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Posted: 5/17/2006 8:25:36 AM EDT
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Here's my setup: A 20" 1/9 and a 20" 1/7, both bushmaster What muzzle velocities can I expect with the following rounds: 75 and 77 gr Black Hills bluebox, Hornady 75 gr. TAP (.223 pressure), and Hornady 75 gr. TAP (5.56 pressure). Also, are the published BC's correct? .35-.390 seems a little high... I'm in the process of selecting the ultimate deer hunting load  , and since I already know the terminal ballistics, I'm starting to compile info for my trajectory calculations. I've got a TA31RCO-A4 mounted, and I'm trying to match the reticle as closely as possible.Thanks for any input you may have to offer. 
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Just got off the phone w/ Black Hills. They said I could expect 2625 fps from a 20" bbl. The published mv is 2750 fps (like on the chart). Are the values on the chart confirmed w/ a chrony? Regarding the BC's, it seems interesting that 73 gr. Bergers are .331, 77 gr. Sierra's are .362, and Hornady claims their 75's are .395. Hornady's figures seem a little off, that's all. |
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I can't answer your questions on the 77gr, but I expect it to be similar to the 75gr. Hornady TAP 75gr #80265 (.223) and BH Red/Blue Box 75gr should run about 2700fps from a 20" barrel. Hornady TAP 75gr #8126N (5.56mm) should run about 2825fps. Of course velocities can vary widely depending on the barrel (chamber dimensions, how used it is, etc), lot-to-lot variances in the ammunition and temperature. IME the Hornady 75gr's BC is more like ~.340 and that's about what a friend of mine got when he punched the dimensions of the projectile into his BC calculator. |
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Thanx for all the input, guys. I wish I had a chrony so I could test the loads myself and post the results. Maybe we should start a tacked thread and compile the actual ballistic info (mv's and BC's done with a chrony and calculator) of various cartridges, particularly the heavy loads. That would prove very interesting to long(er) range shooters. Anyways, thanks for all the help and keep it comin'!
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 Red box: 75 grain Hornday TAP Black box: 75 grain Hornday TAP FPD (for personal defense) The following chronograph data was obtained using an Oehler 35P chronograph, with muzzle velocities calculated from instrumental velocities using Oehler’s Ballistic Explorer software. The weapons used in testing: - Colt 20” A2 government profile barrel with 1:7” twist - Colt 16.1” light weight barrel with 1:7” twist for the 75 grain red box data - Colt 16.1” M4 barrel with 1:9” twist for the 75 grain black box data All strings of fire consisted of 10 rounds. Red box MV 20" barrel 2672 fps SD=20 fps 77 degrees F 16.1” barrel 2551 fps SD=16 83 degrees F |
FWIW, in 2003 Hornady changed their 75gr BTHP. The previous Hornady 75gr BTHP 's BC was more like ~.380 according to the same calculator that gave ~.340 for the current Hornady 75gr BTHP. Hornady claimed the old 75gr BTHP had the same .395 BC too. The boat tail was less angled and longer in length. The projectile itself was longer than all the 77gr projectiles of the time. How our outcome for BC changed and theirs didn't I don't know. |
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