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01-06-2020, 13:16   #16
Mellor
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Originally Posted by clivej View Post
Nothing to do with barrel length. 1in12 means the bullet rotates once in every 12". This equates to, when the bullet leaves the muzzel and travels 12" it has rotated once.
A 1in 8 has rotated once in 8" therefore it has more spin, it has more giroscopic effect and more stability
I think I didn't explain what I meant very well.
I understand that the twist is the rate of rotation of the rifling. 1:10 is same rate regardless of length, therefore the torque is constant being a product of the rate.
Referring to length I meant the bullet is under the influence of the torque for longer. I've always understood that to mean more spin in RPM.

Eg, Take a 22 rifle and a 22 pistol (in the same twist rate). During flight the rifle round will spin faster in terms of RPM. Although come to think of it, that would be mainly a product of increase muzzle velocity I suppose.
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02-06-2020, 07:42   #17
yubabill
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Because the most commonly used ammo seems to be in the 40 to 55gr mark which is ideally suited to a 1:12 twist rate.

1:8 twist rate is only suitable for ammo in the 69gr plus range.

For a long time I was recommending 55gr and above for 1:8 twist barrels and was following that myself.

Then Tack and a few others here were saying they were getting accuracy with 40gr BT's and for a while I was sceptical.

Then I got 3 or 4 45gr Remy JHP's and tried them for pig iron - they were all touching at 50yds.

I bought some 50gr Fiocchi HP's and they're giving me an inch or less at 100yds consistently and I do note that other people have had different experiences with them, but the two boxes I bought have been trouble-free.

I don't know exactly why manufacturers offer a choice between 1:12 and 1:8, maybe the slower twist gives better accuracy with loads of 55gr and less? never got the chance to try that out.

The AR15 was first issued with 1:12 twist for the US 55grFMJ, but were retrofitted with 1:8 when NATO standardised to the 62gr (SS109? can't remember exactly the designation). Maybe that has something to do with it, a kind of tradition thing, but I suspect the reason has more to do with precision.

Maybe some range shooters have compared like-with-like? Say 2 T3's?
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02-06-2020, 09:40   #18
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Originally Posted by alanmc View Post
I've kinda wondered this myself. Mainly when I'm trying to figure out why my .308 barrel is a 12" and not a 10" twist. If you google enough, you'll find holy wars about "over stabilisation". There are "stories" and myths about bullets flying apart due to being over spun. Many refute this and yet many swear it's true. I remain unconvinced.
There are so many variables in play here, not just twist rate (velocity - a function of barrel length, charge weight and powder burn rate, bullet shape - as Cass has explained above, barrel construction and rifling type - button vs cut vs polygonal).

All of these variables contribute to the revolutions per second on the bullet when it leaves the barrel. IMO, there is no substitute to trying bullets in your rifle. The weight vs twist is a good guide, but not always the truth. For instance, my 12" twist .308 barrel on paper shouldn't stabilise a 175g TMK bullet .... but it does.
I read about bullets ripping themselves apart as well from to much rpm. I don't recall exactly however it was in reference to fast .22s and light jacketed varminting bullets.
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02-06-2020, 10:25   #19
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I got the best ever groups with Hornady 40gr VMax from my CZ varmint 223 1in8 twist. 1 x 28mm 3 shot group and 2 x 40mm 3 shot groups at 300m
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02-06-2020, 11:20   #20
yubabill
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Just for the record, to date the best grouping I get from my T3 1:8 is with hornady 55gr Steel Match FMJ - around 0.5inch at 100 yds.

Never tried the 40gr BT's - too rich for my blood - but the 55gr BT's give me around 1 inch at 100 yds (still have some I bought years ago at 25.00 per box).

I just shoot whatever is cheap and reliable and keep the good stuff as ornaments.
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03-06-2020, 15:10   #21
badshot
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any harm asking which model tikka
and where did you order it
can't decide myself between varmint supervarmint or tactical
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03-06-2020, 17:44   #22
zeissman
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Originally Posted by badshot View Post
any harm asking which model tikka
and where did you order it
can't decide myself between varmint supervarmint or tactical
My choice would be the supervarmint.
It costs a bit more than the varmint but I think it's worth it for the adjustable stock and picatinny rail.
The tactical is also a nice rifle but is more expensive.
I would prefer a stainless rifle
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04-06-2020, 16:47   #23
Uinseann_16
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Originally Posted by yubabill View Post
For a long time I was recommending 55gr and above for 1:8 twist barrels and was following that myself.

Then Tack and a few others here were saying they were getting accuracy with 40gr BT's and for a while I was sceptical.

Then I got 3 or 4 45gr Remy JHP's and tried them for pig iron - they were all touching at 50yds.

I bought some 50gr Fiocchi HP's and they're giving me an inch or less at 100yds consistently and I do note that other people have had different experiences with them, but the two boxes I bought have been trouble-free.

I don't know exactly why manufacturers offer a choice between 1:12 and 1:8, maybe the slower twist gives better accuracy with loads of 55gr and less? never got the chance to try that out.

The AR15 was first issued with 1:12 twist for the US 55grFMJ, but were retrofitted with 1:8 when NATO standardised to the 62gr (SS109? can't remember exactly the designation). Maybe that has something to do with it, a kind of tradition thing, but I suspect the reason has more to do with precision.

Maybe some range shooters have compared like-with-like? Say 2 T3's?
Just a tiny thing there the AR-15 was never issued as a main service weapon the M16 was adopted as a service weapon
The AR-15 is a semi auto only civillian rifle
The colt 601, M16, M4 are select fire which are versions of the AR-15 but they are not AR-15s
Basically AR-15s are civvi rifles with no happy switch

But more to your point the original M16a1 was issued with a 1 in 12 twist rate some of the really early rifles had a 1 in 14 twist (the .222 twist rate from which the .223 was developed) they standard round at the time was the 55gr
When NATOs SS109 62gr came out there were issues with the 1 in 12 twist so they changed over to a 1 in 7 twist as theyre were apparently key-holing issues along with accuracy problems etc with the old 1 in 12 barrels
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