Win 284 - can't reach 2800 with H4831SC without ejector marks

[Matt P]

Pat
I've had quite a few 284's and built many for customers, nearly all of them shoot really well and are kind to the brass around the 2770 mark (about 49.7-50 of H4350) the difference between 2770 and 2830 (apparently the go to node) is almost nothing @1000 yards, in fact IMO the difference get lost in the noise of sighting, accuracy difference etc.
The 284 works really well but the case design sucks with the rebated rim which IMO leads to the primer pocket issues.
I'd give the 180 VLD's a go and don't be scared to jam the Hybrids, I have two SAUMS that love Hybrids jammed.
Regards
Matt P

 

[Bart B.]

s with the rebated rim which IMO leads to the primer pocket issues.

Why?

What's the difference in case brass behavior around the primer pocket between 284 cases and 7-08 cases when both are used at their respective SAAMI spec peak pressures?

 

[Alex Wheeler]

PSI, pressure per square inch. Rebated rims apply the pressure of a larger diameter to a smaller case head.

 

[Bart B.]

I don't think a 10% increase in force on the 284's case head from its larger pressure chamber diameter makes that much difference with equal pressure in both cartridges. The 7-08's spec'd peak pressure is about 9% higher than the 284.

 

[RonAKA]

All the new Nosler overbore hot rod cartridges uses the rebated rim design, so I doubt there is an intrinsic design limitation on head pressure.

 

[Matt P]

I did say "IMO". I stand by that and that is a contributing factor to primer pocket failure.
Matt P

 

[RonAKA]

I did say "IMO". I stand by that and that is a contributing factor to primer pocket failure.
Matt P

You are correct in that as the rim head face gets smaller the pressure on the face goes up. The question would be whether or not it goes up enough to yield.

And, as I think about it longer, if the brass in the head extrudes into the ejector pin port, then it has yielded! Now the question comes back to excessive pressure or weak brass...

 

[RonAKA]

View attachment 1012585

That bullet is seated a little deeper than I like to see. I prefer to see the bottom of the bearing surface in a boat tail to be midway up the neck. But, perhaps it is just the angle of the picture...

 

[dskogman]

You can measure the rim size to determine if the case is over pressured and primer pocket is blown. More than +.002 and it's gone.

 

[Matt P]

And, as I think about it longer, if the brass in the head extrudes into the ejector pin port, then it has yielded! Now the question comes back to excessive pressure or weak brass...[/QUOTE]




Ron
I don't think the Lapua 6.5x284 brass is as good a quality as some their other lines and over the years there has been some good batches and quite a few very average batches. I have gone away from the 284 for this reason and now use a 7mmSAUM and down load it to a "fast" 284 velocity. So far great accuracy with great brass life, jurys out on the barrel life (haven't shot enough rounds yet) but throat wear on par with the 284 and visual inspection looks about the same.
Matt P

 

[ImBIllT]

PSI, pressure per square inch. Rebated rims apply the pressure of a larger diameter to a smaller case head.

That's not quite how it works. A 7-08 and .284 will experience the same stress and strain at the primer pocket if fired at the same peak pressure. On the other hand, if the rim were not rebated, the primer pocket would have more support, and therefore would likely experience less STRAIN(and possibly less stress as you move farther toward the rim) when fired at the same pressure. The rebates rim does not cause any disadvantage compared to other large bolt face/large primer cases. It is only at a disadvantage when compared large bolt face/small primer cases, or to magnum bolt face cases.

 

[Alex Wheeler]

That's not quite how it works. A 7-08 and .284 will experience the same stress and strain at the primer pocket if fired at the same peak pressure. On the other hand, if the rim were not rebated, the primer pocket would have more support, and therefore would likely experience less STRAIN(and possibly less stress as you move farther toward the rim) when fired at the same pressure. The rebates rim does not cause any disadvantage compared to other large bolt face/large primer cases. It is only at a disadvantage when compared large bolt face/small primer cases, or to magnum bolt face cases.

It does work that way. When you rebate a rim, you take the lager bolt thrust of a larger diam case and put it on a smaller diam case head.

 

[ImBIllT]

That's not quite how it works. A 7-08 and .284 will experience the same stress at the primer pocket if fired at the same peak pressure.

It does work that way. When you rebate a rim, you take the lager bolt thrust of a larger diam case and put it on a smaller diam case head.

Nope. A pressure a vessel experiences equal pressure at all points. The deformation as a result of that pressure will depend on shape, and thus the larger case head will experience less deformation, but a .284 and a 7-08 will experience equal deformation at equal pressures.

 

[Alex Wheeler]

Nope. A pressure a vessel experiences equal pressure at all points. The deformation as a result of that pressure will depend on shape, and thus the larger case head will experience less deformation, but a.284 and a 7-08 will experience equal deformation at equal pressures.

"A pressure a vessel experiences equal pressure at all points." Agreed. That vessel being the case body. But when you take a larger area and reduce the point of contact you increase the psi. Its simple really. Take a weight say 10lbs. Its supported by a 1" area You have 10lbs pre square inch. Double that area and you have 5lbs per square inch or half the pressure on a given area.

 

[Alex Wheeler]

The smaller head must support more pressure for its area. Basic stuff.

 

[ImBIllT]

"A pressure a vessel experiences equal pressure at all points." Agreed. That vessel being the case body. But when you take a larger area and reduce the point of contact you increase the psi. Its simple really. Take a weight say 10lbs. Its supported by a 1" area You have 10lbs pre square inch. Double that area and you have 5lbs per square inch or half the pressure on a given area.

You took a force and applied it over an area. A pressure vessel contains a pressure applied over an area. It's the same pressure everywhere. That pressure is exerted at right angles to the surface it contacts. It is not magically directed from other parts of the case toward the case head. The pressure that deforms primer pockets does not come from different areas of the case. It comes from inside the primer pocket and is exerted at right angles against the primer and primer pocket. This is what causes primers to flatten and pockets to expand. If the pocket is small or the case head large, then the pocket has more support and experiences less deformation, but the .284 and 7-08 have identical shapes in the areas and will experience identical deformation at identical preasures.

 

[Alex Wheeler]

You took a force and applied it over an area. A pressure vessel contains a pressure applied over an area. It's the same pressure everywhere. That pressure is exerted at right angles to the surface it contacts. It is not magically directed from other parts of the case toward the case head. The pressure that deforms primer pockets does not come from different areas of the case. It comes from inside the primer pocket and is exerted at right angles against the primer and primer pocket. This is what causes primers to flatten and pockets to expand. If the pocket is small or the case head large, then the pocket has more support and experiences less deformation, but the .284 and 7-08 have identical shapes in the areas and will experience identical deformation at identical preasures.

I have to disagree. The pressure forced against the case head is fixed. The diameter of the head changes the force per square inch the head must deal with. A compression force along with the expansive force expand pockets. Its both. If a case head is under compression and brass is going to flow, which way do you think it will flow? In against 60k psi or out against 0?

 

[ImBIllT]

I have to disagree. The pressure forced against the case head is fixed. The diameter of the head changes the force per square inch the head must deal with. A compression force along with the expansive force expand pockets. Its both. If a case head is under compression and brass is going to flow, which way do you think it will flow? In against 60k psi or out against 0?

Disagree all you want.

 

[Whitehunter]

Does this apply to 6.5x284 brass when you expand up to 284 and fire from to 284 Shehane . I have never had a problem with primer pockets with the 284 Shehane. The only problem I have had was some bad brass and this was heaver brass in weight with loss of case volume.

 

[ImBIllT]

Re-examining my stance, the case is probably better represented as a piston than a pressure vessel. In that case, the larger case body diameter would result in more thrust against the bolt face. The smaller case head would thus be subjected to a greater stress. However, that stress would push the brass INWARD toward the primer pocket, and outward as well. The swelling of the primer pocket is still due to pressure from inside the primer pocket, and that pressure is equal to the pressure inside the rest of the case. I stand by my stance that the swelling of the primer pocket in .284 brass would be identical to or possibly less than that of a 7-08 exposed to the same pressure. Leaving the case head at magnum diameter would offer increased support, but the decreased support from the rebated rim simply puts it in the same class as the 7-08.