Twist rate overspin and accuracy

[andym79]

Hi obviously its best to try and match bullet length and rate of twist.

The closer the better, but if you have no choice but to use a bullet significantly shorter than ideal, to what extent can ovetspin potentially damage accuracy?

I am talking about exclusively cast bullets and at velocities that do not push the rpm threshold.

For example I am building a 40-70 single shot, lets say I want to shot Lyman 410663 an 1:18 should do it a 1:16 to play it safe at range. But what harm might a 1:13 do to accuracy if thats what you had to use, I am talking about at speeds of 1200-1500fps.

Thanks

[buckshotshoey]

The only time rpm gets to be a problem is when a lightly constructed bullet will fly apart due to centrifugal forces. Your rpm should be in the 80000 range at 1500fps and a 1 in 13 barrel. Should be fine. Is it needed? No. But it shouldn't hurt.

Any other problems that can occur to cast lead can be minimized or eliminated with proper fit to barrel, and proper alloy used. You might have to run it harder. You are better off with a little too much spin then not enough. Ask the .44 mag rifle guys about that!

[Scharfschuetze]

Bullet stability is really determined by the projectile's length, the calibre and to a lesser extent the velocity of the load.

The military twist rate for the 45/70 in the Trapdoors was 1 in 22" as I recall. It is plenty fast enough to stabilize the 500 grain infantry and Gatling gun load at 1,300 fps MV to way out to 2,000 yards per tests by the Ordnance Corps.

For cast bullets, you are better off with a slow twist for the calibre and to stay away from fast twist barrels. It's all a compromise, but fast twists will generally limit your accurate velocity window while a slower twist will often shoot moore accurately and to a higher velocity with accuracy.

Here is a fairly accurate twist rate calculator that you can make a good prediction from:

http://kwk.us/twist.html

The aforementioned 44 Magnum barrels and older 444 Marlin barrels with a 1 in 38" twist (Marlin, etc.) were way to slow for bullets much over 165 grains. The 250/3000 Savage, 6mm Remington are also examples of rounds that came with too slow a twist barrels from the factory, although they were perfect for the short and light bullets that the manufacturers envisioned the rounds using.

[runfiverun]

none.

[andym79]

Runfiverun are you saying a 1:13 will not harm accuracy?

[Larry Gibson]

A certain RPM is required to stabilize any bullet. The higher the RPM above stabilization the greater the adverse affect the centrafugal force (created by RPM) will have on any imbalances in the bullet created during casting, loading and launching. Under the RPM Threshold it's why we shoot groups instead of one hole. For the most accurate bullets it's why bullet makers strive to make as balanced a bullet as they can.

Also shorter, stubbier bullets for a given caliber have a harder time with "going to sleep" of the pitch and yaw (wobble) in the 1st 100 yards of flight after launch. The higher the RPM the less likely they will go to sleep.

If you're going to shoot 400 gr or less cast bullets in your 40-70 then the 18" twist would be best. If heavier bullets will be used then the 16" twist would be best. Both will also optimise the accuracy of lighter cast bullets.

[andym79]

For a while now I have awfully tempted to have krieger do a 1:17".

[runfiverun]

yes..

[andym79]

Such a tough decision a well made local barrel for $320 in 1:13 or $600 for a Krieger and a 6 month wait. If I lived in America the choice would be simple but the the Krieger would come in only $45 more, but with the export import expense :violin:

Is a Krieger better I would guess yes! Is a 1:17 going to be better than a 1:13 probably, but will it be worth double?

Is the extra accuracy worth $300?

I guess these are not definite answers anyone can give me.

But if one had to take a guess all other things being the same how much damage could the faster twist do to accuracy at 200 yards, 1/8 moa 1/4 moa 1/2 moa more?

I guess if you ruled out cost the you would go with the best, but I want some left for other projects. I guss better to have fewer guns that shot well than many that don't.

On second thoughts if a 1.4" bullet doesn't do well in a 1:17 I could always sell the mould and go a bit shorter.

[buckshotshoey]

Like was mentioned in post 2, a 1 in 22 has worked for 130 years using 400 And 500 grain boolits. I get excellent accuracy with a 1 in 20. If you have the choice, I would go with a 1 in 18, or a 1 in 20. A 1 in 13 shouldt hurt, but it is IMO not necessary. See if the local company you are referring to will do a 1 in 18 for you. You will have a far greater range of alloys you can use. A 1 in 13 might require a bullet that is to the point of being brittle.

[indian joe]

Bullet stability is really determined by the projectile's length, the calibre and to a lesser extent the velocity of the load.

The military twist rate for the 45/70 in the Trapdoors was 1 in 22" as I recall. It is plenty fast enough to stabilize the 500 grain infantry and Gatling gun load at 1,300 fps MV to way out to 2,000 yards per tests by the Ordnance Corps.

For cast bullets, you are better off with a slow twist for the calibre and to stay away from fast twist barrels. It's all a compromise, but fast twists will generally limit your accurate velocity window while a slower twist will often shoot moore accurately and to a higher velocity with accuracy.

Here is a fairly accurate twist rate calculator that you can make a good prediction from:

http://kwk.us/twist.html

The aforementioned 44 Magnum barrels and older 444 Marlin barrels with a 1 in 38" twist (Marlin, etc.) were way to slow for bullets much over 165 grains. The 250/3000 Savage, 6mm Remington are also examples of rounds that came with too slow a twist barrels from the factory, although they were perfect for the short and light bullets that the manufacturers envisioned the rounds using.

Do you have more info on the 44 twist please ? I am approaching a 44/40 build - reading the blurb on Green Mountain barrels and a big hurrah on their website for 1:38 twist - I will only shoot 200grain cast and most will be with blackpowder -lot of commercial 44 barrels now 1:20 -- the 38 twist seems slow but its only a stubby pill - I would rather go with original specs if it works well but ? There is a bit of conflicting info around this - I have no interest = zero = in shooting heavy (240grain etc) in this rifle. thanks for any info. joe