Tell me about twist rate

[dwa12479]

I'm in the market for a 20" upper for a new build. Looking to do a rifle length gas system and buffer, something that I can reliabley shoot to longer ranges, 400-600 yards .... ish... Twist rate isn't something that I've given much thought too in past carbine builds.

As I understand it (and I don't very well), a higher twist rate is needed to stabilize heavier projectiles, but can also overspin lighter ones. Is twist rate something to overly concerned with? Everything I'm looking at is either 1:7, 1:8, or 1:9. If i was to get a 1:8, what range of projectile weight is ideal for that?

[Cgrutt]

Sounds like a question for Mek, LOL...

Anyway, I don't know alot either but do know that all else equal higher velocity will result in higher spin for given rifling so with a 20" barrel you may be better off opting for a slower rifling such as 1:9 or even 1:10 as you will be attaining higher velocities than if you shot from a 16" or 18" barrel. Bullet weight AND length are also factors that go into the equation. Assuming you are talking about .223/5.56. What distances are you shooting at and are you concerned more about accuracy (i.e., grouping on paper) or lethality (e.g., hunting or self defense).

[WARFAB]

For 400-600 yards I'd stay away from any twist slower than 1:8. The rate of twist actually corresponds to stability for a bullet's length, not weight. Since most bullets for a given caliber are usually within a given length based on how the lead fits in the barrel, people just go by weight. Certain VLDs or solid lead bullets might not fit the weight formula though. Berger has a calculator on their website that tells you stability ranges based on bullet length.

http://www.bergerbullets.com/twist-rate-calculator/

I shoot Hornady 75 grain HPBT match bullets across the course through a 1:8 twist barrel. If I re-barrel my rifle I'll probably try to do it with a 1:7.7 twist. Right now my 75 grain loads aren't what's holding me back at 600 yards. Until I improve my wind reading abilities I don't see the point of moving to a heavier bullet. 80 grain Sierra match kings are also more expensive.

It all depends on how accurate you want to be though. I'm sure you could hit a 600 yard silhouette with lighter bullets and 1:9 twist if you so desired.

Shilen has a guide based on bullet weight:

http://www.shilen.com/calibersAndTwists.html

[dwa12479]

The 1:8 is the one I'm leaning towards at the moment. My current handloads are using 55gr Hornady FMJBT, obviously I'd adjust that accordingly for shooting out to 600yds.. It wouldn't be a regular thing to shoot that far due to facility limitations, but the occasional opportunity pops up.

As far as overall accuracy, if I can keep the groups at 1 MOA from a benchrest I'll be quite satisfied.

[WARFAB]

As far as overall accuracy, if I can keep the groups at 1 MOA from a benchrest I'll be quite satisfied.

No small task at 600. Mek will probably be able to say more about the potential problems with over-twisting a bullet. My understanding is that the longer heavier bullets have more surface area in contact with the rifling lands. The push from the case firing is a huge force directly forward, so a bullet without enough contact with the lands might not follow the rifling as it's intended to. That's why I'd stay away from a 1:7 unless you're interested in shooting some of the more extreme 80-90 grain VLDs.

If you want to spend the money progressive twist rate barrels are available. My guess is a progressive twist would be able to handle a larger range of bullet weights, but that's just my theory. I don't know anyone who has one.

[Meketrefe]

IMO 1:8 is a good compromise. It will stabilize the most popular bullets including 70gr solids like the 70gr tsx.
Twist is needed not because of weight but length and bullet shape construction and speed.
There are more problemes with lack of twist than too much twist. Just stay away from hypervelocity rounds with super thin jackets like those at 3600fps 40-50grs to exploding
prairie dogs. FMJs and most 55gr and up will not break apart no matter what speeds even at 7 twist.
Now if you are going to use some of the VLDs in the 80gr to 82gr then you need a 7 twist otherwise the 7 twist is there to stabilize the tracers that are a lot longer due to being filled with magnesium compounds for the tracer burn.

Just check the manufactures recommendations on the ammo and bullets and you will see. Again the weight has some relation many times but the twist is not based on weight and lenght, shape and speed is the determining factor. the further back is the center of pressure of the bullet and perhaps less bearing surface of some long bullets then the harder they become to stabilize. Even lead bullets that are for match that have a large gap of air in the tip are harder to stabilze of another lead bullet of the same weight but with the tipical soft point that is shorter and with more bearing surface. on these the center of pressure and center of gravity are a tad further forward and slightly more apart. this also helps with stabilizing faster. Even a shotgun slug is hollow in the back like a cup so the center of pressure and gravity is moved forward so they will stabilize even w/o rifling in shotguns.

A bit extra twist is a good thing at long range. I helps the bullet stay put as one also migth approach transonic range and it seems to help too with high yaw. A tad more consistent
at LR. but also people normally shoot with even better bullets and loads so it is hard to say how much effect each particular variable has.

But to give you an example I am building myself a supermagnum wildcat basd on the RUM but more similar to the 300 dakota. I will be shooting 230gr Berger VLDS at 3000fps average and I am planning to do it with a 1:8 twist barrel. I know it is crazy for 30 cal (unless subsonic) but when I get a chance and I can get down to PA or AZ I want to bring the rifle and try shooting something 1 mile away. I hope it is something large at least like a car! LOL but I think the extra twist will help keep the super fast and super long VLDS steady w/o jittery trajectory a bit longer.

I think 1:8 will be good for the 223. Never get a 1:9 if you can avoid it. It cannot stabilize some service and popular match ammo and therefore it looses versatility for a caliber that
is supposed to be versatile based on the huge assortment you have.

I hope this makes sense.