Weight vs. Velocity

[TeamTurpin]

I'm working on my first Big Bore gun and it will be chambered for 6.8 SPC. Over the next few months, I'll start tinkering with loads to see what my Contender likes best. I've never worked with this particular cartridge before. I need to order some bullets, but without an unlimited test bullet budget, I need to steer toward a weight range.

So, here's my question. As a new Silhouette shooter, I've never needed to reload specifically for Silhouette and I'm not sure how to approach bullet weight. I get the "accuracy" part, of course, but for knocking metal rams down at 200m, should I be leaning toward lighter bullets that zoom really fast or heavier bullets that bring a little more grunt? And, here in Oklahoma, the wind is always blowing.

The 6.8 (.277) has a range of bullets that run from about 90 gr up to 150 or so. So, assume that my loads are only for BB competition--no whitetail hunting or prairie dog plinking. If you had to buy some .277 bullets solely for match, where would you aim? Low end? Heavy end? Medium? My instinct is to buy the heaviest bullet that will give me good accuracy, but that might be a bad assumption.

There's just not a lot of (non-AR) 6.8 Silhouette shooters out there, so any advice you have would help me to blaze this trail. Thanks in advance.

John
Guthrie, OK

[ole95]

Let me just talk in general (in any cal.)
"most" seam to favor heavier bullets @ lower velocity
"most" seam to favor bullets in the 140 + gr. weight for the rams
if you look @ BB Silhouette loads in general you will see common Velocity vs. bullet weight that seam to work for people. I don't think it make that much difference if that combo is in a a 30 cal or 7mm (or 6.8)
you should fine something in those vel. vs. weight combo that shoot good in your set up
"some" use the guideline of velocity X bullet weight / 225200 and if it = more than 7 that good for rams

that just my $0.02
but remember I just got here

[260 Striker]

ole95 is heading you down the right path. Just look at the Whisper cartridges which are usually successful (nothing is 100%) on big bore rams. They use heavy bullets at slower speeds. Light bullets driven fast may actually blow up on the face of the target without imparting a lot of push where heavier bullets at slower speeds have more "dwell time" on the target and tend to knock over the target. If you can focus on the 140s or 150s and find and accurate load that you don't need a cordless drill to adjust the sights you will probably be more successful with the rams. If a load with 140s or 150s is a little too much for all 40 rounds then maybe 130s for C, P and T then use the heavies for the rams. It is also surprising that some of the heavy bullets at slower speeds don't really recoil as sharp as trying to drive lighter bullets at higher speeds. Recoil is subjective so you will just have to find what is comfortable for you. The SPC would be an interesting cartridge to play with. Should be very conservative on powder if you can just find that right bullet combination. Let us know now your project turns out. Of course you have to remember the OK winds too.

[Richard Pickering]

Building on ''ole 95'' remarks; what he meant to indicate on the formula was to look at loads that generate .7 or greater as being good for the Ram target. Remember you are looking at Mt or Terminal Momentum. You want to look at the momentum value as the bullet nose rests at the Ram. A 150gr bullet impacting the Ram at 1100fps will apply .73 Lb/FT momentum. This is workable in most cases. If your targets have wind from the rear many days or lean forward a few degrees you might have a failure. You realize that to apply momentum you first must hit the targets. So look at your accuracy requirements, look at your momentum values and try to strike a comfortable compromise. RP

[Richard Pickering]

I thought an addendum might be in order for any who may wonder about Momentum. As a simple and quick example think of the number 168,900. That is derived from dividing 225200 by .75. So think of a bullet of weight 168.9gr impacting a target at 1000fps. It generates .75 pounds ft of momentum. In our case Terminal Momentum (Mt).
So, any product of weight x velocity that equals 168900 generates .75. You can see that to do this lighter bullets must impact at greater velocity; heavier ones can arrive slower. If you elected a 6mm 100gr bullet it would need to arrive at 1689fps to generate .75. If you elect a 200gr bullet it can arrive at 844fps and hit equally as hard. To an extreme, if you elected a 50gr bullet it would need to arrive at 3378fps. You can argue that a 50gr bullet impacting a 50lb target may not have the same effect as a 200gr at 850fps. But the math says they are equal in value. Richard

[TeamTurpin]

This is a very cool discussion. It seems like black magic, but really it's just mathematics and physics.

Also as a follow-up, I've been reading up on heavy loads for the 6.8. There's already been some field research on this out there. While bulletsmiths do make 6.8/.277 bullets heavier than 135, I think those must be for the .270 Winchester. Some of the AR/M4 guys have been working on heavy/slower loads for the 6.8, but they seem to max out at 130 gr. And, they've gotten some good results. Since there's no factory ammo out there like this, I'm not sure that I could find velocity numbers for that bullet at 200 meters. (Although, there might be some way to derive that from muzzle velocities and other parameters.)

So, I think I'll order some 115s and 130s and see what I can do with them. A couple of boxes of 6.8 factory ammo (110 gr) just arrived, so they should yield me some once-fired brass soon.

[Richard Pickering]

Since you are going to investigate trajectories I'll submit the following : See www.jbmballistics.com It is helpful in that it contains a library of known bullets in many calibers. I'm sure you'll be interested in .277. You don't need to worry about wind values or angles at present. Just select a desired bullet, its weight, its diameter, give it a desired muzzle velocity, define your desired distances in meters, tell it your elevation and average humidity, and 0 distance. You'll see the velocity decay and bullet path. Give it a few tries. Richard