Comparing the same weight bullet in 2 different calibres

[flap]

Hi all,

I'd like some information on choosing the calibre of a rifle. I'm not actually buying either of these I just want to know for my understanding of things.

Nosler make a Custom Competition HPBT bullet in 168gr weight. (Just using this bullet for the example, could be whatever...)

They do the same "168gr HPBT Custom Competition" bullet in .284 and .308 calibres.

As the bullets are the same weight, what would make you pick one calibre over the other?


There is also the same overlap for their Ballistic Tip bullets, i.e.

.277 150gr Ballistic Tip
.284 150gr Ballistic Tip
.308 150gr Ballistic Tip


I know I'm working on this a bit backwards and you'd pick your rifle then the bullet, not the other way around. Like I said, I'm just asking for my understanding of things.

[Blackened]

Depending on what you're doing this isn't actually a bad way to approach selecting a calibre. Or at least have it play a part in the decision making process.

Obviously if the weight of the bullet stays the same but you increase the diameter for a larger calibre bullet, the bullet has to get shorter to stay the same weight (or vice versa, it gets longer if you make it decrease the diameter for a smaller calibre.

Have a look a the more details specs for the 150gr bullets and you'll see as the bullet diameter increases and the length decreases the sectional density and ballistic coefficient of the bullet also decreased.

If launched at the same velocity a smaller diameter bullet will perform better than a larger diameter bullet of the same weight due to its increased BC.

That won't apply to every situation of course. Different bullets will have different properties, different cartridges will have different powder charges and launch bullets at different velocities. Barrel length plays its part and so...

That's the general physics of it though.

[jackles]

Too many big words. Brain... Hurting...

[Baldrick314]

Basically people will become attracted to the ballistic performance of a particular cartridge and choose to go with that. So even though the projectiles are the same weight the calibres they are shot from may perform drastically different

[Aster]

Too many big words. Brain... Hurting...

We're going to add a quantum mechanics and mathematics section to the forum too.

I volunteer you to moderate it and check everyones work

[Monty]

If launched at the same velocity a smaller diameter bullet will perform better than a larger diameter bullet of the same weight due to its increased BC.

Basically people will become attracted to the ballistic performance of a particular cartridge and choose to go with that. So even though the projectiles are the same weight the calibres they are shot from may perform drastically different.

7mm-08 vs .308 is a good real world example of this for you Flap.

You can do some more research yourself, but the short versions is this...

7mm-08 was developed for silhouette shooting where you need a certain about of force to knock down the target.

As outlined above, shooting the same type of bullet in the same weight, the 7mm-08 round will have a better BC which means it looses the velocity at a slower rate.

Say you're shooting equal pills weight pills from a .308 and 7mm-08 at a ram target at 500m. The .308 is slowing down (and so loosing energy) faster than the 7mm-08. When they both hit the target at 500m the .308 may not have enough energy to know down the target, while the 7mm-08 still would.

That's a cherry picked example, but still valid.

Do a little research on 7mm-08 and metallic silhouette shooting and you'll find plenty to satisfy your curiosity.

[tucked]

What happened to the gold old days when just hitting the target was enough

[Chronos]

If launched at the same velocity a smaller diameter bullet will perform better than a larger diameter bullet of the same weight due to its increased BC.

Basically people will become attracted to the ballistic performance of a particular cartridge and choose to go with that. So even though the projectiles are the same weight the calibres they are shot from may perform drastically different.

7mm-08 vs .308 is a good real world example of this for you Flap.

You can do some more research yourself, but the short versions is this...

7mm-08 was developed for silhouette shooting where you need a certain about of force to knock down the target.

As outlined above, shooting the same type of bullet in the same weight, the 7mm-08 round will have a better BC which means it looses the velocity at a slower rate.

Say you're shooting equal pills weight pills from a .308 and 7mm-08 at a ram target at 500m. The .308 is slowing down (and so loosing energy) faster than the 7mm-08. When they both hit the target at 500m the .308 may not have enough energy to know down the target, while the 7mm-08 still would.

That's a cherry picked example, but still valid.

Do a little research on 7mm-08 and metallic silhouette shooting and you'll find plenty to satisfy your curiosity.

Yep, that's the example I would give to answer the OP.

My 7-08 drops 5" less at 500m than my .308 both shooting 168gr at similar speeds. Given that gravity is the same this demonstrates that the 7mm pill gets there quicker because of lower resistance therefor arrives with more energy. At no point does this become a disadvantage other than actual bullet price and possibly wound channel diameter when hunting at close range.

The other thing a out price is that if you're hunting at under say 300m the 7mm has no advantage therefor ammo price and availability makes the .308 a better choice.

Chronos

[5Tom]

This is how I picked my .243.

1. The way I picked my rifle was determining "what" I wanted to do with it. Target or Hunt? Hunt.
2. What size game will I shoot with a Centerfire rifle? Small to Medium size.
3. Looking at a range of calibres (.22-250, 223, .243, .270), Which offered the best ballistic flight path for my liking? .243 - High energy, flat shooting projectile, great for short and long range.
4. The Rate of twist in a rifles barrel will then determine what size projectile I can accurately shoot. My tikka has a 1:10 RoT so between 70-100gr pills will suit my criteria.

As to this part:

.277 150gr Ballistic Tip
.284 150gr Ballistic Tip
.308 150gr Ballistic Tip

The .277 at 150gr will be a *Longer* projectile than the .308 at 150gr, which will be shorter in length. For the same accuracy between all three of those calibres, the smaller the calibre is, the higher rate of twist is needed.
Stability of a bullet during flight is highly determined by the bullet length and therefore a barrels rate of twist becomes the all important choice.

That's my understanding of it all lol

[Antie]

The other thing a out price is that if you're hunting at under say 300m the 7mm has no advantage therefor ammo price and availability makes the .308 a better choice.

It's important not to overlook that kind of stuff.

So much shooting information is all fine on paper, but doesn't really make much if any of a difference when shooting in the field.

500m silhouette shooting is one thing, hunting is another.

You'd hardly ever, if ever shoot game at those distances. Probably fair to say 100-300 metres is the usual range for the majority of hunters.

If you can use 7mm-08 you can use a .308, and if you hit a deer (or whatever) with either of those calibres inside that 100-300m range it's going down.

All good to talk theory, and understanding this kind of stuff never hurts, but at the end of the day either will do the job fine.

[yoshie]

If they are all the same weight, the .270 will be the longest, and the .30 the shortest.

[flap]

7mm-08 vs .308 is a good real world example of this for you Flap.

You can do some more research yourself, but the short versions is this...

Thanks for the detailed explanation Monty, very helpful and understood