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fmkenner
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Posted: 2/6/2013 3:42:03 PM
[Last Edit: 2/6/2013 3:42:48 PM by fmkenner]
I ordered a 300blk barrel with a 1/7 twist. They just imformed me they had no 1/7 twist but had 1/8 twist. Is there that much a difference? Its going to be a 300blk pistol with a 9.5" barrel, we cant have SBR, and plan to suppress it. Hoping to shot but supers and subs out of it. Should I look else where for the 1/7 twist or go with the 1/8 twist.
jfk
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Posted: 2/6/2013 3:54:19 PM
The 1/7 stabilizes 240 grain bullets better. If you stay with 220 subs, a 1/8 should be fine.
fmkenner
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Posted: 2/6/2013 4:24:21 PM
I understand the heavier bullet stabilize better with a fast twist, but does that apply equally to super-sonic and sub-sonic ammo? Its seems hard to find barrel right now and I'm wondering if its that big a deal or difference. I will be learning to re-load with this caliber too.
Krautling
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Posted: 2/6/2013 4:38:38 PM
[Last Edit: 2/6/2013 4:46:15 PM by Krautling]
I´m in the same boat as you.

I ordered a 1-8 and i´m going to learn to reload on it too....


We should be fine up to 220gr, but heavier bullet weights may or may not work in our particular Barrels. It depends on the particular Barrel and the particular load. Some 1-8 will stabilize heavier bullets then 220gr and some will not. You´ll just have to try it out in your particular Barrel and Play around with some loads.

I decided on a 1-8 because it was available for me and i figured 220gr was enough for me. IF 220gr +++ loads will run, that´s icing on the cake but 220gr is pretty fine already. IMHO

Regarding super and subsonic rounds: 220gr and up are AFAIK always subsonic... you´d probably exceed the max pressure by a wide margin trying to make such a heavy bullet supersonic out of a 300 BLK.



ETA: also, let´s Keep in mind that it´s not the actual weight of the projectile that determines wether a load will run or not, it´s actually the effective lenght of the bullet that touches the grooves and lands! The weight is more of a quick and dirty rule of thumb Thing because usually, lenght and weight will correspondend with each other especially in traditional commercially available projectiles. but it´s not always the case. For instance, a semi wadcutter projectile with a short effective lenght because of the SWC design and with a boattail that wieghs 240gr will probably run fine out of your 1-8 Twist where a different 240gr projectile will/might not....


it ain´t easy.....
fmkenner
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Posted: 2/6/2013 5:21:19 PM
I went ahead and gave the nod for the 1/8 twist. I got the Stainless Steel 9.5", 1/8 twist from Core 15. I can sit on the fence and wait, or get it built. Im looking to get AACs Cyclone and Samson Evo 10" rails and have it slightly under the rails. If it will fit. Thanks for the response everyone.
Fred
Rich_V
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Posted: 2/6/2013 7:33:03 PM
A 1 in 8 barrel will fully stabilize a Sierra 240 g HPBT MatchKing bullet at 1000 f/s.
Play around with this stability calculator to see what you need for stability.
JBM stability calculator
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SR712
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Posted: 2/6/2013 8:06:59 PM
Originally Posted By fmkenner:
I went ahead and gave the nod for the 1/8 twist. I got the Stainless Steel 9.5", 1/8 twist from Core 15. I can sit on the fence and wait, or get it built. Im looking to get AACs Cyclone and Samson Evo 10" rails and have it slightly under the rails. If it will fit. Thanks for the response everyone.
Fred


I got that exact barrel today. It looks nice.
sinsir
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Posted: 2/7/2013 6:57:57 AM
I have a 1:8 micro seven with the 16 inch barrel. The heavy cast 247gr are fine at 950fps,out at 100 yards with a velocity of 650-700 fps the holes is the paper show signs of tail wag. At 125 yards its all I can do to group sub 4 inch diameter, with obvious signs of instability.
The 220 SMK's seem to be fine when compared to the cast during the same test at the same distance.

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fmkenner
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Posted: 2/7/2013 7:20:41 AM
Originally Posted By sinsir:
I have a 1:8 micro seven with the 16 inch barrel. The heavy cast 247gr are fine at 950fps,out at 100 yards with a velocity of 650-700 fps the holes is the paper show signs of tail wag. At 125 yards its all I can do to group sub 4 inch diameter, with obvious signs of instability.
The 220 SMK's seem to be fine when compared to the cast during the same test at the same distance.

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So basicly with 220gr bullets Im fine, even past 100 yards. But 240gr 100yds is about my limits?

Rich_V
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Posted: 2/7/2013 7:31:33 AM
[Last Edit: 2/7/2013 7:34:25 AM by Rich_V]
Originally Posted By fmkenner:
Originally Posted By sinsir:
I have a 1:8 micro seven with the 16 inch barrel. The heavy cast 247gr are fine at 950fps,out at 100 yards with a velocity of 650-700 fps the holes is the paper show signs of tail wag. At 125 yards its all I can do to group sub 4 inch diameter, with obvious signs of instability.
The 220 SMK's seem to be fine when compared to the cast during the same test at the same distance.

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So basicly with 220gr bullets Im fine, even past 100 yards. But 240gr 100yds is about my limits?



fmkenner
Go to the stability calculator I linked above. With this tool you will be able to plug in all combinations of bullet weight/type, barrel twist rates and initial velocity and get a stability readout.
Bullet stability is a function of velocity, twist rate, bullet length and weight. What I think sinsir posted is the 247 g is OK at an initial velocity of 950 fps but if you download to 650-700 fps initial velocity it begins to show instability.
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fmkenner
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Posted: 2/7/2013 7:56:47 AM
[Last Edit: 2/7/2013 8:00:21 AM by fmkenner]
Originally Posted By Rich_V:
Originally Posted By fmkenner:
Originally Posted By sinsir:
I have a 1:8 micro seven with the 16 inch barrel. The heavy cast 247gr are fine at 950fps,out at 100 yards with a velocity of 650-700 fps the holes is the paper show signs of tail wag. At 125 yards its all I can do to group sub 4 inch diameter, with obvious signs of instability.
The 220 SMK's seem to be fine when compared to the cast during the same test at the same distance.

Posted Via AR15.Com Mobile


So basicly with 220gr bullets Im fine, even past 100 yards. But 240gr 100yds is about my limits?



fmkenner
Go to the stability calculator I linked above. With this tool you will be able to plug in all combinations of bullet weight/type, barrel twist rates and initial velocity and get a stability readout.
Bullet stability is a function of velocity, twist rate, bullet length and weight. What I think sinsir posted is the 247 g is OK at an initial velocity of 950 fps but if you download to 650-700 fps initial velocity it begins to show instability.


Thanks. I have the reloading stuff but never have done it yet. I've read what I could, just now I need to find the time and just do it. Forgive my ignorance, I need to find some experience on my own with this. Thanks again.
Fred

ETA: All my ARs have been alot of fun. Each gives me a different thing back. I'm excited about this one as its going to be my first complete build piece by piece, and experience in reloading.


sinsir
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Posted: 2/7/2013 11:31:52 AM
Actually what I was saying is that as the projectile slows down at distance. It is showing signs of instability. If I loaded the cast at higher velocity to start with (1500 fps at the muzzle, and chronographed at 1100 fps at 125) the groups are less then half the dia and the holes are still round.
Please remember that these were hand cast bullets, I haven't found any 240 SMK's to compare against the 220 SMK's for a apples to apple test.

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