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Posted: 3/6/2008 6:16:55 PM EDT
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This question is for anyone, but Molon seems to be the TAP expert here. Why is the muzzle velocity on the 8126N 5.56 75 grain TAp listed as 2665 on the box when the 9760EL .223 75 grain practice ammo listed as 2750 MV? What gives? Hornady is insane about specifying that 5.56 nver be used in a .223 weapon, so how can the get 2750 out of a .223 load. I would be thrilled if the POI was very close of the same between the two loads. Thanks, Andy |
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It's simply a matter of the barrel length and the type of barrel used. For example, a 24" stainless steel barrel with a match chamber versus a chrome-lined 14.5" barrel with a NATO chamber. Also, the amount of erosion present in each barrel used for testing will influence the numbers to some degree. Velocities will also vary slightly from lot to lot of the ammunition, so a fast lot in a tight chamber (with little erosion) versus a slower lot in a NATO chamber with thousands of rounds through it will give you different numbers. Then of course, there are those that will tell you that the rate of twist in the barrel will influence the velocity and I should also mention the varying atmospheric conditions during testing will have an effect too. Mostly, it's about the length of the barrel, mostly.  
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Reading Molon's post I'm not sure if he answered as simply as he could have, (he's really intelligent and explaining in layman's terms is difficult for him  ). I'm not questioning his answer either just putting it another way FWIW.IIRC, the velocity listed on the label of 80265 (.223 pressure red-box 75 grain TAP) doesn't specify a barrel length but in the bound book from Hornady is a 16" or 20" barrel. 8126N specifies on the label that velocity listed is from a 14.5" barrel. Essentially they aren't comparing apples to apples. ETA: Regarding the POI, in my experience 8126N is practically POA/POI at 100 yards and the 9760 will be about 1-2" above POA at that range. My three ARs (20", 16", and 11.5") are all 1/7 if that matters. There are likely small variations given the different barrel lengths hence the "about" in that statement. ETA2: According to Hornady's TAP Test Report and Application Guide, muzzle velocities listed for 8126N and 80265 are 2710 and 2565 respectively from a 14.5" 1/9 Bushmaster. Scott |
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I'd be sure to check the velocity in YOUR barrel rather than take the word of others. I have 2 16" Colt 1/9" guns. #2 is a duty gun with right at 3000 rounds and still gets good velocity. #1 is a game gun for 3-GUN and gets considerably less velocity and this it's second barrel also. There is nearly a 150 fps difference in the two guns.... #1 still shoots MOA with our 55 grain TRU Ballistic tips but doesn't have the velocity... A27257 |
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