muzzle velocity variation

Len,

HA! Usually works like that, don't it. I have the same problem, usually with everything. Mighty fine shoot'in on both your parts.

db
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