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Old March 13, 2000, 09:34 PM   #1
Colduglandon
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I was just wondering. What affect does the shape of the bullet have on the way the bullet behaves while in the barrel. Apart from going out the other end, how does their shape affect their performance. Specifically does the typical FMJ boat tail round expand less in the barrel than the old style flat based bullets. Some bullets are fully covered by the jacket including the bottom, others are not. Any thoughts on the matter. I realize that a lot of the design difference has to do with the areodynamics and what happens when it impacts. How much has to do with what happens up front.
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Old March 14, 2000, 08:12 PM   #2
Art Eatman
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If the bullet diameter is the same size as the bore diameter--and it should be--there can be no expansion within the bore.

If the bullet is a slight bit smaller than the bore, it will be upset slightly by the initial heat and pressure when the powder ignites. In this case, a flat-based bullet will be more likely to expand to bore diameter than any boat-tailed bullet.

One of the bad things in rifles--or pistols--would be for the flame front to be able to get past the bullet. It would erode the bullet in a non-uniform manner somewhere along the jacket as it did this, and thus ruin any hope of accuracy. It would be even worse for lead bullets--which is why lead bullets are commonly one or two thousandths of an inch larger than the bore diameter.

FWIW, Art
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Old March 14, 2000, 09:51 PM   #3
Herodotus
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I surely cannot give the definitive answer on this, but there are (where?) people who believe that a boat tailed bullet wears out a barrel faster than a flat based bullet precisely because it lets more hot gas slip by the bullet. It would take an awsome shooting experiment to verify this.
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Old March 14, 2000, 10:13 PM   #4
Paul B.
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Elmer Keith mentions something to the effect that he did some research on that while working at an arsenal during WW-2. Had something to do with the .50BMG barrels burning out or something. Sorry, I don't remember the details.
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Old March 14, 2000, 11:18 PM   #5
Colduglandon
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I had the notion that boat tail bullets might be more likely to let gas escape. Also aren't the Barnes type bullets more likely to "form" or "shape" themselves to the lands and groves because they are made from a softer material than regular jacketed ammo, expansion is probably not the right term. Like cast bullets the reloading manuals call for less powder with these bullets.
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Old March 22, 2000, 12:38 AM   #6
Colduglandon
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Reloaded some ammo and tried it last week. 150 IMI boat-tails keyhold through the paper at 50 yrds. 150 round nose that I use in my 30-30 shot well using same powder charge. Used a micrometer to check the diameter of both types of bullets, no difference. I would have to work up a load to see if I can group them better. Looks like the old bore does not like the boat tail rounds. Used a collet die to reload. First time, nice not to have to lube the cases.

[This message has been edited by Paul Morceau (edited March 22, 2000).]
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Old March 23, 2000, 09:18 AM   #7
Art Eatman
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If your bullets are keyholing at 50 yards, you have some strange sort of problem. From my '06, I get equally tight groups with 150-grain boat-tails or flat-bases.

I do my first sight-in at 25-yards, and then go to the 100-yard target. I've never had any key-holing, ever.

Do you know the rate of twist in your barrel? I'll do some guessing--correct me if I'm wrong: Isn't the BT bullet longer than the RN? So you might need a faster twist to stabilize the BT? Ever load 110-grain, and get nice, tight groups?

FWIW, Art

FWIW, Art
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Old March 24, 2000, 09:27 AM   #8
Southla1
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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Herodotus:
I surely cannot give the definitive answer on this, but there are (where?) people who believe that a boat tailed bullet wears out a barrel faster than a flat based bullet precisely because it lets more hot gas slip by the bullet. It would take an awsome shooting experiment to verify this.[/quote]

There was a lot of work done in this area by a combine of the US Army (Aberdeen MD.) and the old US Springfield Armory. If memory serves me correctly it was Garands that were used and the results of the test were inconclusive. No appreciable difference between either a BT or FB bullet. It was discovered however that a barrel was more likely to show sings of wear depending on the types of powder sued. Ball powder being easier on barrels, but in my opinion its a lucky man that gets to shoot enough to be affected by the slight difference in the powders.

------------------
Carlyle

[This message has been edited by Southla1 (edited March 24, 2000).]
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