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Taylor
01-18-2007, 07:51 PM
At what point do gas checks become worthwhile? When do they actually work,at what pressure or velocity make them useful? How much of an increase in pressure or velocity do they allow? Is there a formula for this? What is the basic guidelines? I use gas checks of course, and I understand the need for them.But how much more can I expect .

waksupi
01-18-2007, 09:30 PM
Taylor, once you get past around 16-1700 fps, they really come into play. I've shot GC'd bullets to around 2700, and know others have boosted them faster.

Bass Ackward
01-18-2007, 10:22 PM
At what point do gas checks become worthwhile? When do they actually work,at what pressure or velocity make them useful? How much of an increase in pressure or velocity do they allow? Is there a formula for this? What is the basic guidelines? I use gas checks of course, and I understand the need for them.But how much more can I expect .


Taylor,

GCs become worth while when ever accuracy is lost or leading begins. So there is no formula only trial and error in your gun.

The biggest mistake that cast shooters make is trying to think in terms of velocity.

You need to think in term of pressure. The reason I say this is because, if you are using Bullseye, you are going to need a GC a lot sooner than if you are using 4227 and the 4227 stuff is going to be moving down the pipe a lot faster than the Bullseye stuff. Pressure destroys far more lead bullets than velocity.

leftiye
01-18-2007, 11:59 PM
Yes, there is a formula for this. It is called the plastic limit of deformation of the metal, and calculates the pressure at which the lead lets go and extrudes ahead of the gasses. Not exact, but it is something like- 1400 X brinell hardness number. Gas checks are supposed to stop gas getting past the base of the bullet (don't always do it though), AND they will provide support against plastic deformation of the base of the boolit, which is VERY good for accuracy.

leftiye
01-19-2007, 12:06 AM
See thread 1422 X BHN; Relationship btwn BHN and velocity

Bass Ackward
01-19-2007, 07:20 AM
Yes, there is a formula for this. It is called the plastic limit of deformation of the metal, and calculates the pressure at which the lead lets go and extrudes ahead of the gasses. Not exact, but it is something like- 1400 X brinell hardness number. Gas checks are supposed to stop gas getting past the base of the bullet (don't always do it though), AND they will provide support against plastic deformation of the base of the boolit, which is VERY good for accuracy.


Well .... good luck with the formula. Just don't read my posts about on that thread. :grin:

9.3X62AL
01-19-2007, 10:45 AM
With all the variables involved, the "BHn x 1422" is best viewed as a guideline rather than a hard-and-fast rule. That seems to be the thrust of the thread mentioned herein, and I think that's a valid assertion.

Lyman mentions in their publications that 1400 FPS seems to be a "threshold velocity" where the gas check starts earning its wages. I like Bass' viewpoint that pressure is the better barometer of boolit strength, and that the pressure curve's dwell duration characteristics (that of fast powder vs. slower powder) has a role as well. To the extent that velocity is a result of pressure, a load's velocity infers some things about yield strength, but velocity is just a by-product of the impulse.

I place my own emphasis on dimensional integrity between boolit and throat/leade/barrel. Metallurgy comes second--and lube quality is third (I use Javelina Alox). I caution that I don't often run castings past 1800 FPS, so my work isn't real challenging to alloy or lubes. I don't run gas check boolit designs without gas checks, and I generally don't run plain-based boolits past 1400 FPS very much. I also don't run very many plain-based boolits down rifle barrels, either--the 45-70 being an exception to that practice. So, my mode of operating is relatively cautious and conservative in the context of "taxing" yield strength or lubricity. Others here are far more intrepid than I, and would have better info regarding the more extreme applications of cast boolits to the shooting hobby. Call me, OLD SCHOOL--with a vengeance.

sundog
01-19-2007, 10:57 AM
In that last bucket of WWs you might have some that are over 20 years old and some that were on a wheel in a show room last week. You don't know. What you also don't know is percentage of what's in them. So how can you attached a constant to a variable like that? Kinda like the Greenhill formula. It's a guideline or a starting place. A reference mark, so to speak, but not a benchmark. I don't have a problem with that. So from there you put a load together with your other swags and shoot up and down the load to find a sweet spot. Next time around, it may change.

Al, 'old school' is good school. sundog

44man
01-19-2007, 11:43 AM
The gas check can be needed a lot sooner then some think if the boolit is too small in diameter for the throat. It will prevent the base from being gas cut even though some gas can get past it and cause leading. A clean base can exit the muzzle straighter.
For higher velocities, the check will provide the boolit to take the twist without skidding. Soft PB boolits can skid and leave a slot for hot gas to go past and lead the bore.
A hard PB boolit that fits the throats tight and is oversize for the bore will do as good as long as the velocity doesn't get high enough to skid the thing when it hits the rifling.
Loose fitting checks have to be avoided because they can turn on the base until the rifling locks it, when they hit the rifling allowing the boolit to skid too much. You can lose the check at the muzzle too on some but not others, not good for accuracy. I would not be surprised if the loose check can come loose from the boolit as it enters the rifling since we can't ever see what happens.
The check will not prevent leading or bad accuracy if it is too loose, the boolit is too small, the lead is too soft for the velocity or the lube is no good.
Leaving a check off of some boolits can reduce the boolit's drive area that might be a mismatch for the twist rate as well as leaving a path for gas.
There is no one solution the the question you ask, you have to experiment yourself with what you shoot. I will not dig myself into a hole by telling you that you must do this or that. The check is not a cure all that will solve all problems.