Gunsmithing bullet shape question

[obilly]

I posted this in the wrong forum first so sorry for the repost but...

I have had this question since training day 3 Norfolk VA. but never wanted to ask it because it seems like a dumb question. That said, I have used a number of surreptitious means to answer the question with no successes. So screw it, say what you will I'm damn curious…

Why does everyone stop at a boat tail for high end VLD rounds? why don’t we repeat the secant ogive on the back side of the bearing surface? It seems like the smaller the base and bearing surface the higher the BC so why not go to a point or very close?

It can’t possibly be because the gasses need something flat to push on (I sincerely hope)

My only guess is to leave space for powder but that seem like an easy thing to get around. It seems like the maker would need to drastically reduce the length of the baring surface to maintain a similar bullet weight which would mean one might to change seating depth to compensate but not by much and the COAL shouldn’t change much.

I'm thinking the round wouldn’t ever be popular in already over mag rounds like the 300 RUM where a 240 match king is already shoehorned in. However a lot of shooters actually find better accuracy in a 6.5-284 Norma after cooling off their loads and end up with piles of room left over for longer bullets.

I know the numbers would all change but would the round be more efficient? Better? worse? Why?
Thanks for your time.
External ballistics, internal ballistics, projectile, VLD, boat tail,

 

[Sniper Uncle]

I believe that it does have to do with having a surface for the gasses to push on. I have read, I think in a thread here, where a reloader damaged bullets purposefully to see what results he got, and how it affected the shots down-range. One of the most damaging effects was observed when he cut the base off at an angle. The bullet veered off in a few hundred yards, and started to self destruct, if I remember correctly...
Think about it, you are pointing the ogive, to reduce resistance, so if you did this on the back end too, what would the gasses push against??? They would tend to go around the base, and try to wedge in between the base and the barrel....

 

[damoncali]

If you make the boat tail too long, you wind up with a whole mess of stability issues. That's the main reason. That, and they'd be really hard to make.

It's not the area to push against - gas doesn't care and will happily push against a boat tail as as well as a base. Bore diameter is bore diameter.

 

[Sniper Uncle]

If you make the boat tail too long, you wind up with a whole mess of stability issues. That's the main reason. That, and they'd be really hard to make.

It's not the area to push against - gas doesn't care and will happily push against a boat tail as as well as a base. Bore diameter is bore diameter.

Fair enough. I stand corrected, at least partly. From what you're saying, it isn't in the bore, but afterwards where the trouble begins??? I do remember reading about serious stability issues from changing the base from the factory boat-tail.

 

[damoncali]

A long boat tail will necessarily mean a long bullet overall, and require a very fast twist to get gyroscopic stability, which is bad enough. According to Mr Litz's book, you will also quickly run into dynamic stability issues, which are even worse. Throw in any uncertainty in manufacturing such beast, and you get a big wobbly mess.

Where the base area comes into play is that the base drag is less on a smaller area, which is why we want boat tails to begin with. But there is an optimal angle in the 5-10 degree range. If you go past that, the flow doesn't follow the boat tail, negating its effect (you get base drag over the full diameter of the bullet because the flow separates at the beginning of the boat tail rather than at the end). If you don't go far enough, you don't get the benefits of the area reduction.

So long story short, the best boat tail is very approximately about .8 calibers long and somewhere in the area of 10 degrees.

 

[Sniper Uncle]

Very interesting. Thanks for the information.

 

[obilly]

thanks gents. That is exactly what I was looking for.