Thread: Chamber pressure, expansion ratios and muzzle pressure

Well the title says it really

In the old guns world we talk about chamber pressures a lot because certain actions are suitable for this or that chamber pressures in their original configuration.

Then we muddied the water with that new fangled smokeless powder ( it will never catch on ).

But we don't see much about powder expansion ratios and pressures at the muzzle when the bullet exits ?
Or do you smart tech guys have that on tap with internal ballistic programs ?

Why would I be interested in muzzle pressures ?
I am contemplating a very challenging barrel bore and line project for myself.
I have all the data sheets from the European "Individual Proof Of Weapons " ( a mine of info on minimum chamber wall, minimum barrel wall and extractor cut out maximums ) and can make the numbers for chamber and bore minimums easily until I am 1/3 along the barrel and then I would want to step my liner diameter down in two stages.
My rational is that chamber pressure is a very brief moment in time and that internal pressures reduce significantly towards the muzzle.
But with progressive nitro powders the powder expansion ratio is a key factor in pressure at the muzzle

Does anyone have any authoritative information on this subject ?

I do know what I am doing and I haven't blown one of my bore and lines up yet.
I do not include the original bored out barrel in my hoop strength calculations and in this case it will only be 1.25mm anyway

Hopefully this makes sense to someone

so if Im reading that right...your liner will only be inside your barrel for 1/3rd of its length then you want to step down barrel like the Germans did with 8mm mauser etc etc??? so if your liner is still as fat as normal barrel would be at muzzle..there couldnt be any issue as its as strong as a normal barrel.... the savage 7mm mag belonging to a mate has 22" barrel and my 1;20 std threaded thread protector off other rifles can almost start on outside of that barrel which isnt threaded....not alot of meat there and a big grunty powder charge...hope that helps.

Originally Posted by Micky Duck

so if Im reading that right...your liner will only be inside your barrel for 1/3rd of its length then you want to step down barrel like the Germans did with 8mm mauser etc etc??? so if your liner is still as fat as normal barrel would be at muzzle..there couldnt be any issue as its as strong as a normal barrel.... the savage 7mm mag belonging to a mate has 22" barrel and my 1;20 std threaded thread protector off other rifles can almost start on outside of that barrel which isnt threaded....not alot of meat there and a big grunty powder charge...hope that helps.

No the liner diameter will reduce or I would bore through the original barrel

hmmmmm now that really has me thinking WTF are you poking liner into...LOL.

It's on tap for smokeless. Will give you a ring after dinner!

Originally Posted by Micky Duck

hmmmmm now that really has me thinking WTF are you poking liner into...LOL.

7x57R into a very light sporting 303

There is no way you could fit a 303 liner back into the sewer pipe 303 barrel
A 7x57 could just work
A 25-35 would be easy but boring

Not to be confused with 'verified info', but here is a snip of what quickload suggests would be a pressure curve for a 7x57R with a 162gr BTHP and a full case of AR2209 - which may be a useful reference point

Originally Posted by akaroa1

7x57R into a very light sporting 303

There is no way you could fit a 303 liner back into the sewer pipe 303 barrel
A 7x57 could just work
A 25-35 would be easy but boring

something in between too....youve jumped down from 7 and a big half and 7 to 6 and a wee bit.... completely hurdled over the 6 and a halvies ,guess you already have that more than covered with your modern smokepipe.
what about slightly skinnier still,a fastish twist 6mm barrel to take advantage of the heavier end of projectile weights in a suitably sized rimmed case.....there must be something that fits thats criteria that would be unique enough to make you happy.

Originally Posted by Micky Duck

something in between too....youve jumped down from 7 and a big half and 7 to 6 and a wee bit.... completely hurdled over the 6 and a halvies ,guess you already have that more than covered with your modern smokepipe.
what about slightly skinnier still,a fastish twist 6mm barrel to take advantage of the heavier end of projectile weights in a suitably sized rimmed case.....there must be something that fits thats criteria that would be unique enough to make you happy.

Nah ! Needs to be period correct mate.
If you are going to mess with a vintage rifle of some substance and rarity and it can't be put back to what it was then it has to at least be lined to one of its original factory calibers.

That is vastly better than a rebarrel because the roll mark will still be there, the patina will be there, it will be back on face and ready for another 125 years of service.
Only the original caliber marking will be wrong

Sorry MD 6mm and 6.8mm are just too modern

I would put the first step down at 8 inches and the second step at 12 inches from the breech, with both steps being radiused to remove stress points. You should bore the barrel to be only a 1mm wall at the muzzle. The major diameter should leave about a 2.5mm wall at the narrowest part of the barrel shank (ie: the bottom of the threads). Tricky work but quite do-able.

Originally Posted by gundoc

I would put the first step down at 8 inches and the second step at 12 inches from the breech, with both steps being radiused to remove stress points. You should bore the barrel to be only a 1mm wall at the muzzle. The major diameter should leave about a 2.5mm wall at the narrowest part of the barrel shank (ie: the bottom of the threads). Tricky work but quite do-able.

Yes looking at that pressure curve it's 50% @ 10" and 25% @ 20"
So consulting my proof house data and looking at the same bullet diameter in lower pressure cartridges I can calculate the minimum barrel wall thicknesses
It will work just fine

And the bore drills are easy.
I might be able to use some of my existing ones and just grind them to take a live pilot if the existing fixed pilot is too small

303 22?

Originally Posted by Boxton

303 22?

Not a factory chambering in this rifle and maybe not a factory chambering in any rifle

But certainly a worthwhile caliber one day

Using the European Proof Data I can easily compare similar bore size cartridges from cartridges with dergees of lower pressures

So
7x57R 3400 bar pressure = 4.1mm barrel wall

7x72R 2800 bar = 2.7mm

8.15x46R 1650 bar = 1.9mm

These are some of the ways to cross check data and that your assumptions should / could work

@akaroa1 what calibre?