Ok I was just comparing the performance of the 6mm 55gr Nosler BT and the 55gr Sierra Blitzkings and found that the only difference between the two was the velocity. The Noslers where a consistant 100fps faster at 3,800fps. Why? Both loads where identical in powder type wieght and primer type. Accuracy was much the same for both of them though.

Lazo G

Bearing surface (where the bullet actually touches the lands).

Where did you test the speed, Laszo ?

At the muzzle means (an interesting dicussion maybe about) interior ballistics, and at the it target it is a BC consideration again.

[ 08-15-2002, 10:25: Message edited by: waitaminit ]

Like dbBill said - Bearing surface

I measured velocity at the muzzle. Could you elaborate on the effects of bearing surface?

Thanks for the replies.

The bearing surface is the surface of the bullet in contact with the barrel - which means the amount of friction.
It seems evident that a higher friction reduces the speed of the bullet.

I am not so sure about that.
The more surface=friction means also that the bullet is slower in gaining speed i.e. the volume behind the bullet is increasing slowlier.
The more resistance the burning (exploding) powder finds, the higher the pressure will become. Thus the maximum speed of the bullet at pressure maximum will be higher with a bullet which offers more friction.
This initial advantage in speed may be eaten up by the friction of the bullet on its way to the muzzle, but not necessarily.

The unwillingness to start, i.e. to give more room to the explosion gases, is the reason why we reduce the powder charge with raising weight of the bullet.

For you it is easy to find out what happened:

1) same load, same primer, same case: the flatter primer had the bullet with higher bearing surface in front
2) which bullet had the higher velocity ?

So many factors at play, even using the same bullet. For me I am careful to manage chamber temp and chamber time for each cartridge. Is it possible that your firing the other bullets right after the first. Hotter chamber will increase pressure and therefore velocity.

I'd put my betting money on either bullet hardness or distance to lands, before I'd bet on bearing surface. JMO, Dutch.

Thanks again for the replies.

Ok the Noslers had the slightly flatter primer and were faster. As far as chamber temp is concerned I let the rifle cool down between shooting groups. The distance to the land was slightly different between the two bullets I'm presuming because the OAL of the Noslers was a bit longer than the Sierras(but only by about.005). The Nosler is actually a slightly longer projectile (could this be why it has a greater bearing surface?) and by the way has a larger B.C.

Lazo, when I tried to retrieve some data from QuickLoad to bbef up my arguments, I found that Sierra has no 55gr bullets in .243. they start with HP1500 in 60 gr, next is Blitz King 1507 with 70 gr.

You may want to check your bullets before further headscratchings.

Same bullet weight but different shape = difference in bearing surface = difference in MV.
This is also why you cannot interchange bullets of same weight without redeveloping the load from the beginning!!
Or ELSE....

K&B
Niels