Odd question re: chamber pressure vs. barrel length

[Sparky]

My nephew has 7mm Rem Mag built on a Mauser 98 action that was given to him by a family friend. The gun has a 20 inch barrel. He investigated having it rebarreled in the same caliber to a longer 24 or 26 in. tube.
Let me preface what follows by saying I DO NOT know the 'smith who gave him this opinion.
Long story short, he was told the receiver would not handle the increased pressure generated by a longer barrel.
Not being an engineer, physicist or even otherwise remotely qualified to say such information was incorrect, it sort of strikes me as odd.
Why would a longer barrel generate more chamber pressure than a short barrel ?
I understand longer burn time etc., but could 4 to 6 inches more barrel raise pressures to an unsafe level given the rifle is functioning safely with a 20 inch barrel? ( And I might add is UNPLEASANT as all Hell to shoot!)
It doesn't make too much sense to me.
I'd appreciate someone who knows the dynamics of chamber pressures etc. to chime in!

 

[4v50 Gary]

It's not that the longer barrel generates the pressure. Pressure is generated by the burning propellant stored in the cartridge. Whether the barrel is long or short is irrelevant to the cartridge. The distinction is that with the shorter barrel, the gases drop off quicker than it will in the longer barrel. Why? The bullet exits the bore quicker in the shorter barrel, allowing the pressure to drop quicker than in the longer barrel.

 

[Jim Watson]

The pressure peak is reached by the time the bullet has made it only a very few inches from the chamber. Barrel length cannot affect that. Peak chamber pressure is about 60,000 psi in a magnum rifle, muzzle pressure when the bullet pops the cork is more like 5000 psi.

 

[stans]

What Mr. Watson said. It might be more of a case that the gunsmith has tried to rebarrel Mausers in the past and ended up bending the receiver, very easy to do with the Mauser if you are not real careful, and doesn't want to mess with it.

 

[1911Tuner]

Pressures

What Jim said....which of course varies with the burn rate of the powder.

Stans may have also nailed somethin'. While the 98 Mauser action is known for its strength, the receivers do bend if the smith doesn't have the right fixture to remove and replace the barrel...as well as being careful with it.

I'll let this one sit for a while longer and move it over to Rifle Country. Somebody over there will probably know of a good "Mauser Man" close to your
area.

Luck!

 

[Sparky]

Actually Gentlemen, that is pretty much the thoughts I had regarding the pressure. The pressure of firing the round is relatively constantfor the round, loading etc. It did not make any sense to me that barrel length would have any effect which on pressure would render an action unsafe.
I also didn't know 98 Mausers receivers had a tendency to bend in the barreling/re-barreling process. Thanks for that tidbit!

 

[Jim K]

"he was told the receiver would not handle the increased pressure generated by a longer barrel."

Nonsense. If the gunsmith doesn't know any more about working on guns than he does about how they work, I would not want him to do any work for me.

Jim

 

[1911Tuner]

Movin' Along

Sendin' this over to Rifle Country. Anybody know of a good riflesmith in the area?

 

[Swampy]

he was told the receiver would not handle the increased pressure generated by a longer barrel.

What a LOAD of BS..... for reasons already stated.

This is ONE "gunsmith" who would never get his incompetent fingers on one of my firearms.

Swampy

Garands forever

 

[kaferhaus]

This likely started out as a 7X57

military mauser that was later rechambered and shortened. 7mm mag out of a 20" barrel is stupid enough (no offense, I realize you and your friend had nothing to do with it)

7MM Mag needs at least a 26" barrel to realize it's performance potential anything less is just blowing burning powder down range.

The smith is either an idiot or didn't want to work on the gun and figured you'd be too stupid to see through his BS.

If it is an old military barrel re-chamber job and you want to keep the old barrel, then removing the barrel can be an issue.

If the old barrel is gonna be junked, removing it is not difficult and any moron can do it without twisting the receiver.

I will very rarely agree to re-barrel a military barreled Mauser if the customer wants the old barrel back for anything other than a tomato stake.

 

[Preacherman]

Actually, that gunsmith has a point - for SOME Mauser actions, they truly can't handle pressure very well. Others (particularly WW1 or later actions) should be able to cope with just about anything that will fit the action. He may have had some bad experiences with earlier actions, or those designed for lighter rounds, that weren't forged or tempered for higher pressures.

Just my $0.02 worth...

 

[Swampy]

Preacherman wrote:

Actually, that gunsmith has a point - for SOME Mauser actions, they truly can't handle pressure very well.

Sorry Preacherman, but on this particular issue that's not relevent.

Barrel length makes no difference on the PEAK pressure issue.

If the action the guy has is capable of handling the pressure of a 7 Mag at 20", it's capable of handling it at ANY suitable length. As stated above, rifle cartridges reach their PEAK pressure (between 50-60k psi for the 7 Rem Mag) when the bullet is only 2-3" inches out of the brass. It's all downhill after that. By the time that bullet has reached the muzzle on a 20" bore the pressure is down below the 10k psi point by far... maybe as low as 5k psi. It will be even lower as it reaches the end of a 24-26" tube.

Best to all,
Swampy

Garands forever