Thread: New rifle, scope question

Originally Posted by timattalon

I was trying to say that the energy the bullet has leaving the muzzle is the main determinant of recoil going the other way. The reason the rifle is slower going rearwards is that it is 300 to 400 times heavier than the projectile thus reducing speed. If he wants lighter recoil then choosing between identical models of 308 vs 7mm08 (say a T3) then the weight difference between the rifles is negligible, and the energy of the bullet at the muzzle is close enough to identical between the calibres, thus there is almost no difference in recoil between the two. To make any significant drop in recoil, he would either have to sacrifice speed / energy by using lighter loads or make the rifle heavier to slow the recoil.

As a side note, I use the term slow the recoil because the actual amount of recoil in a heavier rifle scenario would actually be the same but the slower transfer of energy to the shoulder will make it feel like less recoil.

Short and sweet. Recoil difference between 7mm08 and 308 is close enough to not matter.

Mmmm so how does a big fat slow bullet vs a small fast bullet smack me around more or less in the same rifle?

Read what good old Chuck has to say on the topic and check out his table
Rifle Recoil Table

Originally Posted by Gibo

Mmmm so how does a big fat slow bullet vs a small fast bullet smack me around more or less in the same rifle?

The powder/primer charge gives the energy to the projectile according to 1/2mv². From this, very roughly, we get the velocity of the projectile. Then as the projectile leaves us on it's little journey the momentum, mass * velocity (mv), comes into play which imparts the velocity and energy back into the firearm, with roughly the same momentum. This energy is dissipated into the firearm and shooters shoulder. There are other things in play but these are the main bits.

Originally Posted by gadgetman

The powder/primer charge gives the energy to the projectile according to 1/2mv². From this, very roughly, we get the velocity of the projectile. Then as the projectile leaves us on it's little journey the momentum, mass * velocity (mv), comes into play which imparts the velocity and energy back into the firearm, with roughly the same momentum. This energy is dissipated into the firearm and shooters shoulder. There are other things in play but these are the main bits.

No I was asking Tim, based on his previous reasoning that I quoted.

Originally Posted by Gibo

No I was asking Tim, based on his previous reasoning that I quoted.

Multiply the mass by the velocity for both projectiles. The bigger result will give you the bigger slap. Divide the result by your rigs weight and that will give you the initial velocity of your rig that needs to be absorbed. 1/2 mv² of your rig will be the recoil energy that you need to absorb.

I'm talking exactly the same info as Tim. He just used Einstein's energy equation for particles travelling at the speed of light which is double what we expect normally and leads on to the theory of relativity. Yes, I'm a geek that has done post graduate level physics.

Originally Posted by gadgetman

I'm talking exactly the same info as Tim. He just used Einstein's energy equation for particles travelling at the speed of light which is double what we expect normally and leads on to the theory of relativity. Yes, I'm a geek that has done post graduate level physics.

Still no. This was what I was referring to.

If he wants lighter recoil then choosing between identical models of 308 vs 7mm08 (say a T3) then the weight difference between the rifles is negligible, and the energy of the bullet at the muzzle is close enough to identical between the calibres, thus there is almost no difference in recoil between the two

Originally Posted by Gibo

Still no. This was what I was referring to.

If he wants lighter recoil then choosing between identical models of 308 vs 7mm08 (say a T3) then the weight difference between the rifles is negligible, and the energy of the bullet at the muzzle is close enough to identical between the calibres, thus there is almost no difference in recoil between the two

Isn't that what they were saying? That's what I read at least.

Originally Posted by Nibblet

Isn't that what they were saying? That's what I read at least.

Dunno, I wasn't listening, I'm just being a c unit

Originally Posted by Gibo

Still no. This was what I was referring to.

If he wants lighter recoil then choosing between identical models of 308 vs 7mm08 (say a T3) then the weight difference between the rifles is negligible, and the energy of the bullet at the muzzle is close enough to identical between the calibres, thus there is almost no difference in recoil between the two

Look at Core-lokt and the usual packet velocities.

308, 150gr, 2820f/s - mxv = 423k units of momentum

708m, 140gr, 2860f/s - mxv = 400k units of momentum

So about 5% difference = bugger all.

223, 55gr, 3240f/s - mxv = 178k units of momentum

Less than half.

Originally Posted by gadgetman

Look at Core-lokt and the usual packet velocities.

308, 150gr, 2820f/s - mxv = 423k units of momentum

708m, 140gr, 2860f/s - mxv = 400k units of momentum

So about 5% difference = bugger all.

223, 55gr, 3240f/s - mxv = 178k units of momentum

Less than half.

I think my point was, my 308 can have quite varying kick depending how I load it, this is from the same weight rifle.

Originally Posted by Gibo

Mmmm so how does a big fat slow bullet vs a small fast bullet smack me around more or less in the same rifle?

Not quite sure what you mean. Are you shooting a big fat bullet combo as well as a small fast bullet combo in the same rifle? Do you use the same powder charge for both?

If they are both have similar energy (ft/lbs) at the muzzle, they wont.

Generally bigger bullets have bigger cases to hold more powder which means they get more energy and more recoil. This is not the case when comparing 7mm08 and 308.

Simply put the energy itself is generated by the powder/primer reaction. The difference in recoil between a big fat bullet and a small fast bullet would depend greatly on the amount of powder used to generate the energy.

In the cases of 7mm08 and 308 being the same case and the charges and energy at the muzzle being so close, as you lighten the bullet, it travels faster, as you increase the weight, it travels slower. But as the charges and case sizes and energy values are so similar between these two calibres, there recoil and overall performance is very very similar. The 7mm08 can shoot lighter bullets hence they go a bit faster / flatter trajectory, but the as mentioned just over 1/2 inch at 300 is close enough to nothing for a new shooter and for an experienced shooter, it should not cause any issues either.

For example I use a 43gr load of 2209 in my 303British. With the 215gr projectile it is accurate with, I get 2176fps (+/- 15fps) , with a "smaller faster" 123gr projectile I get more speed, the same recoil and a flatter trajectory,. (But due to other barrel / projectile issues average accuracy at best....). The energy imparted into the Big fat bullet and the small fast bullet in this case is the same because the powder charge is the same.

Originally Posted by timattalon

Not quite sure what you mean. Are you shooting a big fat bullet combo as well as a small fast bullet combo in the same rifle? Do you use the same powder charge for both?

If they are both have similar energy (ft/lbs) at the muzzle, they wont.

Generally bigger bullets have bigger cases to hold more powder which means they get more energy and more recoil. This is not the case when comparing 7mm08 and 308.

Simply put the energy itself is generated by the powder/primer reaction. The difference in recoil between a big fat bullet and a small fast bullet would depend greatly on the amount of powder used to generate the energy.

In the cases of 7mm08 and 308 being the same case and the charges and energy at the muzzle being so close, as you lighten the bullet, it travels faster, as you increase the weight, it travels slower. But as the charges and case sizes and energy values are so similar between these two calibres, there recoil and overall performance is very very similar. The 7mm08 can shoot lighter bullets hence they go a bit faster / flatter trajectory, but the as mentioned just over 1/2 inch at 300 is close enough to nothing for a new shooter and for an experienced shooter, it should not cause any issues either.

For example I use a 43gr load of 2209 in my 303British. With the 215gr projectile it is accurate with, I get 2176fps (+/- 15fps) , with a "smaller faster" 123gr projectile I get more speed, the same recoil and a flatter trajectory,. (But due to other barrel / projectile issues average accuracy at best....). The energy imparted into the Big fat bullet and the small fast bullet in this case is the same because the powder charge is the same.

Oh gee thanks for that...

Originally Posted by Biggun708

Oh gee thanks for that...

Sorry. I did not mean to get all wordy. Just the differences between these two calibres are not very much. Both have pro's and con's but whichever one someone gets , they can know that both are very well suited to hunting here in NZ.