Reduce loads for flat base vs boattail

[milprileb]

Thank You for that focus.

Not sure what weapon you use ...bolt or semi auto but my experience is BT bullets are the only way to go once you leave 600 yd line. That goes for 3006 , 308, 7.5 Swiss and 303 Calibers.

Generally BTHP are more accurate at 300 /600 for me but the accuracy (groups) gained by flat base bullets can be quite respectable if the rifle and hand load agree. Case in point: shooting clay skeet birds at 300 and 600 yds, my 7.5 Swiss K31 (scoped) will bust single birds with flat base 150 gr FMJ bullets if I hold steady, otherwise, I dance rounds all around the clay bird. If that was on a paper target, I'd be easily in 10 ring.

[Unclenick]

Flat base open nose bullets (not the open base types) tend to be more accurate at shorter ranges for several reasons, but once you get out to where wind deflection is getting significant, the lower BC that boattails have keeps them from deflecting as far in the wind. That is is why I find boattails easier to keep on target from about that same 300 yards distance and beyond.

As to why a solid flat base would shoot better at shorter ranges:

• Bryan Litz points to the fact it is easier to form a perfectly symmetrical square base from the bottom of a jacket cup than it is to make a perfectly symmetrical boattail from it.
  
• It is also the case that a flat base bullet the same weight as a boattail typically is the shorter of the two, giving it a higher gyroscopic stability factor for the same velocity in the same rifling pitch as its boattail counterpart gets.
  
• That same-weight flat base bullet typically has a longer cylindrical bearing surface portion, making it tend to straighten in the bore more completely than its boattail counterpart does, mitigating drift from the eccentric spinning of the center of mass that an in-bore tilt causes.
  
• Initial yaw in flight can be smaller with a flat base, as it clears the muzzle all at once and is not exposed to muzzle blast irregularities from crown irregularities for as long as a boattail base is while clearing the muzzle from heel to toe.
  
• Finally, a flat base bullet tilted during initial yaw exposes less angled surface area to the muzzle blast stream, offering less opportunity to acquire a lateral drift component to its trajectory from that cause.
  
• But no question, once the wind rears its ugly head, a smaller BC means more windage clicks are needed to keep on top of it.

All the above assumes same-shaped ogives on both bullets. Secant ogives seem to be more easily tilted in a bore and require more attention to producing a concentric cartridge.

[M829A3]

Thanks everyone!

[Unclenick]

Incidentally, looking at the two interior ballistics programs, there is no argument for bearing surface length in either one, suggesting it has not been found to affect pressure predictions significantly. All bullets of similar construction have the same start pressure in these calculations. On rethinking it, I suspect that once the leading edge of the bearing surface starts into the rifling, the remaining portion is met while the bullet speeds up, at which point the difference is likely rather smaller than the inertial reaction force from accelerating the bullet mass, so its difference is lost in the noise of normal shot-to-shot pressure variation.

[M829A3]

This was my initial conclusion as well. I was looking for validation of it, but nothing was conclusive. It seems to be similar to the AP is harder on the bore, because it doesn't "squeeze" as much. But nothing other than hearsay for evidence.

[britrifles]

I was curious of how flat base and BT bullets of the same weight shot thru my No. 4(T) with my standard match load, particularly what the difference was for measured velocity. Not that velocity really means much, but if the longer bearing surface might affect it.

I did the test in two ways:

1. Pull the 174 gr Mk VII flat base bullet from service rounds and sub the 174 SMK, compare to service load.
2. Load the pulled Mk VII flat base service bullet with my standard match load, 40.0 gr. Varget, and compare to my match load with 174 SMK.

I measured about 50 fps higher MV with the SMK in both comparisons. Now, that might be because the SMK is .311 and the MK VII is .312 diameter.

The Mk VII bullet is a hard point open base bullet. It actually has a respectable BC because core of the bullet nose is aluminum, making the bullet longer than the SMK.