7mm PRC

[Dave Hoback]

I get what you are trying to express, however it really doesn’t need to be overthought. Heavy projectile at higher speeds is going to cause much more backward thrust on the lugs than a lighter projectile or at lower speeds. Have you ever fired say a 30-06 with a 150gr vs a 220gr? The difference in recoil is quite noticeable. And the bolt head/Action lugs, Action Screws & Recoil lug are all being exposed to very same force difference. Right? So, yes… exactly.

What you are talking about is only evident in a larger diameter case with a higher powder charge. Given if there two cases which are the same length but one is 1.5x wider, & they both have equal powder charge, the larger diameter case will exhibit lower pressure. Thus lower projectile velocity & subsequently less energy applied to the lugs & all all other parts.

[Robinhood]

I assumed the surface area of the case exhausted some of the the thrust and the smaller area of the case head reduced it as well. PSI means surface area effects the amount of force applied.

[charlie b]

Force applied to the bolt face, and then the lugs, is related to recoil, but, not exactly. The force on the bolt is due only to the chamber pressure applied over the area of the bolt face. So, for the same chamber pressure a larger dia base means more force on the bolt.

There is also the rate at which the force is applied, but, that gets a bit complicated for these discussions. As long as you are in the elastic range of the material it only affects fatigue failure.

Recoil is determined by the acceleration of the mass of powder and bullet down the barrel. Again, related to chamber pressure, but, not a simple relationship.

You can have a max chamber pressure with a small bullet traveling fast and a lower chamber pressure with a heavier bullet traveling not so fast. The heavier bullet can have a higher recoil than the smaller, faster bullet, even at a lower chamber pressure.

Note: last portion of this was wrong so I deleted it.

[Bleeb]

Yes it is a 7x7. Pretty crazy......

[efm77]

Force applied to the bolt face, and then the lugs, is related to recoil, but, not exactly. The force on the bolt is due only to the chamber pressure applied over the area of the bolt face. So, for the same chamber pressure a larger dia base means more force on the bolt.

There is also the rate at which the force is applied, but, that gets a bit complicated for these discussions. As long as you are in the elastic range of the material it only affects fatigue failure.

Recoil is determined by the acceleration of the mass of powder and bullet down the barrel. Again, related to chamber pressure, but, not a simple relationship.

You can have a max chamber pressure with a small bullet traveling fast and a lower chamber pressure with a heavier bullet traveling not so fast. The heavier bullet can have a higher recoil than the smaller, faster bullet, even at a lower chamber pressure.

Note: last portion of this was wrong so I deleted it.

This is what I was thinking but you said it more eloquently than I could have. Although the PRC’s are different lengths, and have different powder charges, and bullet sizes, they’re all the same case diameter and operate at the same MAP of 65k psi. So the bolt thrust should be the same with all 3 of them.

[Dave Hoback]

Well, yes & no. It doesn’t quite work nice & neat like that. Cartridge PSI is a factor, but not the only part. A more powerful cartridge is forcing the bolt back with more power. The recoil we feel is the same thing the bolt & lugs are taking the force of. If the three were the same, the recoil would be same between all three. And it certainly is not!

Edit: I understand there is all kinds of math & who’s it’s & what’s it involved…. But it remains a fact that a more powerful cartridge cause more stress on the firearm. Including the Lugs. The Pressure, weight of projectile & diameter of the case/bolt face are all intertwined in this. But I was just trying to keep it simple. Which it is.

[Dave Hoback]

Oh yeah, and as for the math: Bolt Thrust equals the cartridge pressure X the Area of the Case Head. ​However, as has been said, it’s not so simple as that.

[charlie b]

The complex part is it happens over time. Integrating the area under the pressure curve gives the total, but, recoil feels different if that force is applied over a shorter or longer period of time.

[efm77]

As I said, my 375 Ruger and 300 PRC both have 1/2” locking lugs, so apparently the savage engineers thought that was plenty for that size cartridge. And the 375 has a lot more recoil than any of the PRC’s, I have the eyebrow scar to prove it lol. I still find it strange that every other PRC (including the 6.5) and 375 Ruger I’ve seen in a Savage had the large shank, yet the 7mm is a small shank. Just weird some of the things Savage does.

[efm77]

So then, if more recoil contributes to more force on the bolt lugs, does a muzzle brake that reduces felt recoil, also reduce the force applied to the bolt lugs?

[charlie b]

Not in gross terms. When looking at the rifle as a 'system' there is a balance of forces from the bullet and ejected gasses. Since some of those gasses go to the rear the net linear force to the rear is reduced.

When looking at the chamber/barrel interior as a 'system' it still sees the same pressure curve.

BUT..the gasses ejected at the muzzle may present more 'resistance' to the bullet and gas path that could raise the chamber pressure slightly (kinda like the bullet resistance function in the internal ballistics programs). I've not looked at the analysis for that. I recall a section of the Army Internal Ballistics documentation that dealt with muzzle brakes. May have to look that up.

[Dave Hoback]

This was an interesting discussion. But I fear it’s become a runaway tangent. LOL! Good talk guys.

[efm77]

Not in gross terms. When looking at the rifle as a 'system' there is a balance of forces from the bullet and ejected gasses. Since some of those gasses go to the rear the net linear force to the rear is reduced.

When looking at the chamber/barrel interior as a 'system' it still sees the same pressure curve.

BUT..the gasses ejected at the muzzle may present more 'resistance' to the bullet and gas path that could raise the chamber pressure slightly (kinda like the bullet resistance function in the internal ballistics programs). I've not looked at the analysis for that. I recall a section of the Army Internal Ballistics documentation that dealt with muzzle brakes. May have to look that up.

Makes sense. Thanks.