Velocity difference with 155 gr bullets??

[FORESTBARBER]

Well, I am enjoying being a new reloader. I have loaded several combinations for my .338lm and found one that shoots better than my factory ammo. Good.

First loading today of .308.

Used new Lapua brass. Winchester primers. 44gr of IMR 4064. 155gr Lapua Scenar.

Also did above loading with Sierra 155 bthp.

The Scenars run a little over 100 fps faster on average than the Sierra's.

Does this mean the Scenars have a better ballistic coefficient?

I only ran 10 each of the above and neither seemed really accurate, but it is quite windy here today.

Thanks for all the good info.

Forest

 

[vman]

Re: Velocity difference with 155 gr bullets??

You ran these loads over a chrony?

Lighter grain projectiles run faster than heavier grain, but the 155's i would expect to shoot at a similar FPS.

Perhaps the bearing surfaces on the two projectiles are different?

 

[Grump]

Re: Velocity difference with 155 gr bullets??

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: FeedsTheNeed</div><div class="ubbcode-body">....
Used new Lapua brass. Winchester primers. 44gr of IMR 4064. 155gr Lapua Scenar.

Also did above loading with Sierra 155 bthp.

The Scenars run a little over 100 fps faster on average than the Sierra's.

Does this mean the Scenars have a better ballistic coefficient?
Forest </div></div>

No.

BC won't affect velocity to the tune of 100 fps until a few hundred yards downrange. BC effects from the muzzle to the typical 5-10 foot midpoint of your chrono screens is mathematically measurable but so small as to be of absolutely no practical difference.

Two things are at play here: usable case capacity and bearing surface. Just look at the bullets side-by-side after marking the approximate ogive-to-shank point by lightly twisting the nose in a sized case mouth.

You didn't say which Sierra 155. What I describe below is of greater effect with the older 2155, less so with the 2156.

1. The Scenar is noticeably longer. Loaded to the same OAL on the front end, the heel of the bullet leaves less space for the powder. This typically raises pressure and velocity.

2. The Scenar has a longer bearing surface, and perhaps a stiffer jacket or jacket/core combo. This increases barrel resistance, also increasing pressure and velocity (within limits, I'm sure).

The jacket/core factor, and a very small difference in bearing length, is most likely why 155 A-Max bullets go about 30 fps faster than 155 SMKs (old 2155s) or 155 Nosler CCs (those two are interchangeable in my rifle).

 

[FORESTBARBER]

Re: Velocity difference with 155 gr bullets??



The Sierra's are the 2155's and measuring as you suggest the Scenar's measure 0.400" vs the Sierra's 0.300".

Makes sense.

Many thanks,

Forest

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Grump</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: FeedsTheNeed</div><div class="ubbcode-body">....
Used new Lapua brass. Winchester primers. 44gr of IMR 4064. 155gr Lapua Scenar.

Also did above loading with Sierra 155 bthp.

The Scenars run a little over 100 fps faster on average than the Sierra's.

Does this mean the Scenars have a better ballistic coefficient?
Forest </div></div>

No.

BC won't affect velocity to the tune of 100 fps until a few hundred yards downrange. BC effects from the muzzle to the typical 5-10 foot midpoint of your chrono screens is mathematically measurable but so small as to be of absolutely no practical difference.

Two things are at play here: usable case capacity and bearing surface. Just look at the bullets side-by-side after marking the approximate ogive-to-shank point by lightly twisting the nose in a sized case mouth.

You didn't say which Sierra 155. What I describe below is of greater effect with the older 2155, less so with the 2156.

1. The Scenar is noticeably longer. Loaded to the same OAL on the front end, the heel of the bullet leaves less space for the powder. This typically raises pressure and velocity.

2. The Scenar has a longer bearing surface, and perhaps a stiffer jacket or jacket/core combo. This increases barrel resistance, also increasing pressure and velocity (within limits, I'm sure).

The jacket/core factor, and a very small difference in bearing length, is most likely why 155 A-Max bullets go about 30 fps faster than 155 SMKs (old 2155s) or 155 Nosler CCs (those two are interchangeable in my rifle). </div></div>