Bullet weight vs. Velocity

[D1gger]

If all things are equal, shouldn't 2 separate bullets that weigh the same have the same velocity at the muzzle?
The reason I ask is that I have some 175gr seconds for my 308 that do not group well, but the first run bullets do.
Can I do load workup around velocity using the seconds and apply that data to the good bullets? Wouldn't this same principle apply regardless of the bullet type, as long as they weigh the same?
In theory, I think they should have the same velocity if V=MA, but nothing would surprise me when it comes to this stuff.

Thanks!

 

[Olen_4504]

but all things aren't always equal. just because they are the same weight doesn't mean they are the same shape. and that has a big effect

example: a buddy of mine has two different lots of 105g hyrbids. one lot is 40 thou longer than the other. bearing surface is different

the longer ones were 20-30fps slower than the shorter ones. with the same powder charge and seating depth - they performed drastically different at 1k yards.

 

[Milo 2.5]

but all things aren't always equal. just because they are the same weight doesn't mean they are the same shape. and that has a big effect

example: a buddy of mine has two different lots of 105g hyrbids. one lot is 40 thou longer than the other. bearing surface is different

the longer ones were 20-30fps slower than the shorter ones. with the same powder charge and seating depth - they performed drastically different at 1k yards.

Pay attn ^^^ to this.

 

[Huskydriver]

Bearing surface makes a huge difference between to bullets of the same weight.

 

[RmeJu]

example: a buddy of mine has two different lots of 105g hyrbids. one lot is 40 thou longer than the other. bearing surface is different

Yikes... on Bergers? I just switched over to Berger for the exact reason that I didn't want to have this kind of problem. Was I being too optimistic?

 

[MK20]

I like Hornady. They are cheap, consistent, and you can buy them in lots of multiple thousands.

 

[Olen_4504]

Yikes... on Bergers? I just switched over to Berger for the exact reason that I didn't want to have this kind of problem. Was I being too optimistic?

I think you are fine. They are one of the most popular bullets right now in PRS. Within the same lot they are very consistent. But lot to lot they can vary same - as I’d imagine with most manufacturers. Just double check when you get a new lot. Test them and see if you need to tweak anything

 

[supercorndogs]

Yikes... on Bergers? I just switched over to Berger for the exact reason that I didn't want to have this kind of problem. Was I being too optimistic?

Lot to lot variations are the norm, except probably with the solids being cut on a lathe.

 

[Ledzep]

Yeah, lot to lot variance is a real thing. No way to get tooling exactly the same and you're dealing with material flow (drawing, swaging, etc..)... Changes in surface finish alone on the tooling can cause differences to crop up.

Buy enough same-lot to last the barrel from the get-go if you don't want to chase your tail.

 

[clcustom1911]

Bearing surface makes a HUGE difference. For example a few months ago, I loaded 10 each of 130 grain Sierra Tipped Matchkings, Norma Golden Target, and Nosler RDF. Same case, same primer, same powder charge, same distance to lands. I did not sort bullets by weight, but they all weighed between 129.9 and 130.1. Bullet lengths were different, and you could see all 3 bullets had different bearing surface lengths.

Nosler had the shortest bearing surface, was about 2900 fps, no pressure signs

Sierra and Norma have longer bearing surfaces, were moving 2970, and had stiff bolt lift on 7/10 rounds fired.

Scientific or statistically significant findings? Not really, just general empirical information for others and me to stew over.