Twist question?

[SamSpade]

I shoot a 6mm/284 Rem 700 with Shilen 1/12 twist barrel. Shilen and everybody else says that you can shoot up to an 85gr bullet in this twist which I knew when I put the barrel on. The 58gr Vmax will shoot 1/2" at 200 yards and 85gr Game Kings with shoot 1" at 200 yds. I tried to push the envelope a little and tried 90gr Bergers and they won't stay on the paper at 200 yds. The ones that print are leaving a round hole. My question is why? I expected the groups to open up some but not to that extreme. I fully understand that this twist obviously won't stabilize the bullet but why with only a 5gr increase?

 

[James Jones]

its not the weight thats killing the accuracy its the length the 90gr Berger is a long bullet longer than alot of 100gr hunting bullets , truefully the 1-12 may not stabilize a 70gr VLD style bullets but will probably stabilize a 100gr round nose due to it being short for the weight.
the 75gr V-max has a pretty good BC for what it is and they will give stellar performance when driven fast , I shot them out of a 243Ai and loved them they seem to do at 500yds what a 22-250 does with a 55gr v-Max at 300

 

[SamSpade]

Thanks, that's interesting it gives me a better understanding of the twist rate vs bullet. If I understand correctly, it has to do with the length of the "bearing" surface of the bullet that is touching the rifling? Even after 50 years of shooting you can learn something everyday!

 

[James Jones]

Not realy the "bearing" surface but the over all length for instance that 85gr bullet your shooting probably has alot more bearing surface than 95gr Berger VLD due to the VLD bullets having such a long slender ogive which is what helps with the BC , its just that the longer overall bullet has to be spun a little faster then the shorter one.

What relay screws some guys up is when their buddy is shooting a 75gr A-max out of his 1-9 twist custom barrel and his won't even think about stabilizing them out of his 1-9 twist , I've seen this a couple times and checked the twist of both guns and they are the same but differant barrel makers!

 

[SamSpade]

Thanks, JD

 

[davewilson]

excluding all of the minor things that affect twist rate,the main thing is the ratio of the length of the bullet as compared to the diameter.the longer and thinner it is, the faster you have to spin it.

 

[SamSpade]

A picture of the 3 6mm bullets I tested. The 90gr wouldn't stay on the paper, the 87gr Hornady grouped about 2" and the 85gr Sierra about 1".

JD, as you stated the Berger is quite a bit longer. I have a better understanding of the twist vs length not weight!

So I need short fat bullets with a 12" twist! /ubbthreads/images/graemlins/grin.gif

 

[davewilson]

it wouldn't surprize me if the 12 was on the hairy edge of stability with the 85-87 bullets.i'll bet a 10 or 11 might shoot them more accurately.an example would be the 58's are shooting half inch.might just be the bullet, but it might just be a stabilization issue

 

[Mikecr]

I wouldn't think that it's twist causing your inaccuracy. Unstable bullets tumble immediately. They won't stay on 8x12 paper @50yds. Maybe your just moving further from an accurate barrel node with heavier bullets and slower velocity?

 

[James Jones]

Thats not totaly true Mikecr , I have seen 75gr A-max bullets shot out of a 1-10 Encore in 22-250 that would group around 3" at 100yds but 90% of the bullet hole were percect sidways cuts and the other 10% being very elongated holes. I have also see guns shoot bullets into 2" oblong holed groups at 100yds and then shoot the same load into 2" groups with perfect bullet hole at 200yds.

On another note if you notice that the 87gr bullets is only slightly longer than the 85 but the ogive on the 87 is noticably longer and more streamlined.

 

[edge]

IMO, how hollow is the hollow point makes all the difference in the world! A bullet with lead sticking out but the same shape as a hollow point will normally be more stable. Put tungsten in the nose of a bullet and you will need almost no twist at all, it is the fact that the heavier tail section wants to overtake the lighter front half of the bullet.

edge.

 

[davewilson]

it's also a fact that the hollow point is there to bring the center of gravity towards the rear because it makes it shoot more accurate.