Bearing surface and other mysteries?

[maverick5582]

[45r]

I've tried 3 bullets in my Grendel.Sierra 85HP,95 V-max, and 123SST.
My most accurate and favorite one is the 123 SST 10 thou off the lands.
I seat them to 2.245 with 27.6 AR Comp and Rem 7 1/2 primers.
My barrel has a short throat.
It shoots close to 1/2 MOA at around 2560 fps.
That is better than I thought I could get.
It seems to me the 123's are the way to go.
When I shoot up the 85's and 95's I'm going to buy 123's only since they shoot so well.
The heavier weight and bearing surface of the 123's seem to be the best for the Grendel even though I've got the shorter,lighter bullets to shoot sub MOA and go over 2800 fps.
The 123's shoot under 3/4 inch consistently with bullets touching or near touching with AR Comp.
The groups that go in the .5's look very good.
The lighter bullets tend to string vertically and have higher ES.

[nincomp]

I have been doing quite a bit or reading in the archives on o.a.l, jump, and bearing surface and I need some clarification.

Well Maverick, I can give you info but I doubt that the your buddy, "bearing surface" will listen.

The most common definition of "bearing surface" is the area of the shank between the places where the lands of the rifling first touch the bullet and where they stop touching in the back. If you look at a recovered bullet, the part where you see the rifling engraving is the bearing surface. If you have a Grendel barrel with the top of the lands at 0.256," then any part of the bullet with a diameter larger than 0.256" would be part of the bearing surface (at least by the common definition).

A certain amount of bearing surface is needed to keep the bullet from tipping sideways in the barrel. One of the few useful rules is that bullets with a small proportion of bearing surface tend to be sensitive to the distance to the rifling. Berger bullets has some useful info about seating long-ogive bullets.

Too much bearing surface can be a problem. Have you ever looked at the Barnes bullets and noticed the grooves cut into their shanks, that is partly to reduce bearing surface.

Here is where it can get complicated and keeps competition shooters busy between matches -some say having the exact same bearing surface on each bullet used for a competition makes a small difference in accuracy. Others say don't worry about the small variations from one high-quality match bullet to another.

In the real world, the actual effect of bearing surface is determined by many things. For example, the alloy of the jacket, the core of the bullet inside the jacket, the exact shape of your rifling, the ratio the rifling lands vs grooves, whether your second cousin (twice removed) ate beans that day .. ..well, you get the picture.

Here is the part that is almost like the "I told you that story so that I can tell you this one" that comedians use:
Figure out what kind of bullet you want to use (varmint, deer hunting, etc) then test until you find one that works well in your rifle. Don't try to squeek out those last few fps because: a) a few fps doesn't make a whole lot of difference, b) it will drive you crazy and c) adding more and more powder may kaboom your gun and not be so good for you either.

I hope that this is useful, even though the final analysis is a bit of a dissappointment.

[maverick5582]

I guess then that it is just the matter of considering the amount of "jump" in question. I thought you needed the diameter of the bullet (.264) inside the case for accuracy. Hence, which works better here .215" inside the brass or .260" inside the brass. So I will concern myself with the jump and let the rest slide. Oh well, on to the range for another bang bang session.

[nincomp]

You do need enough of the .264" inside the case to hold the bullet strongly enough that it stays in place through the recoil and feeding. In that respect, it does affect accuracy, since if the bullet moves a all, the jump will change. If a bullet gets "set back" in the case too far, the effective cartridge volume-to-powder ratio may change enough to be dangerous.

My earlier post is an example of what my Dad used to say : Ask Jim what time it is and he will tell you how to build a clock.

-Jim

[LR1955]

I guess then that it is just the matter of considering the amount of "jump" in question. I thought you needed the diameter of the bullet (.264) inside the case for accuracy. Hence, which works better here .215" inside the brass or .260" inside the brass. So I will concern myself with the jump and let the rest slide. Oh well, on to the range for another bang bang session.

Maverick:

With the very light bullets (very short bearing surface) you are probably better served by seating the bullet inside the case enough for the bullet to stay in place during the loading part of the cycle. I would seat the bullet so the bearing surface is more than .264 inches. .3 or .35 probably. In the case of these short bullets, opt for seating it deep enough over how much it stands off from the lands.

That said, if you seat it too deeply and the cartridge doesn't have enough OAL, you may run into feeding problems.

If you get too anal about things, you will spend all of your time wondering if .01 inches makes a bit of a difference. It may in the bolt gun that Stokes is trying to sell but it would be hard to prove. It won't in a gas gun. Go for perfect function and good accuracy with a gas gun and it will never fail you. Go for pure accuracy with a gas gun and you open yourself up for function problems.

LR1955

[Bwild97]

Maverick5582

I am assuming here that the more bearing surface, the more friction or drag the bullet has as it travels along the bore of the barrel, hence more bearing surface will result in less velocity. I am thinking about the 108 Scenar compared to the 107 SMK.
This is where I get confused on bearing surface and the seating depth of the bullet.

Mav, I don't mean to confuse you further, but your assumption is a bit backwards. A longer bearing surface will (in most cases) increase friction, but does not necessarily reduce velocity. Velocity is a product of pressure within the system.

While the difference in velocity across a selection of projectiles of similar weight (with varying bearing surfaces) may be negligible, the difference in pressure may be great. My experience here is with the 130g GameKing in comparison to the 129g SST, 130g Berger, 130g Norma.

I have never considered bearing surface when choosing a seating depth, I have always used my chamber OAL -.010". I believe the general rule for a suitable neck tension is to use a calibers width of bearing surface, when possible.

[LRRPF52]

I look at bearing surface in terms of building pressure, not engagement with the neck, except in some very rare cases like the 75gr A-MAX in the .223 Remington.

The examples of the 107gr SMK and 108gr Scenar are great for this discussion because you would think they are relatively similar bullets, when they really aren't.

The 108gr Scenar is a 123gr Scenar with less lead in it. It has the same external profile, using the same jacket, but is just a little lighter. From an analysis of the load data, it behaves very much like a 123gr Scenar in terms of building pressure, but you can get a little more velocity from it.

The solids are another great example of where we look at bearing surface, and the relief bands cut into them to relieve bearing surface-induced pressure, as pointed out by Bwild97.

The short answer is that we never assume that load data for one projectile of the same or similar weight is interchangeable with another. 107gr SMK is not = to 10gr Scenar, and 120gr TSX is not equal to 120gr NBT, for examples.

[maverick5582]

[QUOTE=Bwild97;91614]Maverick5582


Mav, I don't mean to confuse you further, but your assumption is a bit backwards. A longer bearing surface will (in most cases) increase friction, but does not necessarily reduce velocity. Velocity is a product of pressure within the system.

Bwild97:
Thanks for the clarification. I should have thought that one out more clearly. I tired coated bullets in my 6.5x284 bolt gun. It reduced the friction and lost velocity. I was always adding back .2-.3 grains of powder to get the velocity back.

[maverick5582]

I look at bearing surface in terms of building pressure, not engagement with the neck, except in some very rare cases like the 75gr A-MAX in the .223 Remington.

The examples of the 107gr SMK and 108gr Scenar are great for this discussion because you would think they are relatively similar bullets, when they really aren't.

The 108gr Scenar is a 123gr Scenar with less lead in it. It has the same external profile, using the same jacket, but is just a little lighter. From an analysis of the load data, it behaves very much like a 123gr Scenar in terms of building pressure, but you can get a little more velocity from it.

The solids are another great example of where we look at bearing surface, and the relief bands cut into them to relieve bearing surface-induced pressure, as pointed out by Bwild97.

The short answer is that we never assume that load data for one projectile of the same or similar weight is interchangeable with another. 107gr SMK is not = to 10gr Scenar, and 120gr TSX is not equal to 120gr NBT, for examples.

LRRPF52:
Since I had read what you wrote about the 108 Scenar and the 123 Scenar before, I bought 200 of the 108 Scenars to try. With 28.3 grains of 8208 XBR they gave me a velocity of 2654 f.p.s today with my 24" pipe. I appreciate your clarification it helps me keep things straight in my mind. The gas gun stuff is still very new to me.

Mav