Twist rates for .223 rifles


PDA






JimUCD
June 9, 2003, 02:23 AM
This is something I had always wondered about....... I understand that a quicker twist rate (ie 1:9 compared to 1:12) is not good for heavier bullets, due to the extra rotation and "over" stabilization that is put on the bullet............but is it the same the other way around?

example:
-dont use high grain 223 bullets in a 1:9 twist, bullets will be over-rotating.

question:
-can you use high AND low grain bullets in a lower twist rate barrel, like a 1:12? Would this "under" stabilize the lower grain bullet?

Thanks guys.

If you enjoyed reading about "Twist rates for .223 rifles" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
106rr
June 9, 2003, 03:11 AM
I have a .223 with a 1 in 12 twist. It shoots the light 40 and 45 gr bullets pretty well and it will make one and a half inch groups with the 64 gr Winchester "deer bullet" I thought that the 40 gr would be unstable but it wasn't.
These are 100 yd three shot groups. I suspect that a flat base 64 gr will be the longest projo that will stabilize in that twist.
You will have to try the different loads in your rifle. I wish mine was a 1 in 9 twist.

JimUCD
June 9, 2003, 03:20 AM
why do you wish yours was a 1:9?

Jim Watson
June 9, 2003, 09:37 AM
JimUCD:

YOU UNDERSTAND WRONG!

A "faster" twist - one full turn in a smaller number of inches is what is needed to stabilize long heavy bullets. The gyroscopic effect has to work harder on a long skinny object. Examples from .22 history:

16" twist - . 22 lr 40 grain; .22 Hornet 45 grain bullets.
14" twist - most .22 centerfires with 50-55 gr bullets, will handle up to 60 grain flatbase spitzer. Heavier if semipointed or roundnose, it is really the length that counts.
12" twist - early M16 barrel for M193 55 grain boattail in cold climate.
10" twist - I think Ruger used it in Mini-14 for a while, usually ok for SS109 62 grain and 68 grain.
9" twist - early .223 target barrel for 68-69 grain BTHP.
7", 7.5", 7.7", 8" twists - current .223 target barrels for 80 grain VLD.
7" twist - current GI barrel for M856 tracer, a 9" is enough for M855/SS109 ball.


"Overstabilization" of a light bullet in a fast twist is a bugabear seen only with very extreme differences. A very light bullet in a very fast twist might not be real accurate. You can actually blow up a bullet in flight by driving it too hard and an excessive twist for the weight will make it worse. But a little extra twist is helpful with mediocre bullets. A 9" twist will often be more accurate with cheap FMJ econo-ball than a 12".

My 9" twist barrels are accurate with everything from 50 to 68 grains and that is enough for me, but I am not a long range shooter who needs the really heavy bullets (see below.) I have a 12" .223 and a 14" .22-250 that are most accurate with a 60 grain flatbase and a 9" that prefers a 52 grain. The only way to know for sure is to shoot them.

An "understabilized" bullet will soon become an UNstabilized bullet and will not hit anything. A friend of mine has a .223 match rifle made when 68 grain bullets were the heaviest available. It has a 9" twist and is very accurate with them and has plenty of barrel life left. But it will not shoot the 80 grain bullets now preferred for the 600 yard line and is somewhat obsolescent, especially on a windy day.

BigG
June 9, 2003, 11:23 AM
What Jim said: the higher the grain the faster the twist.

JimUCD
June 9, 2003, 12:40 PM
What would the "SAFE" grain range in bullets if being shot out of a 20in barrel with 1:12 twist rate?

ddc
June 9, 2003, 12:48 PM
FYI, FWIW: This issue is addressed in part over at www.ammo-oracle.com.

Steve Smith
June 9, 2003, 12:50 PM
I have no experience with light varminting bullets. I do know that a 52 grain SMK will hold in a 1:7" twist barrel. 1:9 or 1:10 would be a good safe twist if one wanted to throw frangibles...I THINK. I personally would not go slower than that because I'd want the ability to shoot heavier bullets occasionally. If I already had a 1:12 I would probably throw a few 62's downrange to see if they would even group, and if not just start stepping down till I found something that did. The 52/53 SMK bullets ought to work, as well as the mid-40 range varmint bullets.

Jim UCD: as Mr. Watson said, you need a faster twist to stabilize a longer object. Of course the diameter doesn't change (they're all .224") but the length changes. Imagine a child's spinning top. If it is really short, it hardly has to spin at all to stay upright, wheras a long top would need a lot of spin to keep it from toppling over.

As far as overstabilization goes, theoretically, it can have detrimental effects, but recent experimentation with 1:6.5 twist barrels (developed for 90's) and 77 gr SMKs show that they work VERY well when slightly "overstabilized."

JimUCD
June 9, 2003, 12:58 PM
Thanks Steve and Jim, your info helps a lot..............another: can you shot a large grain range in a higher twist right? Say if my .223 was a 1:9, would I be stuck using only the heavier bullets?

Steve Smith
June 9, 2003, 01:07 PM
Not at all! See my comment about 77's in that 1:6.5 twist. However, I could shoot a 45 grain varminter bullet in my 1:7.5 Pac-Nor and blow it up (the bullet) in flight. The construction of the bullet has a lot to do with it too. I can shoot a very light (52 grain) bullet in my fast twist rifles, but they have to be solid or match type bullets. If I try to shoot a really light thin walled bullet, its centrifugal force will cause the bullet to disintegrate in flight.

Poodleshooter
June 9, 2003, 01:21 PM
I've never heard of anyone blowing even a 40gr varmint bullet up in a 1x9" twist barrel. My 1x7" Colt will destroy most soft or hollowpoint bullet under 60 grains if they are driven faster than the 2700-2800fps range. As Steve said, you can shoot the Sierra 52/53gr match bullets in a 1x7" since their jackets are thicker and more consistent.
1x9" is a great twist provided you don't want to shoot full course highpower or other long range matches with 75-80gr match bullets. That's essentially the only reason for a twist tighter than 1x9".

thisaway
June 10, 2003, 05:27 PM
I wonder if anyone has ever tried a "gain twist" in .223?

Steve Smith
June 10, 2003, 05:32 PM
Yes.

Bostonterrier97
June 10, 2003, 06:36 PM
While in flight, a bullet has 2 different types of momentum.
It has linear momentum (its moving downrange at a certain velocity and the bullet has a certain amount of "weight" or mass) and it has angular momentum (It is spinning at a certain speed and it has a certain amount of mass).

Most of the forces acting on the bullet are due to the "wind" resistance as it is moving down range. In fact there is more "wind" resistance - resisting the bullet's linear momentum than there is due to the bullet "spinning".

Because of this the bullet "wobbles" (or precesses) like a spinning top. As the bullet loses its linear momentum (ie. slows down) it wobbles less because the bullet's "spin" hasn't slowed down as much as the bullets linear velocity..so Gyroscopic stability increases.

The longer the bullet, the faster twist rate you will need in order to stabilize the bullet.

If you wish to shoot 62 grain .223 rounds you will need a 1-9 twist.

If you plan to shoot 80 grain VLD's then you will need a 1-7 twist rate.

makarov
June 11, 2003, 12:43 PM
Does barrel length come into play at all in stabilizing heavier bullets? For instance, a 1:12 twist barrel comparing a 16" and a 26" barrel, would you be able to stabilize a heavier bullet with the longer barrel?

Jim Watson
June 11, 2003, 05:21 PM
Only as barrel length affected velocity. Longer barrel = higher velocity (usually) = more RPM to stabilize bullet. But someone worked through the math and found that bullet stability was proportional to the fourth root of muzzle velocity, so the difference would be small and would only help - or hurt - a marginally bad case.

Kobun
June 11, 2003, 05:36 PM
This is a very interesting topic!

Thinking about RPM's:
Will a short barrel (7" or 12") benefit from a higher rate of twist, (lets say 1:7 instead of a 1:10), to make up for the reduction in RPM's, compared to a 20" barrel with a 1:10 twist?

Bullet weights from 55gr and up.

makarov
June 12, 2003, 02:12 PM
Thanks. The reason I asked was that some people have reported pretty good results with long barreled bolt actions in 1:12 and bullets up to 68 grain or so. I just wondered how much effect the length of the barrel had to do with it. Thought maybe the faster twist rate was originally to help short barrel AR's or something. Glad I ordered the 1:9 now!

bad_dad_brad
June 13, 2003, 01:27 AM
1 twist in 9" is sort of considered the defacto compromise. Most Bushmasters use that ratio.

If you enjoyed reading about "Twist rates for .223 rifles" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!