Ian
Notorious member
So you cast a bullet with a perfectly sharp, uniform base edge, being careful to maintain ideal sprue plate temperature, pour rate, venting, and sprue puddle size. You select a mould which requires the least amount of resizing for the application in order to minimize base flashing. You cull every dinged base. Then you stuff the bullet in a case and blow it through a rifled tube with enough pressure to permanently deform the whole bullet slightly....and probably expect the thing to fly true.
I know I do.
But what about when it doesn't work? What do we change? What is the failure mode? Plain-based or flat-based bullets are still a bit of a mystery to me in how they manage to group well at all. Any base defect from casting or handling will cause the bullet to diverge from the path of its more perfect brethren, probably as a result of gas force acting upon the irregular base and inducing a non-linear force vector, i.e. tipping or yaw. Regardless of how uniform the bullet base may be when the nose is introduced into the rifling, it doesn't remain so as the barrel lands slice across the base band and displace metal. We are left with a little bump and a trailing flag or nib at each land engrave point of our bullets, and there is so much to go wrong with this process with regard to uniformity. If the circle of the bullet base is the least bit eccentric with the groove center, more displacement will occur on one side than the other. Some of the nibs will be less robust than others, potentially breaking off altogether, leaving our bullet with tail fins in one place and not in others, compounding the yaw at muzzle exit and exacerbating dispersion down-range.
Assuming the irregular displacement being the cause of at least some woes, how would we mitigate this with our bullet design? A bevel? A convex radius? A step? A boat tail?
If you were thinking "gas check", well, so was I. I've recovered a lot of gas checked bullets and at the times when I was able to correlate them reliably with fired groups with various loads, the ones with the least base edge deformation and least trailing edge displacement invariably shot the best. Also, placing a hard, thin card or plastic wad disc beneath a plain-based bullet almost always vastly improves groups, especially with an alloy and powder combination that tends to cup the whole bullet base. Buffers also tend to make a similar improvement to plain-based bullet accuracy. But what if the application is of relatively low pressure, low velocity, large caliber, and an extremely tough alloy and/or gas check isn't required or desired? Is there some design feature we can add to a bullet's base to make it shoot better, or is a flat, sharp base the best possible configuration?
I know I do.
But what about when it doesn't work? What do we change? What is the failure mode? Plain-based or flat-based bullets are still a bit of a mystery to me in how they manage to group well at all. Any base defect from casting or handling will cause the bullet to diverge from the path of its more perfect brethren, probably as a result of gas force acting upon the irregular base and inducing a non-linear force vector, i.e. tipping or yaw. Regardless of how uniform the bullet base may be when the nose is introduced into the rifling, it doesn't remain so as the barrel lands slice across the base band and displace metal. We are left with a little bump and a trailing flag or nib at each land engrave point of our bullets, and there is so much to go wrong with this process with regard to uniformity. If the circle of the bullet base is the least bit eccentric with the groove center, more displacement will occur on one side than the other. Some of the nibs will be less robust than others, potentially breaking off altogether, leaving our bullet with tail fins in one place and not in others, compounding the yaw at muzzle exit and exacerbating dispersion down-range.
Assuming the irregular displacement being the cause of at least some woes, how would we mitigate this with our bullet design? A bevel? A convex radius? A step? A boat tail?
If you were thinking "gas check", well, so was I. I've recovered a lot of gas checked bullets and at the times when I was able to correlate them reliably with fired groups with various loads, the ones with the least base edge deformation and least trailing edge displacement invariably shot the best. Also, placing a hard, thin card or plastic wad disc beneath a plain-based bullet almost always vastly improves groups, especially with an alloy and powder combination that tends to cup the whole bullet base. Buffers also tend to make a similar improvement to plain-based bullet accuracy. But what if the application is of relatively low pressure, low velocity, large caliber, and an extremely tough alloy and/or gas check isn't required or desired? Is there some design feature we can add to a bullet's base to make it shoot better, or is a flat, sharp base the best possible configuration?
But I'll play. I don't think we can say flat out one is always better. I really like bthp bullets, but have some insane flat base loads too. I think to determine which is better for any one design would require making a mold with each base configuration with all else being the same. 
