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Best load data for 1:9 twist 16" barrel?


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#1 OFFLINE   knot28

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Posted Sep. 26 2009 - 09:06 PM

Hello all, I just joined your site but have guest visited many times in the past, I have some questions and hope you can help. My new AR15 (S&W M&P15T) is a blast but it needs better ammo, I have less than 100 rounds through it but realize the PPU 55gr fmj is not very accurate, it shoots 6" higher from 100 yds to 200yds.

The shooting times article I read was excellent at explaining why this is happening (thanks to whoever posted that link) but now I hope someone can get me good load data for when I reload.

I'm thinking to use Hornady 68gr BTHP Match or 75gr BTHP Match bullets, any thoughts on which will perform better out to 200 yds and 300 yds?
What powder and what grains work well?
I've noticed hollow point is more accurate than ballistic tip but can't understand why, it seems it should be the other way around.


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#2 OFFLINE   TigerStripe

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Posted Sep. 26 2009 - 09:16 PM

:welcome: to the Armory! Most of the time a 1:9 barrel won't stablize a 75gr bullet, but every once in a while one will. Beyond that, the load doesn't mean much to me as I haven't gotten into reloading yet. I'm sure you can find a lot of info in the Reloading Armory.


TS




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#3 OFFLINE   Proudtexan

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Posted Sep. 27 2009 - 07:00 AM

:welcome: to the armory and the disease!


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#4 OFFLINE   +Zeke+

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Posted Sep. 28 2009 - 02:08 AM

The 1/9 16" barrel was designed by the military to accommodate the 62gr M855 round when it was introduced as a replacement for the M193. The older guns used a 1/12 twist.

Since that barrel was the result of trying to optimize for that 62gr round that would be where you most likely will find the best load results.


#5 OFFLINE   knot28

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Posted Sep. 28 2009 - 03:33 PM

The 1/9 16" barrel was designed by the military to accommodate the 62gr M855 round when it was introduced as a replacement for the M193. The older guns used a 1/12 twist.

Since that barrel was the result of trying to optimize for that 62gr round that would be where you most likely will find the best load results.



Thanks, I'm hoping it will shoot the 68 or 69gr as well. Maybe I'm asking too much out of this gun but have always liked the heavier grain bullets because of the retained energy.


#6 ONLINE   jchtrh

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Posted Sep. 28 2009 - 03:45 PM

You got some great answers already. The 1:9 twist will do nicely with 55 gr to 69gr. It will take some experimenting to find out which one your gun shoots best, but that just gives you an excuse to go shooting.


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#7 OFFLINE   TigerStripe

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Posted Sep. 28 2009 - 04:06 PM

The lighter TAP rounds should shoot well out of a 1:9 barrel. You should have several 55 grain factory loads that will shoot well and a few in the 62 grain weight and one or two up to 69 grains.

What some people call "rack grade" barrels will have a deviation from a true 1:9 twist. Some will be closer to a 1:8 and possibly stabilize the heavier grains of 70 to 77. Others will be closer to 1:10 and shoot lighter bullet weights better. It takes some searching, and as JC said, shooting...



TS


#8 OFFLINE   TomJefferson

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Posted Sep. 28 2009 - 04:56 PM

Ok I'll try to explain this.

All a twist does is get the bullet spinning. Each bullet has a band or sweet spot from here to there on spin. How fast a bullet spins in a given barrel depends on how fast it goes down the barrel. The .223/5.56mm has a very small case which limits how much pressure is in there. It takes more pressure to push a heavy bullet than a light one so heavier bullets in .223/5.56mm are going to be naturally slower. The shorter a barrel is then the slower the bullet will come out.

That's why the AR has so many different twist rates. Faster the twist rate, like a 1:7, then the faster the bullet will spin for a given (slower) speed.

The inverse is also true. Too fast a spin rate, too fast the bullet, too short the barrel, then a bullet will skirt too much of the rifling or spin too fast making it yaw to one side direction.


The 55 is like the middle of the road in bullet weights which works well in most barrel lengths and twist rates.

Now if you follow this then a 69 grain HPBT loaded for a 16" 1:9 will naturally be at the higher end of the load data than the lower end because you need to speed it up to make up for the slightly slower twist and shorter barrel. Load data is typically the longest barrel lengths.

When you consider a 50 fps delta for every inch of barrel length then compare the load data for a 75 grain bullet, then you can see it will be awfully hard to speed it up enough to stabilize it in a shorter barrel with a slower twist. We only have so much pressure to work with.

Obviously your best bet is a 5.56mm case which can take 10,000 psi more pressure than a .223 and using a very slow rod, like Varget, that because it burns more down the barrel than a fast powder gets more muzzle velocity than a fast powder per PSI.

Its pretty close but not there. I've stabilized the 75 in a 20" 1:10 but not the 16" in 1:9, but then I'm not about to over pressure my case either.

Tj


#9 OFFLINE   rvrrat14

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Posted Oct. 04 2009 - 12:03 PM

good info, mine is 1"9 and all i've been shooting is 55gr. FMJ. haventy had a bunch of trigger time on it yet to tell what it likes or how well it will shoot.....


#10 OFFLINE   bfausett84

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Posted Oct. 11 2009 - 09:36 PM

The lighter TAP rounds should shoot well out of a 1:9 barrel. You should have several 55 grain factory loads that will shoot well and a few in the 62 grain weight and one or two up to 69 grains.

What some people call "rack grade" barrels will have a deviation from a true 1:9 twist. Some will be closer to a 1:8 and possibly stabilize the heavier grains of 70 to 77. Others will be closer to 1:10 and shoot lighter bullet weights better. It takes some searching, and as JC said, shooting...



TS


On that note, is there a way to check ur twist rate? one that's fairly accurate?


#11 OFFLINE   TigerStripe

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Posted Oct. 11 2009 - 09:49 PM

The twist rate is typically stamped on the barrel either on top of the barrel in front of the front sight base or beneath the handguards. If it's not there, you can follow KennyOhioHunter's method described here.


TS







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