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  #1  
Old 08-02-2020, 01:30 PM
K22 K22 is offline
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Default Cup Base vs Flat Base

I never understood the significance of a cup base bullet over a flat base bullet. Does one have an advantage over the other?
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  #2  
Old 08-02-2020, 01:36 PM
Bill K Bill K is offline
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I will take a stab at your question. My understanding is that the idea is the cup base will flare more than a flat one and cause better fit in the bore and give better sealing into the lands and grooves.
But I would also say, the difference is ever so slight to really make a difference.
A carry over from soft lead bullets first being done that way in the old muskets for tighter fit in the bore. Bill K
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Old 08-02-2020, 01:43 PM
df06 df06 is offline
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Default Cup base

I agree with Bill, especially the “carryover” comment. I doubt it make a noticeable difference at all.
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Old 08-02-2020, 02:48 PM
Dean2 Dean2 is offline
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One more vote for the perspective that cupped bases originated with lead bullets, originally from the days of Black Powder. It was done for two main reasons, bullet diameters varied during production as did the barrel diameter, both in production and as they wore from being shot. On cast lead, even in modern rifles, they still have a place in ensuring better obturation (the necessary blocking of the bore to ensure it is sealed so the expanding gases provide maximum thrust on the bullet). With round balls, this was accomplished by the patch, with some older style bullets they also "Paper Patched" them by wrapping thin paper around the bullet. Especially with muzzle loader type guns, the cupped base sped up loading greatly and made ramming the projectile home much easier in fowled rifle barrels. The cupped base was a way to provide the same gas containment as patches in a much easier to make and load bullet. A cupped base on a jacketed bullet would be a complete waste of time.

Last edited by Dean2; 08-02-2020 at 02:51 PM.
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Old 08-02-2020, 03:33 PM
pertnear pertnear is offline
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You don't hear anything about it anymore, but I remember a warning from the gun writers saying boat tail bullets would reduce barrel life because of hot gases cutting around the bullet before it sealed in the bore.
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Old 08-02-2020, 03:49 PM
Bill K Bill K is offline
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Quote:
Originally Posted by pertnear View Post
You don't hear anything about it anymore, but I remember a warning from the gun writers saying boat tail bullets would reduce barrel life because of hot gases cutting around the bullet before it sealed in the bore.
Maybe you don't hear much about it anymore, because the gun writer was all wet. Bill K
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Old 08-02-2020, 04:17 PM
pertnear pertnear is offline
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Quote:
Originally Posted by Bill K View Post
Maybe you don't hear much about it anymore, because the gun writer was all wet. Bill K
That's for sure!...LOL
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Old 08-02-2020, 04:34 PM
Daryl Daryl is offline
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The exposed base of a Nosler partition bullets actually do obturate somewhat. Due to this, the 150gr. and 180gr. Nosler Partitions actually shoot quite well in the smaller bored .303's - like the .311"/.312" groove dia. Ruger #1's.
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Old 08-02-2020, 08:45 PM
K22 K22 is offline
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Now that makes a lot of sense. RWS Subsonic 22lr. was where I first seen the cup base.
I think I remember seeing cup bases on Rem. 17 cal. bullets also.
So why would bullet makers offer jacketed flat base and cup base bullets in the same bullet weight if they don't offer much if any advantage.
I have some Blackhole 37gr. 20 cal. bullets that are cup base, but he also offers the same bullet in a flat base.
I should add that Blackhole bullets shoot great in my 17 Rem. and a Kimber 204.
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Old 08-02-2020, 09:49 PM
Bill K Bill K is offline
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Mostly a simple reason K22, marketing. Some like them and will buy them.
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