Meplats vs Velocity

[KnifeMaker]

Not Scientific I know, but I am having very-very good results with the .257 NOE 80 gr flat point after I milled down the mold to throw a 59.9 bullet. It seems that it is right in your sweet spot with your figures!

It is accurate, and really packs a punch on game with it's wide metplat shooting it abut 990 fps at the moment.

More testing was on the table for this weekend, but sadly, the valve collapsed. But it has had quite a lot of rounds through it.   

So far, so good!

Knife

[Nomadic Pirate]

nothing beats a good Meplat for hunting

[dyotat100]

Okay I received my new copy of the Lyman 311316 from accurate.
It's a little on the light side. Drops at 112.3-5 gr. Original is 116.5 gr.  I had Tom shoot for a .309" diameter and it is a little over .310". Original is .312"+.

I went up to do some testing. Reg is set at just over 3000 psi and it is shooting 970 fps at the muzzle and 899 fps at 100 yds with the 112.3 gr. I forgot my 118 gr pointed on the bench because I pulled them out to compare them to the new 311359 I got.

Lyman is not very consistent in what they make. The new 311359 is dropping at 128 gr instead of 118 gr like my old mold. After comparing the 2 it is .050" longer and all of it is in the gas check. So when I have Eric hollow point the mold I will have him deck the mold down .048" to get it to 118 gr. I stuck a couple in a collet on my mill and took of that much to get it at 118 gr.

I tried shooting the .257 through the chrony at 100 yds also but for some reason the numbers weren't consistent through the chrony at 100 yds. .308 was all within 7 fps. At the muzzle it was right at 1100 fps with 67 gr and 1065 fps with the 74 gr.

Now I will send the 311359, 311316 copy, and my NOE 80 gr to get them hollow pointed. I have around 600 of the 311359 to shoot until it comes back and I have a 4 hole 311359 mold.

Next I'm having NOE make a shorter version of there 80 gr. Going to send them a few sample of the ones I have from a mold that has been cut down.  They will make it smaller by just not putting the cutter in as deep.  The 67 gr works real good at 1100 fps.

[rsterne]

The only data I can see in there with two velocities is the 112.3 gr. Lyman 311316.... I assume your MV is taken at 1 yard, making the distance between the two measurements 99 yards?.... Using that  distance, 970 fps at the muzzle and 899 fps downrange, and using the standard G1 ballistics model, the Ballistics Coefficient works out to 0.223 using ChairGun and 0.228 using the JBM calculator, which is quite good for a Sectional Density of 0.169.... If your MV was measured further from the muzzle than 1 yard, the BC will be lower.... I understand that the Lee C309-113F is very similar (and it shoots well in my .308 cals) and it has an advertised BC of 0.199, but that would likely be supersonic, so no direct comparison....

I will be very interested when you get the chance to test the .308 Spitzer.... and the .257s as well....

Bob

[dyotat100]

That was pretty much the muzzle  or a couple of inches. I was tethered so I just sat it on the table and stepped back a little to shoot thru it.

Nothing like shooting through your chrony at 100 yds. Wind was blowing and moved the chrony a little so I think that is why the .257 was off.

I'm going back up there Wednesday to weld his trailer and I will bring the guns also.

[rsterne]

The interesting thing about actual testing to determine the BC is that it doesn't matter what ballistics model you use to calculate the numbers.... You have to use the same model for all bullets you are comparing, of course, but if they start at the same velocity, the one travelling faster at 100 yards has the better BC.... Whether the number you get is 0.200 using the G1 model or 0.160 using the RA4 model, or 0.088 using the G7 model, if another bullet has a higher downrange velocity, it will have a higher BC.... Since the G1 model is the "standard" I think it makes sense to use that though....

Bob

[KnifeMaker]

Dyotat, don't forget that you usually use an LDC, so even though you'r just a few inches from the chrony, you need to figure the distance from the actual muzzle rather than the end of the LDC.

I can't help but wonder if your having the same problem with your valve that I am.  When the valve stem starts loosing strength, it swells near the ports. This gives wildly varying FPS numbers.

Is the 67 gr. mold you are testing. (NOE) the one I milled down for Cedric?  I was hoping that you or Cedric would be able to let NOE know how it worked. I need to call them about my 60 gr. version for lower powered AG's,

Cedric said that he was having accuracy problems with it, but later told me that after trying it again, and cleaning the barrel, it did shoot a LOT better. My 60 Grain'er is shooting very well indeed!

Knife

[dyotat100]

The inconsistent shots were at 100 yds with the .257. At the muzzle the fps is consistent. The chrony shifted a little from the wind so I was shooting thru it a little angle. I could see the screen and see some shots weren't even reading.

.308 has no shroud yet. .257 does and it makes it about 7" longer than the barrel.

I talked to NOE on Tuesday and they said it's not a problem to make the bullets shorter. They just need a sample to see how deep to go with the cutter.

[PakProtector]

Thanks for the 311316 numbers. I am thinking that the comparable BBT will have a slight advantage... I just need to get the 308 to shoot them in.
cheers,
Douglas

[rsterne]

There has been a recent discussion about large Meplats in another thread, and I revisited this data and noticed the trend to bigger Meplats as the length of the bullet decreased and decided I needed to do the data for bullets that were only 1.5 calibers long, as bullets of that shape are quite common for the larger calibers.... Here is the data for flat base bullets, as before....



As before, subsonic at only Mach 0.5 there seems to be no difference in the drag, but at Mach 1 the Form Factor peaks at a Meplat of 75% of the caliber.... I would think that as we get into the transonic region, above Mach 0.8 in particular that would also be the case, so that makes a strong case for using bullets of that shape for the relative short bullets necessary to keep the Sectional Density low enough to achieve decent velocities in the large calibers such as .45 cal.... This is a strong aerodynamic argument in favour of using bullets with a 70-75% Meplat in the larger caliber airguns.... to support their excellent performance on big game....

Bob