OK. I know that the boat tail has a better BC and therefore makes a better long range bullet. What I don't understand is what advantage the flat base bullet has over the boat tail at closer range. Does it deliver more power somehow? Thanks in advance for the vast sums of knowledge I'm sure will be downloaded into my brain.
Splain Please: Boat tail vs flat base
- Thread starter Brasso
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DMK
Member
I always thought that the BT just had better aerodynamics by creating less turbulance at the tail. 
P95Carry
Moderator Emeritus
Ok first .. I am not and do not claim to be a ''ballistician''. I am just a humble ole engineer.
However my take is this ....... boat tail has better rear end airodynamics .. if Schlerian (?spl) photography is used .... it appears that the turbulance and vortices are smoothed out .. and so probably contributing to reduced drag and stability ... long ranges more so IMO. Better BC as you said.
Now a plain base has one big advantage as I see it .. and that is to do with obturation. The gas seal is confined to the bullet base and there is no ''base burn-past'' as with a boat tail but airodynamically that plain base has more problems with vortices and turbulence.
I think it is all relative .. for instance I load a .243 round with a Speer 105 Spitzer ... plain base JSP. Now that bullet at 100 yds is tight as tight for groups ... better than a factory round with boat tail. And I doubt I'd have to push it beyond 200 yds. So no disadvantage for me there.!
However my take is this ....... boat tail has better rear end airodynamics .. if Schlerian (?spl) photography is used .... it appears that the turbulance and vortices are smoothed out .. and so probably contributing to reduced drag and stability ... long ranges more so IMO. Better BC as you said.
Now a plain base has one big advantage as I see it .. and that is to do with obturation. The gas seal is confined to the bullet base and there is no ''base burn-past'' as with a boat tail but airodynamically that plain base has more problems with vortices and turbulence.
I think it is all relative .. for instance I load a .243 round with a Speer 105 Spitzer ... plain base JSP. Now that bullet at 100 yds is tight as tight for groups ... better than a factory round with boat tail. And I doubt I'd have to push it beyond 200 yds. So no disadvantage for me there.!
Hmmm. I guess I just assumed that there must be some advantage to the flat base since it is still being used. If the boat tail was so much better I would think that there would be no need or demand for flat base bullets anymore. But I don't know, that's why I asked.
shoobe01
Member
Trans-sonic = Long range
Flat bases work fine (actually, great) at supersonic velocities (ie: right out of the muzzle on rifle calibers). Shock waves come cleanly off the trailing edges, just like the tip (and somewhere around 1/3rd of the way down the ogive).
As the bullet goes downrange it looses speed slowly. The trans-sonic region (Mach .95-1.05 or thereabouts) is fairly unstable, with shocks forming and dissipating. A sharp base will get shocks randomly off different sides, causing it to slew off course which equals inacuracy. This also causes more drag and it will drop speed more quickly, so less maximum range.
A boat tail is tapered enough at the back to smooth out those trans-sonic effects, and even fly nicely at subsonic speeds, so retains accuracy and gets some extra actual range as well. I can't recall exactly, but for .308 it seems this trans-sonic zone is past 800 yds, so most of us don't care. 1000 fps is /vry roughly/ the speed of sound, so look in your reloading manual ballistic tables for speeds at range to tell if it matters to you (if you are lucky enough to have a 1000 yd rifle range).
As far as I can tell, some BTs are popular just because they are on good all around bullets. There is nothing wrong with them for short ranges, so there ya go.
Flat bases work fine (actually, great) at supersonic velocities (ie: right out of the muzzle on rifle calibers). Shock waves come cleanly off the trailing edges, just like the tip (and somewhere around 1/3rd of the way down the ogive).
As the bullet goes downrange it looses speed slowly. The trans-sonic region (Mach .95-1.05 or thereabouts) is fairly unstable, with shocks forming and dissipating. A sharp base will get shocks randomly off different sides, causing it to slew off course which equals inacuracy. This also causes more drag and it will drop speed more quickly, so less maximum range.
A boat tail is tapered enough at the back to smooth out those trans-sonic effects, and even fly nicely at subsonic speeds, so retains accuracy and gets some extra actual range as well. I can't recall exactly, but for .308 it seems this trans-sonic zone is past 800 yds, so most of us don't care. 1000 fps is /vry roughly/ the speed of sound, so look in your reloading manual ballistic tables for speeds at range to tell if it matters to you (if you are lucky enough to have a 1000 yd rifle range).
As far as I can tell, some BTs are popular just because they are on good all around bullets. There is nothing wrong with them for short ranges, so there ya go.
You're forgetting something, Dave.
Flat-base bullets have the advantage of departing the muzzle with less chance of being skewed by off-axis propellant gases. The flat bullet base makes a clean departure break from the muzzle, there's no boat-tail in the bore that could cause tipping. The results? Greater accuracy. It turns out to be the darling of the benchrest competition crowd, at least out to 200 meters.
Boat-tail bullets, like the Sierra MatchKings and even nicer Lapua Scenars, fly through the air with considerably less drag than their flat-based cousins. This drag is stated in terms of the bullet's Ballistic Coefficient, and the higher the BC number, the better. The less drag a bullet creates, the longer it retains velocity over it's flight path. The boat-tail bullets, while susceptible to tipping or yawing at the muzzle, maintain their velocity further downrange than a comparable flat-base bullet.
So it's really a trade-off of accuracy vs. sustained velocity. For the 200-meter benchrest folks, they have plenty of velocity all the way to the target, so they'll gladly trade the aerodynamics of the boat-tail bullet for a more yaw-resistant bullet like the flat-base. For the 1000-yard Palma folks, the necessity of keeping the bullet supersonic all the way to the target outweighs the yaw-resistance and higher drag of the flat-base bullets.
That was probably about as clear as mud, wasn't it?
Flat-base bullets have the advantage of departing the muzzle with less chance of being skewed by off-axis propellant gases. The flat bullet base makes a clean departure break from the muzzle, there's no boat-tail in the bore that could cause tipping. The results? Greater accuracy. It turns out to be the darling of the benchrest competition crowd, at least out to 200 meters.
Boat-tail bullets, like the Sierra MatchKings and even nicer Lapua Scenars, fly through the air with considerably less drag than their flat-based cousins. This drag is stated in terms of the bullet's Ballistic Coefficient, and the higher the BC number, the better. The less drag a bullet creates, the longer it retains velocity over it's flight path. The boat-tail bullets, while susceptible to tipping or yawing at the muzzle, maintain their velocity further downrange than a comparable flat-base bullet.
So it's really a trade-off of accuracy vs. sustained velocity. For the 200-meter benchrest folks, they have plenty of velocity all the way to the target, so they'll gladly trade the aerodynamics of the boat-tail bullet for a more yaw-resistant bullet like the flat-base. For the 1000-yard Palma folks, the necessity of keeping the bullet supersonic all the way to the target outweighs the yaw-resistance and higher drag of the flat-base bullets.
That was probably about as clear as mud, wasn't it?
P95Carry
Moderator Emeritus
shoobe01 ... welcome!
Good first post! I should have remembered the trans-sonic aspect .. because had read about that. Nicely summed up.
Good first post! I should have remembered the trans-sonic aspect .. because had read about that. Nicely summed up.
Art Eatman
Moderator In Memoriam
For hunting, the difference in velocity loss within 300 to 400 yards is negligible. There is not enough difference in trajectory to matter.
One thing I have found with Sierra bullets--and this was verified by the Sierra people--is that in 30-caliber, the lighter (150-grain) boat tail has a bit of a thinner jacket than the flat-base. I found that the 150-grain BT will come apart when above around 2,800 ft/sec at the time of impact. This is less the case for their 165- and 180-grain BT bullets. (I shot a mule deer in the neck at 30 yards; the bullet didn't exit.)
I've noticed that the .243 Sierra 85-grain HPBT is rather explosive at closer ranges. Here again, a somewhat lighter bullet for the caliber...
FWIW, Art
One thing I have found with Sierra bullets--and this was verified by the Sierra people--is that in 30-caliber, the lighter (150-grain) boat tail has a bit of a thinner jacket than the flat-base. I found that the 150-grain BT will come apart when above around 2,800 ft/sec at the time of impact. This is less the case for their 165- and 180-grain BT bullets. (I shot a mule deer in the neck at 30 yards; the bullet didn't exit.)
I've noticed that the .243 Sierra 85-grain HPBT is rather explosive at closer ranges. Here again, a somewhat lighter bullet for the caliber...
FWIW, Art
Tony Williams
Member
From 'Basic Ballistics' on my website:
"The first problem is that the FF is different at subsonic and supersonic velocities, because shapes which work best at subsonic speeds are not the best at supersonic velocities. At subsonic speeds, the drag caused by the low-pressure area created at the back or base of the projectile is significant, and major reductions in drag can be made by tapering this to some extent (boat-tailing). At supersonic speeds, it is the nose shape that is critical; finely pointed noses are needed, but the back end doesn't matter so much. Some taper towards the base is useful, but the optimum taper angle is different from that at subsonic velocities. The benefit of boat-tailing at very long range can be demonstrated by two .30-06 bullets, both weighing 180 grains (11.7g) and fired at 2,700 fps (823 m/s). At sea level, the flat-based bullet will travel a maximum of 3,800m, the boat-tail 5,200m."
Tony Williams: Military gun and ammunition website and discussion
forum
"The first problem is that the FF is different at subsonic and supersonic velocities, because shapes which work best at subsonic speeds are not the best at supersonic velocities. At subsonic speeds, the drag caused by the low-pressure area created at the back or base of the projectile is significant, and major reductions in drag can be made by tapering this to some extent (boat-tailing). At supersonic speeds, it is the nose shape that is critical; finely pointed noses are needed, but the back end doesn't matter so much. Some taper towards the base is useful, but the optimum taper angle is different from that at subsonic velocities. The benefit of boat-tailing at very long range can be demonstrated by two .30-06 bullets, both weighing 180 grains (11.7g) and fired at 2,700 fps (823 m/s). At sea level, the flat-based bullet will travel a maximum of 3,800m, the boat-tail 5,200m."
Tony Williams: Military gun and ammunition website and discussion
forum
One of the earliest documented uses of BT was after WWI, the Army studied MG tactics as it was high tech. Julian S. Hatcher found that the stated range of the flat base .30 Ball ctg was quite a bit less than the book answer. Thru experimentation they were able to add some 1,000 yards to the range before the bullet augered in by changing to a BT bullet. This would be something to think about in aircraft applications for example where you might see a target at quite a long distance. Every bit of extra range would help. JMTC
Edward429451
member
Tell us what you know about rebated boat tails, those with experiance beyond reading about them.
I've only read about them and do have a box of Hornady 168g RBT Match bullets. Haven't loaded any yet. From what I understand, they're supposed to be the best of both worlds. A BT for it's aerodynamic advantages, and rebated for less tipping as it exits the bore. Do they work? Why didn't they catch on?
I've only read about them and do have a box of Hornady 168g RBT Match bullets. Haven't loaded any yet. From what I understand, they're supposed to be the best of both worlds. A BT for it's aerodynamic advantages, and rebated for less tipping as it exits the bore. Do they work? Why didn't they catch on?
Steve Smith
Moderator Emeritus
Gewehr's got it. The flat base causes some cavitation and therefore drag. Up to about 350 yards it's inconsequential, but beyond that, the bullet slows too quickly with the flat based bullets. Hence the boat tail.
sumpnz
Member
Everything I've read, and it's pretty much just that, read, is that the boat tails pros are: easier to seat in the brass when reloading, better long range (400+yard) performance; cons are: yawing at exiting of muzzel, inferior gas seal compared to flat base. Terminal performance differences should be negligable assuming all else is equal (overall construction type - i.e. FMJ, hollow point, or soft point; jacket thickness; velocity at impact; shot placement; etc).
At normal hunting ranges (<400 yards) the advantages of the boat would seem to be inconsequential from a ballistics standpoint. Otherwise, what matters more to you - ease of seating the bullet in the brass, or a better gas seal as the bullet is accellerated down the barrel. Not being a reloader, the former doesn't matter to me, and the potential detrimental effects of the latter don't matter too much more.
Bottom line, for me anway (YMMV), is that I don't give a hoot if my bullets are flat base or boat tail. I'll choose which ever seems best suited for a given application based on experimental results.
At normal hunting ranges (<400 yards) the advantages of the boat would seem to be inconsequential from a ballistics standpoint. Otherwise, what matters more to you - ease of seating the bullet in the brass, or a better gas seal as the bullet is accellerated down the barrel. Not being a reloader, the former doesn't matter to me, and the potential detrimental effects of the latter don't matter too much more.
Bottom line, for me anway (YMMV), is that I don't give a hoot if my bullets are flat base or boat tail. I'll choose which ever seems best suited for a given application based on experimental results.
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