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  #1  
Old 10-16-2019, 11:54 AM
M829A3 M829A3 is offline
 
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Default Reduce loads for flat base vs boattail

If weight is equal, or perhaps slightly lower for the flat base, (such as M2AP vs BT of similar weight), do you reduce the powder charge to compensate for the increased bearing surface between the two?? Seems to some conflicting opinions, and some load table don't list flat base bullets in some of the weights.
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Old 10-16-2019, 12:47 PM
HighpowerRifleBrony HighpowerRifleBrony is offline
 
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If near max charge, I'd back off at least 0.5gr.
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Old 10-16-2019, 12:53 PM
ceresco ceresco is offline
 
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What is your objective? Do you want the same velocity, same pressure, what? M2 loads are so far down from maximum that the differences between flat base and boat tail have no safety implications and few other significant aspects. It is more a question of how that particular barrel likes various bullets. Good Shooting. ...
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Old 10-16-2019, 01:28 PM
rcolarco rcolarco is offline
 
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[QUOTE=M829A3;1877337]If weight is equal, or perhaps slightly lower for the flat base, (such as M2AP vs BT of similar weight), do you reduce the powder charge to compensate for the increased bearing surface between the two?? /QUOTE]

No
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  #5  
Old 10-16-2019, 11:23 PM
Bml Bml is offline
 
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In physics the amount of bearing surface is not a factor in friction force. However for a bullet, several other things may have a impact on pressure developed such as cutting the rifling over a longer distance and if the bullet construction allows pressure to act internally on the bullet jacket. That is why load development working from near starting loads is a recommended practice.
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Old 10-17-2019, 07:00 AM
bruce bruce is offline
 
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"load development working from near starting loads is a recommended practice." Bingo! No matter what, when developing a new hand-load, always start with a recommended starting load from a reputable source. Work up with an eye to accuracy and pressure indications. That is the golden rule of hand-loading. It applies across the board with all firearms. Do that and you'll have no worries with flat based or boat tail bullets. Sincerely. bruce.
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Old 10-17-2019, 02:53 PM
M829A3 M829A3 is offline
 
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It's not particularly aimed at M2, it was just an obvious comparison. I have an assortment of bullets, including some M2, and others, that are flatbase, and in weights that don't seem to fall into the common load tables, because they aren't boattail.

Now I have seen both yes and no to the question on the 'net and here. Just from observation, most flatbase have 2x the bearing surface. Not looking to reinvent the wheel nor looking to "Clark" a rifle. I think load development is pretty much sorted out as far as M1 rifles, so just looking for advice or experience, on a non typical scenario.
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  #8  
Old 10-17-2019, 04:22 PM
Unclenick Unclenick is online now
 
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It depends on whether you concern yourself with safety or with ballistic equivalence. There are three basic influences on the pressure to take into account. First in line is the bullet jump to the throat of the barrel. If a bullet is very close to or touching the lands, the peak pressure is increased as much as 20% or so over what the same charge weight of the same powder would produce with a spitzer nose bullet about 0.030" off the lands or so. Conversely, if you seat a bullet so deeply that it takes up a lot of case volume, the pressure will also be increased because of the powder charge starting its burn in a tighter space. Somewhere between those two extremes is a pressure minimus seating depth. Dr. Lloyd Brownell's 1965 study at the U. of Michigan that DuPont funded has plots of this effect for round nose bullets. The bottom line, though, is the effect of getting too close to the throat or contacting it is the same as raising start pressure. It lets the powder build more pressure before the bullet starts to move than happens when it is seated a few hundredths deeper. That speeds up the powder burn a little so it reaches the peak before the bullet has travelled as far down the tube as it would be loaded with some bullet jump.

The second influence already mentioned is how long it takes to finish extruding the bearing surface of the bullet into the throat until the rifling marks are complete. That, too, can raise starting pressure. It is this resistance that is much reduced by having bullets with a dry lube on them, such as moly or hex-form boron nitride. The loss of engraving resistance causes the powder to take longer to reach peak pressure, which lets the bullet move further and expand the volume behind it more before the peak occurs, and that results in lower peak value. Coated bullets usually need a few more tenths of a grain of powder to match velocity with uncoated ones and even more to arrive at the same peak pressure. The greatest increase in engraving force comes from changing bullet construction to a solid copper or gilding metal or bronze material, as these are harder than a thin jacket backed by soft lead.

The third influence is bore friction. When a bullet is under acceleration force from the propellant gasses, it experiences the same effect it would from sitting upright on a stationary platform about 50,000 miles from the surface of a neutron star; about 173,000 g at the pressure peak for an M2 bullet with 50,000 psi at its base. As you might imagine, with its weight 173,000 times greater than it is on earth, gravity would tend to distort the bullet to a shorter and fatter form. Well, that's what it experiences in a gun, except the steel sides of the bore constrain it from getting fatter. The force needed to constrain the sides of the bullet from getting fat are what produces the peak friction coincident with peak pressure. This is why copper accumulates fastest near where the bullet bearing surface is when the pressure peaks; usually an inch or two beyond the throat in high power rifle.

For ballistic equivalence, the easiest thing to do is use QuickLOAD or Gordon's Reloading Tool to model the load with your first bullet and then figure out the seating depth of the second bullet so it has the same jump to the lands the first bullet did, and then see what the software predicts for it by comparison. The model won't hit your exact pressure or velocity, but it does relative differences very well, so you can adjust your powder charge in the software to give the same velocity or the same peak pressure or the same barrel time, whichever you are going for, and get a pretty accurate charge adjustment. Nonetheless, you want to back the actual charge down by 10% off the published maximum and work up from there to your newly calculated value while watching for pressure signs. That's just normal reloading prudence.
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Last edited by Unclenick; 10-25-2019 at 09:46 AM. Reason: typo fixes
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  #9  
Old 10-18-2019, 09:03 AM
milprileb milprileb is offline
 
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Is this a M1 rifle question for 100 , 200, 300 yd accuracy, for service rifle XTC 200/300/600yd use ? I ask that because there is a ton of great loads for BT and Flat Base bullets to serve effectively. Also...you want score ring accuracy or do you want tight shot groups/ precision results.
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  #10  
Old 10-19-2019, 12:08 AM
M829A3 M829A3 is offline
 
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Not loading for competition. Looking for reasonable loads for 200 600 and maybe 1k. As I have stocks of various .308 bullets, I was just looking for some guidance as to BT , flatbase, interchangeability as far as powder charge.
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