2500 fps w/gas check .308 What Alloy will I need????

[sawinredneck]

Thank you both, most certainly you, Larry! I get it now. As a machinist I see it! Chuck up some 1/2" alum in a lathe sticking out 6" from the chuck at 2400rpm, life's fine. Do the same with 4' sticking out and it's going to bend and you WILL have a bad day! Watched a guy lose a nearly new Kennedy toolbox pair because he had too much material hanging out the back of the lathe and with the high rpm it "whipped" and landed on his toolbox!
I hadn't factored in the helix! Damn, you are smart!

[Larry Gibson]

You might also find this helpful;

http://castboolits.gunloads.com/show...velocity-chart

[gtgeorge]

This all made me think but what I am having trouble wrapping my mind around is if the RPM threshold is reached spinning a bullet then why is it downrange at extra distances the problem gets worse. If accuracy is fine att 50 or 100 yards wouldn't the bullet spin have slowed down along it's flight to 200 and 300?

[Oklahoma Rebel]

larry, thank you for the support, one question... what boolit design do you think woud hinder me in this quest lol? this is the on I have picked out-http://www.accuratemolds.com/img/bullets/detail/37-285C-D.png

[Oklahoma Rebel]

i also plan on getting the same design in 250gr, in a combo mold i think 2300 for the 250 and 2100 for the 286gr is a reasonable goal...and should really put the smackdown on some hogs, and one day i want to take it to go moose and brown bear or grizzley bear hunting. ps i plan on using h-4895, i would go for 4350 but with a 20.5 in barrel i want the powder to burn fast enough to be efficient and completely burn by the time the boolit leaves the barrel.

[sawinredneck]

You might also find this helpful;

http://castboolits.gunloads.com/show...velocity-chart

No, its best you keep it simple for many of us, getting 3-6hrs of sleep for many yrs, heavy narcotics on top of that, well, 7yrs and you get me.

[Hickok]

Keep it going. I am not here to detract from anything. I am here to learn!!!

[Hickok]

I have been planning to shoot my .308 loads out at 200 yards. Getting 2100 fps and around 2 inch groups at 100, the groups could be 8 or 10 inches at 200, (or Worse) since the RPM threshold causes groups to be non-linear. I believe this is how I understand it.

[Larry Gibson]

larry, thank you for the support, one question... what boolit design do you think woud hinder me in this quest lol? this is the on I have picked out-http://www.accuratemolds.com/img/bullets/detail/37-285C-D.png

Both of those bullets are "bore riders" having less than 50% bearing surface (that which is "groove diameter" in the bore) in relation to the bullet OAL. That means there is a whole lot of basically unsupported nose mass that can and will bend or tilt the bullet while in the bore under the intense acceleration needed for HV. Even though the bullet may be a tight fit in the bore most barrel bores have less than 50% (usually 35% +/-) bore area, the rest (50 - 65%) is groove area with .004 +/- side movement possible.

A bullet for HV cast bullet use is best served by 65%+ bearing surface with minimal nose. The NOE 30 XCB, Lyman 311466 and Accurate 30 XCB-FN are good examples. The problem with long bearing surface cast bullets is that most often the chamber throat is to short thus the GC gets seated below the case neck. That's not good at the high psi, high gas volume needed to push the bullet to HV. The larger % of the bullets mass that is in the nose the greater leverage it has to bend or tilt the bullet under acceleration.

That's why the 30x57/30XCB cartridge and the 35x57/35 XCB cartridges were designed with '06 length necks to utilize bullets with minimal bullet mass in the nose and long bearing surfaces. The GC and lube grooves are contained within the neck. With short necked cartridges the chamber should be throated so the bullet is seated with the GC at the base of the neck. The front drive band should then just kiss the leade.

Of the 2 Accurate bullets asked about I would select the 37-285D IF GC'd as it has the longest bearing surface in relation to the bullet OAL. With that bullet well cast and using the proper slow burning powder in the 14" twist you get pretty high velocity, at least to the velocities you have mentioned.

[sawinredneck]

This all made me think but what I am having trouble wrapping my mind around is if the RPM threshold is reached spinning a bullet then why is it downrange at extra distances the problem gets worse. If accuracy is fine att 50 or 100 yards wouldn't the bullet spin have slowed down along it's flight to 200 and 300?

Took me a bit to grasp. It's not the rotation from the rifling, it's the bullet being out of balance mixed with the rifling. The bullet creates two circles leaving the barrel, that created by the twist, and that by out of balance. The farther from the barrel you get, the bigger that outer circle gets.

[Smoke4320]

^^^^in a manner of speaking correct ..

[Larry Gibson]

Took me a bit to grasp. It's not the rotation from the rifling, it's the bullet being out of balance mixed with the rifling. The bullet creates two circles leaving the barrel, that created by the twist, and that by out of balance. The farther from the barrel you get, the bigger that outer circle gets.

Smoke4320 is correct......."in a manner of speaking correct .."

[gtgeorge]

Took me a bit to grasp. It's not the rotation from the rifling, it's the bullet being out of balance mixed with the rifling. The bullet creates two circles leaving the barrel, that created by the twist, and that by out of balance. The farther from the barrel you get, the bigger that outer circle gets.

^^^^in a manner of speaking correct ..

Okay if that is the case it still doesn't seem right that the group or lack of group would grow non linear. I must still be missing something here. I wiull go back and read Larry's other posts on the subject to see if I can grasp this idea better.

[oldblinddog]

Okay if that is the case it still doesn't seem right that the group or lack of group would grow non linear. I must still be missing something here. I wiull go back and read Larry's other posts on the subject to see if I can grasp this idea better.

It's not like a wobbling football. It is like one of those bottle rockets on the 4th of July that goes spiraling out of control (and somewhere else than you aimed it). The football may still be on target despite the wobble but the bottle rocket never will be.

[gtgeorge]

It's not like a wobbling football. It is like one of those bottle rockets on the 4th of July that goes spiraling out of control (and somewhere else than you aimed it). The football may still be on target despite the wobble but the bottle rocket never will be.

But then how can you have a good group at shorter ranges? That's the part that is confusing me is the need for 200 and 300 yards etc. to show the groups growing non-linear. Maybe as the speed of the spin starts slowing down it is no longer stabalizing enough and the wobble grows worse?

I wish someone with high speed cameras would be able to capture it to show exactly what is happening.

[OS OK]

Think of it liken to the old washing machines going into a spin cycle...if they were out of balance they'd start wiggling and wobbling and the faster they went in the spin cycle, eventually they'd be whacking the dryer or the wall behind the washer if you didn't go redistribute the clothes.

Same comparison...the faster that tub spun (RPM) and the longer it ran (going downrange) the wider the circles it made (the more open the groups).

Now, that makes perfect sense to me...but . . . being a Californian, I might not reason like the rest of ya.

[sawinredneck]

Smoke4320 is correct......."in a manner of speaking correct .."

I was trying to simplify it.

[oldblinddog]

But then how can you have a good group at shorter ranges? That's the part that is confusing me is the need for 200 and 300 yards etc. to show the groups growing non-linear. Maybe as the speed of the spin starts slowing down it is no longer stabalizing enough and the wobble grows worse?

I wish someone with high speed cameras would be able to capture it to show exactly what is happening.

The bottle rocket analogy was meant to give you a visual to compare to the wobble that you had pictured in your mind previously. The bullet remains stable in its spin throughout, but arcs in a spiral trajectory which grows larger as the bullet goes downrange. This is caused by the center of gravity acting against the center of pressure of the bullet in flight. The rpm imparted by the rifling will not decrease appreciably, at usable ranges, to destabilize the bullet. It is the imperfections of a cast bullet that work against accuracy and the faster the bullet the more exaggerated the imperfections become. There are many possible imperfections that work against success with cast at high velocity. Many of them have been pointed out here by Larry and others. Each one must be addressed to achieve success.

[mnewcomb59]

I've got some stupid questions about the RPM limit and shot groups, so please bear with me.
Why does the RPM threshold affect cast more so than jacketed?

Why do you think groups open up so much more going from 100 to 200yds because of the RPM threshold?

I really appreciate the work Larry has done, as well as many others here. Lead alloys are softer than jacketed bullets. RPM is a convenient umbrella term for two main issues: bullet skidding, and nose slump. I believe Larry's "RPM limit" is a very real thing, but I would guess that different calibers and different sectional density bullets within those calibers have different RPM limits.

bullets can skid on the rifling, and this lets gas blow by. Or if the skid is stopped by the gas check, there is still metal displaced on the first few driving bands. Over spun bullets usually elongate, sometimes into a corkscrew shape.

All rifled lead bullets experience nose slump. When it nearly instantly gets accelerated to 100-200k rpm, the nose slumps sideways towards the barrel walls. Kind of like when you were a kid on the merry-go-round and your head sucks towards the outside from the centrifugal force.

[Oklahoma Rebel]

[There is a video on u tube that would show him exactly what it looks like in high speed footage, I would look up de-stabilized bullet or something