Can you over stabalise/rotate cast bullets?

[ovenpaa]

I predominantly shoot jacketed bullets at longer distances and I can probably recite all of the required twist rates for various bullets and calibres off by heart dependant on weight and make, however when it comes to cast bullets I am an absolute novice so my question is can a cast bullet be over stabilised/rotated?

I am looking at various twists using Millers formula for a choice of barrel and am seeing Sg numbers of 4.0 and more for the short cast bullets I have selected. Normally 1.4 or more is acceptable for jacketed bullets. I have witnessed jacket separation on a few occasions with jacketed bullets, obviously cast bullets are not jacketed however would higher rotation lead to excessive leading or any other issues?

Thoughts anyone?

[dromia]

There is a lot of discussion and disagreement in cast bullet circles round what is called the RPM threshold.

Basically the RPM threshold is where the rotation causes the alloy to strip upon entering the rifling to such an extent that is affects accuracy.

Proving this and applying it across the board is difficult as the conditions especially alloy composition is very variable, so the RPM threshold is moveable feast.

If you look for RPM threshold over on Cast Boolits you will see much discussion testing and heated debate, Larry Gibson is the RPM threshold guru and has done a lot of work on it although there are others who challenge him on it.

Overall the RPM threshold in my view has merit although it is a moveable feast.

When I was using cast bullets at 1000 yrds I made my alloy tough, not hard, to increase the RPM threshold and used the heaviest bullet I could get away with to maintain velocity at the longer distances.

Didn't do any science around it but it worked, and my averages increased with cast.

[ovenpaa]

It looks to be quite an emotive subject across some of the other forums. The good news for me is my choice of barrel twist seems to be quite comfortable for the rifle with a fair spread of bullet weights however the only real way to confirm this is to fit the barrel and shoot it.

[bewildered]

I shoot 195g gas checked lead bullets at about 1550fps through my 1 in 13 twist target rifle at 200 and 300 yards with great success but the same rifle doesn't like 175 fmj bullets.
I'd have thought the lead bullets needed a faster twist than 1 in 13 but maybe as I'm shooting at shortish range it doesn't matter.

[dromia]

Until you reach an rpm threshold then cast are no different from jacketed, I am sure that jacketed will have their own rpm threshold, however in relation to cast bullets with those velocities I wouldn't even think about twist being a problem with that weight and slow twist.

What velocities re you driving your jacketed at?

[Laurie]

I shoot 195g gas checked lead bullets at about 1550fps through my 1 in 13 twist target rifle at 200 and 300 yards with great success but the same rifle doesn't like 175 fmj bullets.
I'd have thought the lead bullets needed a faster twist than 1 in 13 but maybe as I'm shooting at shortish range it doesn't matter.

Your lead bullets will be short in relation to a similar weight or as in this case lighter jacketed type and it's primarily bullet length in relation to calibre that determines the required spin rate. Weight is used (often incorrectly) as a broad guide as all other things being equal (type / material / shape) a heavier bullet is longer than a light one and therefore requires a faster twist / higher rotational speed.

In fact for two bullets of identical length and the same calibre but with significantly different weights, the heavy model actually manages with a substantially lower rotational speed and therefore gets away with a slower rifling pitch. Then throw in base type as well - two otherwise identical weight and length bullets in the same calibre but one boat-tail, the other flat-base (like your gas checked cast bullets) will see the BT example need a full inch faster twist than the FB type. So relatively heavy and short blunt FB bullets manage with surprisingly slow twist barrels. A good example is Sierra's 60 and 63gn SMP (semi-pointed) FB models in 0.224 calibre, antediluvian designs for small / medium game in the old .222 Rem with its 1 in 14-inch twist barrels. However, the 62gn SS109 type FMJBT ball projectile loaded in current 5.56mm military ammo has a minimum twist requirement of 1 in 9.1" and an optimum one of 1 in 8.2" (Bryan Litz), and the military actually use 1 in 7" in modern assault rifles. (But that's dictated partly by planning for Arctic winter campaigns, cold dense air needing more stabilisation and also by the need to handle the even longer tracer bullet.)

[bewildered]

Many thanks for the replies boys.
In answer to your comments Dromia, the 175s were loaded by a friend of mine (I don't know how fast they were going). At 600 yards my reloaded 145s and 155s were great but the 175s weren't so good. They shot well in my friends rifle though with a faster rate of twist. I should mention these were all 308.

Laurie, many thanks for the explanation. My cast bullets are about 28mm in length and the 175s are about 31mm with the lead bullet having a greater surface area in contact with the rifling of that matters.

[Mike95]

I successfully shoot many types of cast in 30 cal rifles up to 2000fps. ....bullet must fit the bore...2 thou over is the norm. A loose fitting bullet will lead to leading of the barrel. GC normally fitted. Alox lube. No problems experienced with all weights to 190gn and twist 1/10, 1/12... I have only had a problem with the 6.5 Swedish Mauser which had a twist of 1/7.25....could not get the relatively long cast bullets to stabilise/group well. The Loverin bullet designs tend to be more accurate, small ogive, lots of ridges into the grooves. Bore riding designs can work well providing they are oversize..the Swiss 7.5x55 requires special attention!
One alternative to cast bullets is the H & N High Speed plated bullet. The 30 cal RN have 6 thou of total copper plating with a soft lead core and a non foul plastic coating. They shoot well in most calibres to 2000fps and I suspect they might have been the answer to my 6.5 Swede! A sort of "half way house" between FMJ and cast. If you intend to shoot cast I would suggest buying the Lyman cast bullet reloading manual.

Mike95

[dromia]

The issue with the 6.5 swede isn't bullet stabilisation but bullet deformation before it is fully supported in the rifling, they can be got to shoot at over 2000 fps with accuracy but there are a few wrinkles and loading processes that need to be applied in detail to get that performance out of it with cast.

[FredB]

many years ago I was working in Germany and I picked up an English translation of an article from a German shooting magazine. It stated that groove diameter was not the issue with cast bullets: what matters is throat diameter. If you have ever tried to load for a Martini Henry, you will know that this is true.
Fred