Truncated cone or bore riding trucated cone ?

del1863
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Truncated cone or bore riding trucated cone ?

Postby del1863 » Tue Mar 18, 2008 9:28 am

I am designing a 300 gr bullet for my 460 S&W, it has a 70% meplate and a .050 foward band, my design know is using a truncated cone, would using a bore riding truncated cone ".001 step" give any advantage ? or would it be better to have the angel of the normal truncated cone hitting the throat forcing cone insted of the .001 ledge ?
Also, if i want to size to .452 what what should i set up my "as cast" dia. for ? how long is the wait once a mold is ordered ?
I am very exited about this mold and can;t wait to get on on order.

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mtngun
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Re: Truncated cone or bore riding trucated cone ?

Postby mtngun » Tue Mar 18, 2008 10:34 am

Depends on what you are trying to do.

If you are trying to maximize powder space and bullet weight, then a long bore riding section can be a useful way to pack weight onto the nose.

On the other hand, if powder space is not an issue, or if you have excess powder space, then a short truncated cone will, for a given weight, force the bottom half of the bullet to be longer, which can be a good thing up to a point because it eats up excessive powder space, increases bearing length, and increases lube capacity.

If your goal is to ensure reliable chambering, then either a truncated cone nose or a bore riding truncated cone nose can serve that purpose.

A 0.001" step is pretty meaningless since there will be 0.002" tolerance, plus it is common for the nose to be unintentionally bumped up by the lubrisizer. I'd suggest making a wheelgun bore rider about 0.002" larger than bore diameter, which might be 0.444" - 0.446" for the 460 S&W.

Is the 460's chamber so short that there is only 0.05" between the case mouth and the 0.452" throat? The reason I ask is that a 0.050" long front band will be troublesome to fill out with the humble wheelweight alloy that most of us use for big bores..

The Star lubrisizer supposedly works better when the bullet is a snug fit in the sizing die, so Star owners often like their bullets to drop about 0.002" over the sizing diameter.

RCBS/Lyman lubrisizers are relatively puny and the die quality is often poor, so these machines are not happy doing heavy duty sizing. If you plan to size in one of these, I would suggest spec'ing the bullet to 0.452"+.

"Bucket droppers" -- people who heat treat by letting the hot bullets fall from the mold into a 5 gallon pail of water -- like the bullet to drop out of the mold at the size-to diameter because the bucket dropped bullets don't like to be sized. Sometimes the perfect as-cast diameter is wishful thinking, though.

I normally let my bullets air-cool, then size in a push-thru die, then oven-treat, and then lube in a lubrisizer. Since the soft air-cooled bullets are easy to size in a good push-thru die, I prefer the as-cast diameter to be plenty big. The oversize bullets allow experimenting with different sized diameters, and it insures that the bullets will be "big enough" despite normal casting tolerances. If this were my bullet, I would spec it 0.453"+.

del1863
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Joined: Tue Mar 18, 2008 9:16 am

Re: Truncated cone or bore riding trucated cone ?

Postby del1863 » Fri Mar 21, 2008 9:24 am

Thanks for the good information. i have been casting my own bullets for a long time, but this is the first time i have designed a custom mold for a spacific gun. I am not a bucket dunker, I size then oven-harden the lube, and i do use a RCBS luber-sizer

The alloy i use is 96% lead, 6% tin and 8% antimony, any thoughts on this alloy.

what do you feel is the idea "free bore" distance between the foward bearing area of the bullet and the start of the cylender forcing cone ??? keeping in mind that the 460 s&w id a very high pressure cartridge. I'm in the prosses of measuring it but the distance betwen the forcing cone and the case mouth seems to be quite sort. thank you for you imput, i look foward to hearing from you.

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Re: Truncated cone or bore riding trucated cone ?

Postby mtngun » Fri Mar 21, 2008 12:58 pm

Did you mean 86% lead, 6% tin, and 8% antimony? That is in the neighborhood of linotype. If you can get wheelweight in your neighborhood (it's getting hard to find in my neck of the woods !!!) I'd suggest using wheelweight for your 460 and saving the 86/6/8 for 22 caliber through 7mm. Oh, your alloy will cast great in my XL aluminum mold at your 300 grain weight, and it should shoot fine, too, but it seems like a spendy alloy for a handgun bullet. But suit yourself.

I want wheelgun cartridges to drop into the chamber effortlessly. If you have to give the cartridge a push to get it all the way in, that is not acceptable, in my book, for a hunting/defense weapon. I would aim for 0.01" - 0.02" "jump" before the front band contacts the constriction in the chamber. Bear in mind that you will always have some variation due to case length, crimp die adjustment, and residual lube in the crimp die. I like to err on the side of reliability.

Remember to add a portion of the crimp groove, about 0.015", to the length of the front band, when calculating the bullet's fit in the chamber. For example, if the front band on a TC nose is 0.100" long, then the front band will protrude about 0.115" above the case mouth.

The program's "rifle throat length" estimate does not work on wheelguns. The rifle throat length calc is based on bore diameter, whereas a wheelgun's throat is approximately groove diameter.

An alternative approach is to size the bullet 0.0005" smaller than the throats. In theory, this allows the bullet to easily enter the chamber with a glove fit. It may not work out so great in practice, though. It is common to unintentionally "bump" the nose in the lubrisizer, making it bigger than intended. Even if you ensure to size the nose, there will still be some diameter variation. I gave up on the "glove fit" approach because I could never get it to chamber reliably 100% of the time, and the "glove fit" bullets didn't shoot any better than a deep seated TC.


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