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Posted: 3/27/2005 3:49:24 PM EDT
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This is my 1st post. I hope it will be helpful. First, some background on myself. I have been reloading since highschool (1969), bought my first AR15 in 1973. I shoot NRA Highpower "F" class and IDPA. I have noticed that many of you put a lot of faith in factory ballistics and that your "whatever" barrel length rifle/carbine will have the same velocity as someone elses rifle/carbine or close to it. I think this is a dangerous assumption when it comes to a firearm/ammo combo for defense use. Why???? 6 DPMS M4 14.5" barrel carbines XM-193 ammo---pact chrono 10' from muzzle air temp 35 degrees--the average muzzle velocity for the 6 M4's was 2978 fps, the high rifle was 3010 fps, low was 2950 fps. Using a Sierra Infinity Five Ballistics calculator and a 2700 fps minimum for effective fragmantation, here are the following distances to the 2700 fps: mv 3010 fps 80 yd velocity is 2710 fps mv 2978 fps 70 yd velocity is 2716 fps mv 2950 fps 60 yd velocity is 2726 fps Next 2 16" bushmaster carbines XM-193 same lot (24), air temp 80 degrees--average velocity for 2/5shot groups 3120 fps. Air Temp 45 degrees--3050 fps average velocity for 2/5 shot groups. Range to 2700 fps: mv 3120 fps 110 yd velocity is 2714 fps mv 3050 fps 90 yd velocity is 2711 fps and finally my best example 14.5" Bushmaster M4, 40 degree air temp XM 193 lot 24. Average for 2/5shot groups 2984 fps, Q3131A average for 2/5shot groups 2822 fps, range to 2700 fps: XM193 mv 2984 fps 70 yd velocity is 2721 fps Q3131A mv 2822 fps 30 yd velocity is 2711 fps (lot ?, bought case from Midway) I guess the point of this post is if you don't chrono your defense loads in your rifle, you really don't know what your velocity/ammo performance is. I chrono 5% of rifle ammo when I buy 500 rounds or more and handgun and rifle ammo by the 50 round box I do 10%. Just some food for thought..... |
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This is true, and that's one of many reasons why fragmentation range can only be an estimate. Environmental conditions, altitude, lot-to-lot variations in the powder load, and many other factors affect the velocity you'll get out of your rifle, and there can be variations in the construction and performance of the bullets as well. The ammo Mods and myself always try to give realistic estimates, and acknowledge the fact that there are individual examples of rounds that perform outside of those estimates (both better and worse, depending on the situation). That's to be expected. -Troy |
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