tight chambers and pressure

WhelanLad

WhelanLad

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hey cgnrs,
say you have a tight chamber , and a 'standard' length throat an all things normal does the pressure then equate to higher muzzle velocity ? for the 'same load' but in a different rifle with the same things normal... but say a slightly normal to larger chamber??


example, a book says the min load shows 2400fps..... but in this 'tight chambered rifle' the min load actually shoots 25xxfps ... is that often the case?


thanks
 
Velocity (other things being the same) is a function of pressure. If the velocity is higher, the pressure must be higher.

Higher pressure would be due to a tighter neck, shorter throat or smaller bore.

A variation of 100fps between "identical" rifles is not unusual.
 
yeah I cant say I have chronoed the actual rifle an loads im talkin about but played with the ballistic software a couple times for a guestimate with some ranges 1,2,300 poi's ..


true about tighter neck, I guess also bullet jump plays a part...
throat makes sense , manufacturers vary much in this department though per cartridge?
 
hey what about Rifling Twist? that must change a bit of pressure?? or not?
I noticed the example in the book is with a different twist...? what does that alter?
 
Chamber should be reamed to sammi spec.
So any variances should be minimal however there is of course a tolerance.
I doubt it would be a 100ft/s I crease though however that's entire cartridges and load dependent.
Bullet jump will of course affect chamber pressure.
But if your asking is a "tight" chamber will affect pressure yes it will. However how much would actually depend on how loose it it.
Sammi spec is as tight as it's gonna get. So any tolerance will be a bigger chamber,not smaller.
Smaller than sammi means it won't chamber the round.
So your numbers are based on the best case scenario.
 
WhelanLad said:
hey cgnrs,
say you have a tight chamber , and a 'standard' length throat an all things normal does the pressure then equate to higher muzzle velocity ? for the 'same load' but in a different rifle with the same things normal... but say a slightly normal to larger chamber??

example, a book says the min load shows 2400fps..... but in this 'tight chambered rifle' the min load actually shoots 25xxfps ... is that often the case?
Click to expand...

Unless the neck has been chambered too tight, the only difference will be due to slight differences in powder case capacity. That is more likely going to be due to differences in the brass dimensions than differences in the chamber. If you have a program like QuickLoad you can enter the case capacity difference and see what it does. Generally if you keep powder charge the same, then reducing case capacity will increase peak pressure and velocity. However if you exceed maximum pressure, you have to reduce powder charge and your velocity goes down.
 
Ganderite said:
Velocity (other things being the same) is a function of pressure. If the velocity is higher, the pressure must be higher.

Higher pressure would be due to a tighter neck, shorter throat or smaller bore.

A variation of 100fps between "identical" rifles is not unusual.
Click to expand...

P.O. Ackley once wrote about this... he said they tested two identical 30-06 factory rifles with consecutive serial numbers and they chronographed about 100 f/s difference... both before and after chambering to Ackley improved. They rifles were carefully examined and they appeared identical... but 100 f/s difference. He gave no explanation other than barrels were individuals.
 
I have a Leeper-built 308 Norma Magnum at home with a very tight chamber.
Because it is a custom barrel, there may be other reasons for the fact that
it takes 1½ less grain of IMR7828 in it to reach 3100 with a 180 Partition than
it takes in 2 other 308 Norma Mags. Dave.
 
Some rifles beat the book with the starting loads and exceed it easily without any indications of pressure.
Others don't make speed no matter what you do. In a general sort of way the pattern I've found is that quality barrels with decent chambers make speed and welfare grade sewer pipes don't. The difference between the over achievers and the under achievers can be rather extreme. There's more difference between some
Barrels that there is between many cartridges, that get argued over endlessly.
 
WhelanLad said:
hey what about Rifling Twist? that must change a bit of pressure?? or not?
I noticed the example in the book is with a different twist...? what does that alter?
Click to expand...

Rifling twist, number of lands, profile, and angle of lead in will all effect your pressures. Projectile material will affect pressures in relation to rifling design as well. Bore diameter will affect pressures. Based on all variables, many of which have nothing to do with the gun such as the ammunition parameters, it is super easy to end up with 100fps of difference between two "identical" guns.

-J
 
WhelanLad said:
hey cgnrs,
say you have a tight chamber , and a 'standard' length throat an all things normal does the pressure then equate to higher muzzle velocity ? for the 'same load' but in a different rifle with the same things normal... but say a slightly normal to larger chamber??


example, a book says the min load shows 2400fps..... but in this 'tight chambered rifle' the min load actually shoots 25xxfps ... is that often the case?thanks
Click to expand...

The other part of your question refers to what a book says. I've read that some data for reloading manuals is actually done by testing loads for pressure and velocity in different steps. Measuring velocity of a load in a test rifle, then testing the same load in a pressure test barrel to get pressure information. So the book info could be coming from 2 different rifles that may or may not have even produced the same pressures and velocities.
 
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The longer I mess around with rifles, the less likely I am to make any blanket statements regarding influences on velocity. Just when I think I have a pretty good handle on things, I come across an example which contradicts everything I thought I knew. When I was younger, I was much more certain of things. Maybe I was smarter or maybe senility is setting in.
 
Leeper said:
The longer I mess around with rifles, the less likely I am to make any blanket statements regarding influences on velocity. Just when I think I have a pretty good handle on things, I come across an example which contradicts everything I thought I knew. When I was younger, I was much more certain of things. Maybe I was smarter or maybe senility is setting in.
Click to expand...


I hear you and agree completely. The more I learn and experience as I get older, just makes me realize how much more there is to learn. Kind of opens the mind
 
I guess not to mention that most "Books" are done in 24-26 inch barrels , an no one really has a concrete number on how many fps is lost per inch when we try to guesstimate what our loads are doing until we actually chrono them... then we think we know until we test another rifle LOL

yeah, I get it!.
 
Some of them are done in pressure barrels which are going to be on the tight side. The Speer manual has the loads developed in the pressure barrel, then chronographed in one of their house rifles then they publish that velocity. Others work out the loads in rifles
and take an swag at what the pressure is. It's no wonder why loads and velocities vary from manual to manual.
 
Kelly Timoffee said:
I have a question , maybe someone knows it.

How many chambers are cut on a factory reamer before it is tossed?
Chamber No.1 can no way be the same as No.99
Click to expand...

Perhaps they are ground to cut maximum SAAMI, and then chucked when they reach minimum SAAMI? And may even be sharpened between those two points...
 
I have cut fifty chambers with the same reamer with no significant change in dimension. Most of the time, dimensional variations are the result in set-up errors or variations. When a reamer is re-ground it is ground on every edge. This means that the result is a reamer which is as new. As diameter is reduced, the shoulder is also ground back so the reamer form is simply moved back on the blank.
 
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