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Discussion Starter · #1 ·
I have a chance to buy a case of 5.56 for a pretty good price, but is it 75 gr rounds. I have never shot anything over 62, would those rounds fire out of my hbar ok? Anybody use this grain before, what did they shoot like?
 

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They should be fine. Accuracy may even be better depending on the brand and bullet make.
 

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I'm assuming you have the standard 1:9 barrel.

If so, IN GENERAL, 75gr is pushing the envelope for reliable longer range accuracy, if you have one of the "classic" early run XCR's with a 1:8 barrels you are in a better position. Rule of thumb, YMMV

The only real way to tell if you're barrel is going to be happy with 75gr is to shoot 'em.
 
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I got a bunch of that new wolf steel case 75gr, and it shoots fine out of my 1-9 XCR. Not the most accurate, but it aint tumbling at 100 yet.

I think you are probably pushing it tho as the velocity on the wolf is a bit low to allow really good stabilization in a 1-9.

I use some HSM 75 gr that's runnin 2850 FPS and it does fine as far as I can tell.
 

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Discussion Starter · #5 ·
Thanks guys, it is the wolf that I was looking at and I might pass on it but I will look into the HSM though
 

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75 only sometimes works in 1:9. It may work at higher temps but if it is cold out it could fail. You just should not do it.
 

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What has temperature got to do with it? Accuracy and performance is about velocity and rifle twist vs bullet length.
 

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rsilvers, it's good to see you here. I believe you are XCRforum's first Industry Professional and Subject Matter Expert (SME) outside of Robarm

I look forward to reading the insights you have offer. Many here can learn a great deal from you, I'm sure
 

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rsilvers, it's good to see you here. I believe you are XCRforum's first Industry Professional and Subject Matter Expert (SME) outside of Robarm

I look forward to reading the insights you have offer. Many here can learn a great deal from you, I'm sure

Welcome, rsilvers SME.

Subject - AAC Marketing ;) :ninja: ;D










Go Gemtech!
 

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What has temperature got to do with it? Accuracy and performance is about velocity and rifle twist vs bullet length.
I am the author of an iPod app called BulletTwist. It computes a formula devised by Mr. Miller that is more advanced than Greenhill. Temperature effects air density. This feature is also in my other app, BulletFlight -- a ballistic computer I did for KAC.

The Hornady 75 grain AMAX is 1.070 inches long. A Sierra 77 is .985. A Barnes TSX 70 is 1.030.

Here is a scenario:

Hornady 75, 2400fps, 1:9 twist, 0 degrees F, 30.42 InHG air pressure - stability factor is just 0.90. NOGO.
Change temp to 90 degrees F, and 29.92 pressure, and stability is 1.1. That is marginal, but it would look normal to a shooter.
A shooter would post on an interweb forum "Works fine, you can shoot it." They would be giving bad advice because it won't work for someone after the temp drops.

I like to check stability at 0 degrees F and a higher than normal atmospheric pressure to determine worst-case.

Now back to 0 degrees F, 1:8 twist is 1.16 stability. Marginal. Not good enough.

1:7 twist, stability goes to 1.51. Excellent.

If you shoot AMAX 75 grain bullets at around 2400fps, you want a 1:7 twist, period. TAP 75 NATO ammo is
perhaps the single best load for defense so that is why I want 1:7.

If you shoot the Sierra 77, then 1:8 is fine If you shoot Sierra 69, you can do 1:9.
 

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Ok, that I can understand, but air density can be affected by more than just temperature. Does your application take into account humidity and it's affect on air density?

In real world terms what is the difference between the marginal values and the acceptable? Also what is the effect on grouping or POI? Given the varience in actual lengths of production bullets, what are the tolerances in these results?

I too would like to see a 1:7 twist barrel.

I am also interested in understanding the differences between 1:7.7 twist Kreiger tube and the general 1:7 and 1:8 tubes. My Kreiger barrel shoots the 77 SMK extremely well. Half MOA is easy for the first five or six thousand rounds until the barrel starts to wear. It would be interesting to note the differences between the 1:8 and 1:7 with respect to accuracy AND terminal balistics on impact as a result of the differeing levels of stability.
 

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My app only does temp and air pressure. There are diminishing returns to these values where the entire range of 0% humidity to 100% would not make a significant difference so there is no point in overloading the user.



A 7.7 twist sounds excellent.
 

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What has temperature got to do with it? Accuracy and performance is about velocity and rifle twist vs bullet length.
I am the author of an iPod app called BulletTwist. It computes a formula devised by Mr. Miller that is more advanced than Greenhill. Temperature effects air density. This feature is also in my other app, BulletFlight -- a ballistic computer I did for KAC.
I have the bulletflight app on my phone, I will have to take a look at the twist app now.
 
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