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 Rifles, Bullets, Barrels & Ballistics

#15
05-03-2009, 03:54 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 4,223

From QT Sierra 240mk G1 BC is .699 under ICAO conditions from 2150fps
My math would predict .703 from measurements given to me, same conditions.
Now this is based on a calculated uniform specific gravity of 10.3, an OAL of 1.600", a meplat of .062, and a .760" nose with a 7.8cal tangent radius ogive.
A good enough twist for this at ICAO would be 9.5

If I drop USG to 8.9, changing only length to make 240gr weight, ICAO/G1BC = .695 and same performing twist would be 7.5
So BC would be lower and twist reqs go tighter...
If I change no dimension, weight goes to 208gr, BC to .608, twist to 8.5
I doubt there is any gain here...
Note, this 8.9 specific gravity, assumes copper only, and not a poor performing alloy I suspect Lost River may have used(if more brass-ish USG would drop toward 8.2)

But a turned bullet would likely be enhanced with a smaller meplat;
If I drop the meplat to .030" and lengthen as before for a 240gr copper SMK, I get a BC of .723 with 7.5 still a good twist.
If I use this meplat in the lighter bullet example(208gr), BC would be .634 and a good twist is still 8.5

So I conclude there is potential(if we ran out of lead), but not much, and who knows how well it would shoot...
#16
05-03-2009, 06:53 PM
 Platinum Member Join Date: Jan 2003 Location: The rifle range, or archery range or behind the computer in Alaska Posts: 3,830

Quote:
 Originally Posted by RockyMtnMT Did some loading yesterday to go out and do a little playing at distance. While loading saw this; 180g Nosler Ballistic Tip B.C.= .507 180g Nosler E-Tip B.C.= .523 Michael, does this go along with what you are saying, or not? I know that last year I chose to use the E-Tip over the 200g AB because the increased velocity of the 180g E-tip out performed the 200g AB out as far as I could shoot. I would have to run the #s again but I think about a 1000 yards. Not looking for a dispute, looking for knowledge. Steve
No dispute taken. Actually that is a very good question and a very good observation.

1st I will start by saying that mathematically the ACCUBOND or ballistic tip have a slightly higher BC than the E-tip. Slightly meening .001-.003

2nd, I have not shot them side by side over double chronies so I cant prove that is the case in reality. I can say however, I shot the 180 BT's over double chronies last week and found them at .524 This also corosponded with my drop tests yesterday. I was using 3220 for FPS, the current air density of both sessions and found .524 spot on both times using both methods.

In reality, both are most likely scary close to eachother. IMO, Nosler never had the BC of the ballistic tip or accubond right in the first place. The BC for the E-tip is most likely very close to .523 (published) where I believe the BT and AB is closer to .523 also.

The only way to know for sure is to shoot them both over two chronies to see which one is stronger. My bet is that they will be very very close if not t he same. The lower SG of all copper will lower the BC, the added length and subsequent weight of 180 grains will bring its BC back up. The length will only make up the difference in BC as far as the weight will allow. Both are still 180 grain bullets of very similar nose and boattail designs, thus they should give BC's dang near the same despite one being longer than the other. The added length only makes up for the lack of SG but does not exceed it.
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Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
#17
05-03-2009, 08:51 PM
 Official LRH Sponsor Join Date: Mar 2007 Location: Montana Posts: 2,979

That is very interesting. I only have a few of the E-tips left. Makes me want to test the two chronys with both bullets. I wasn't planning on purchasing more E-tips, wasn't unhappy with them, quite the opposite. Just was moving on to the next thing to try for this next hunting season. I took for granted the BC's published by Nosler. You are correct that the published BC's for the E-tip are very close. I don't have the ability to use conditions at location for my drops, I make them at home using my best guess for altitude etc. I have found them to be about a half moa or so low, but always figured that that was due to me and my lack of equipment.

Causes a lot of food for thought. I do however, very much like the on game performance of mono metal bullets. So much less blood shot, and none of the animals shot had a chance to complain about my choice of bullet.

Anyway thanks for letting me hi jack the thread.

Steve
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To hunt... or not to hunt...? What a stupid question.
#18
04-09-2011, 10:33 PM
 Silver Member Join Date: Sep 2007 Posts: 127

help guys, I made myself some copper bullets, I took the form of a 308, 178 grainers(a-max) and made them nearly identical, the same form and size, and ended up weighting 166.2 grains. if the amax bullet has a G1 of 0.495, how much bc should the 166.2 bullet will have?

I also made some "copies" of the nosler hpbt in 140 grains in 6.5mm and ended up weighting 123.5 grains. these ones shot sweet....very nice groups...same speed as the barnes....

how can I test for BC?

some guys are using brass to make their own bullets, is there any specific alloy that could be better?

Joe
#19
04-13-2011, 06:31 PM
 Silver Member Join Date: Sep 2008 Posts: 327

the problem in 30cal is the std twist rate barrel being limited 1:10, if you have a tighter twist rate barrel then you can design a copper bullet with a higher BC than existing lead bullets such as the 240gr SMK. To get the same SD in copper the bullet has to be longer and requires more rpm to stabilize it.

The problem is tighter twist rates and more rpms generally hurt accuracy as it increases the mechanical forces accociated with slight inconsistencies in material density and concentricity of the bullet. you will note competitive benchrest shooters generally use the slowest twist rates they can get away with largely for this reason.

Someone mentioned using tungsten inserts to improve the density of the bullet so that you avoid the stability problem... well thats fine but you run into the problems accociated with concentricity and axial center of mass much quicker. The more complex the bullets construction is the more places tolerances will create accuracy problems. Unless you can make the insert absolutely dead center, the slightest error in this regard will create very poor accuracy.

And, its of no advatage drilling a small center hole in a copper bullet and using a thin rod insert of tungsten thru the center. Sure you can achieve the same weight and length as the lead bullet, but your not improving the axial inertia by doing it like this. The bullet will still be less stable than the lead bullet. The heavy material needs to be at the axial extremities to get the improvement, so you end up with basically a copper jacket over a thick tungsten core like barnes has done and you end up with a VERY expensive bullet that doesnt shoot very well because the tungsten doesnt swage in the barrel like lead does...

SD isnt the only means of improving BC, the other half of the equation is bullet drag related to its form factor. There is much room for improvement in this area compared with the 240SMK... there is a far better argument to produce a bullet with same BC as the 240SMK, but only weighs 200gr which is entirely possible with improved design and reductions in drag.
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#20
04-14-2011, 12:02 AM
 Silver Member Join Date: Sep 2007 Posts: 127

having less drag is the hole purpose of my designs. in one of the designs I made what looks like a rebated boatail, its the one that works best, but I have not shot long distances with it...

If I need a hunting bullet I would try a design I saw in a french hunting magazine in a bullet made by Sauvestre where the put a hole in the rebated boatail and created a special cavitation efect that worked best for them. tough I never saw that ad ever again...

on the other hand how big should I leave the meplat?

this next week I'll start to work on a new design for a 338 bullet.. just to play with different ideas...

Joe
#21
04-14-2011, 09:59 AM
 Platinum Member Join Date: May 2008 Location: South of Canada and North of Wyoming Posts: 6,068

Joe, I think it's great you're making your own bullets! Wish I could do the same.
I have recently been experimenting with GS Custom and Cutting Edge mono metal copper bullets. Specifically in .308 cal, GS 177'a and CE 180's.

If you look at the diagram of the GS bullet in the link I have provided, you'll see they a have a unique (patented) driving band design. This allows them to get higher velocities than other bullets. The GS driving bands look similar to the Barnes TSX's, but the TSX's are not driving bands. They are just displacement grooves cut into the bearing surface of the bullet. I am getting about 100-150 fps more velocity with the GS bullets, than other bullets of same weight, including the CE bullets. The bore riding shank of the bullet (surfce between the driving bands) is .3005" in diameter. The Driving bands, which provide a pressure seal in the bore are .3075" in diameter.

GS has changed the 177's a little since this diagram. The nose is longer and the driving bands are located more to the rear and one less driving band and the meplat is a little narrower, about .068 by my measurement. The CE bullets meplat is about .050.

From what I've read the Ideal meplat for BC is about 15% of the diameter of the bullet which for a .308 would be .046 The question is, what's necessary for reliable expansion from shot to shot.

GS CUSTOM BULLETS - Specifications for use

Just some things to think about.

-Mark

Last edited by MontanaRifleman; 04-14-2011 at 10:30 AM.

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