RPM threshold, gas cutting and BHN!

[andym79]

Hi guys, I just wondered if there is any link between RPM threshold and BHN?

Does using a harder cast bullet allow you to push the RPM threshold higher for a given twist?

The main reason I ask is because I hit a brick wall at around 1600fps with the Swedish Mauser and I would like to push it faster whilst maintaining accuracy!

I was using a Hardball alloy of 2/6/92, could I achieve much more with Linotype or is that wishful thinking?

[waksupi]

Oh, man, you sure know how to stir a pot!
Throwing together RPM threshold, and the Swede in one topic always starts a Donnybrook! Do a search on the Swede, and prepare yourself for days of reading and arguing!

[44man]

Harder will resist skid as the boolit wants to turn. Not much to do with the "threshold" because every alloy just changes that.

[Bullshop]

waksupi
But this time you can just watch and enjoy.

[beagle]

Might try alloying with some copper content Babbitt. This toughens the bullet and probably will allow a little higher velocity without stripping. Just how much will have to be determined. 2400 FPS in the .223 seems to be the threshold with air cooled WW alloy. By adding Babbitt, I was able to retain accuracy at 2600+. Haven't messed with it anymore but the results look promising. Now, with the 6.5 and it's long bullets and weird twists, you'll just have to experiment./beagle

[NVScouter]

Just cast up some ice boolits while you're at it.

Seriously though its all in that gigantic thread. I had to read it about 5 times to get most of what was being said. Most 6.5 thread are garbage here loaded with bias and hurt feelings. If a 6mm, 25 (basicly a 6.5) and 7mm cast and shoot fine a 6.5 will be fine.

[Idz]

material strength in psi is approximately:
Compressive strength = 1422 * BHN
tensile strength = 500 * BHN
shear strength = 300 * BHN

for a properly fitting bullet that fills the barrel the shear strength is the important number.

the shear on a bullet rifling can be estimated by:

shear (psi)=1.232 * (Pmax*D^3)/(Tw*N*X*L)

where Pmax = max chamber pressure in psi, D = caliber in inches, Tw=twist in inches/rev,
N=number of grooves, X=groove width in inches, L= axial length of driving bands in inches

I believe the biggest problem people have is an ill-fitting bullet that allows the hot gases to leak around the bullet. If the bullet surface is heated the material strength plummets rapidly and shearing can occur.

[dragon813gt]

Did someone say RPM Threshold?????


This is always fun to watch :laugh:

[pdawg_shooter]

Easy cure ... paper patch. Ignore the rpm theory, use a bhn of 18+ and load them to whatever velocity you wish. Or use AACOWW and stop around 2550/2600fps. Try it, It works.

[Larry Gibson]

andym79

Obviously you understand the RPM threshold because you have found it (I hit a brick wall at around 1600fps which is the top end of the RPM Threshold) in your 6.5 Swede. PM me including bullet and load information for the answers to your questions and how you may push it a bit higher.

Larry Gibson

[45 2.1]

for a properly fitting bullet that fills the barrel the shear strength is the important number.
The boolit is not under tension......... and the same BHN can be achieved thru different methods with vastly different alloys which will not give the same results.
I believe the biggest problem people have is an ill-fitting bullet that allows the hot gases to leak around the bullet. If the bullet surface is heated the material strength plummets rapidly and shearing can occur.

Ill fitting boolits are the problem, they miss align when booted too hard. That's the problem.

Did someone say RPM Threshold?????


This is always fun to watch :laugh:

I've got some popcorn on, join me.........

Easy cure ... paper patch. Ignore the rpm theory, use a bhn of 18+ and load them to whatever velocity you wish. Or use AACOWW and stop around 2550/2600fps. Try it, It works.

Now ain't that just the truth............

[blikseme300]

Did someone say RPM Threshold?????


This is always fun to watch :laugh:

Here we go again! I just re-read the thread on the threshold earlier today - it's going to be interesting, maybe!

[youngda9]

[TXGunNut]

Yes and maybe. The threshold does indeed exist but it can be pushed around in a number of ways; BHN, boolit fit, boolit alloy, boolit design, paper patching and a carefully built rifle. The threshold is not a static point, it is determined by several factors for each individual rifle.
Youngda9, you didn't bring enough popcorn!

[sthwestvictoria]

The main reason I ask is because I hit a brick wall at around 1600fps with the Swedish Mauser and I would like to push it faster whilst maintaining accuracy!

Is RPM the only limiting factor you could be up against?
Are you using a fast powder like AR2205 (H4227) or faster? A fast powder can cause pressure problems on the bullet base without much velocity.
Is you lube a standard recipe known to perform well or a custom lube that you have pushed faster in other calibres?
Have you tried heat treating the alloy you have?

[Driver man]

what is AACOWW I cant figure it out

[andym79]

I am not certain, I think some of it is an abbreviation for air cooled wheel weights!

[sthwestvictoria]

COWW is clip on wheel weights.
Not sure what the AA part is - air cooled/air aged? to seperated it from WQ (water quenched) or HT (heat treated)?

AACOWW is not listed in the abbreviation sticky:
http://castboolits.gunloads.com/show...-on-This-Forum

[andym79]

Is RPM the only limiting factor you could be up against?
Are you using a fast powder like AR2205 (H4227) or faster? A fast powder can cause pressure problems on the bullet base without much velocity.
Is you lube a standard recipe known to perform well or a custom lube that you have pushed faster in other calibres?
Have you tried heat treating the alloy you have?

I have been using H4198, CCI LRs, Winchester brass, a 140GN GC projectile cast in Hardball (BHN 16) I just use liquid Alox and have pushed the same alloy with that lube close to 2000fps in the 30-30 with pretty good accuracy!

The 30-30 has a 1:12" twist the 6.5 a 1:7.8". Maybe I could rebarrel the Swede with a slower twist, jacketed bullets would be the cheaper option! I am however determined to get it shooting cast!

[sthwestvictoria]

I have been using H4198, CCI LRs, Winchester brass, a 140GN GC projectile cast in Hardball (BHN 16) I just use liquid Alox and have pushed the same alloy with that lube close to 2000fps in the 30-30 with pretty good accuracy!

The 30-30 has a 1:12" twist the 6.5 a 1:7.8".

Not much to suggest changing then. That is a very fast twist the built into the Swede.