How does chamber dimensions affect cartridge pressure?

[thefitter]

Been thinking about pressure, and what affects it...some are easy to understand:

Powder speed

Powder volume

Bullet weight

Bullet length

Case volume

Case length

Bullet Seating depth

Shouldn't these be enough to accurately predict pressure?

What about chamber dimensions? Or barrel dimensions such as diameter? We always hear "look for pressure in your rifle".

How exactly does chamber dimensions or other mechanics unique to a rifle affect pressure?

Thanks

 

[Puzzaz01]

Good question. I'm going to follow this.

 

[pogson]

Shouldn't these be enough to accurately predict pressure?

What about chamber dimensions? Or barrel dimensions such as diameter? We always hear "look for pressure in your rifle".

How exactly does chamber dimensions or other mechanics unique to a rifle affect pressure?

Every variable affects peak chamber pressure, even the hardness of lead or jacket. Slight differences in throat, rifling, cartridge wall thickness, ... It's just too many to "accurately predict" peak pressure. The best we can do is select the most important factors and make a prediction from that. That's what programmes like Quickload attempt to do. They are close but not perfect.

It's usually enough for most high-pressure cartridge shooters to have reasonable starting and maximum charges and to start low and work up until satisfactory performance or high pressure signs appear, whichever comes first.

The low pressure cartridges often don't show signs of high pressure. e.g. shotshells. If you flatten a primer (say 50KPSI) you are probably holding a fragment of a shotgun if you are still standing. Those cases and actions are designed for less than 20KPSI with most loads being around 10KPSI.

Peak pressure is usually what concerns us. Too high and things break. Average pressure, mostly related to charge/volume of case/bore, is important as it relates directly to energy of the bullet at the muzzle. The peak pressure is reached when the powder has been totally converted to gas and the bullet has not moved or the bullet sweeps out new volume at the same rate that the propellant can burn. It's a balancing act we control mostly by charge, seating depth, and weight of bullet. The peak pressure is usually reached in microseconds. The pressure at the muzzle is still thousands of PSI in many cases which is why muzzle-blast affects hearing so much. With a slow powder the bullet may have to move several inches before the powder is all consumed. With a very fast powder, the bullet may be barely moving before the powder is consumed. It's complicated and you can see in old reloading manuals versus new manuals how much recommended charges have changed. In the old days, they used "copper crushers". Today they usually use pressure transducers to measure peak pressure. The copper crusher was way less accurate and much slower.

Predictions are useful but it is dangerous to rely on them. Use judicious testing to confirm.

The manner in which chamber/cartridge/bore dimensions affect pressures are several. One is that, roughly, pressure = some constants X amount of propellant consumed X temperature of the propellant gases / volume behind the bullet. At first the bullet is stationary so the volume is determined by chamber volume and cartridge thicknesses and seating depth. As the bullet moves, the volume increases slowly at first and then more rapidly as the pressureXarea of the bullet - friction with case and bore/rifling is the force able to accelerate the mass of the bullet. The peak pressure depends on many things from primer, to starting temperature, these volumes, and even the hardness or slipperiness of the bullet. A soft bullet will swage into the bore some plugging it very tightly whereas a hard bullet will cut itself on the rifling and slide along the bore. There's also some force from the rifling being an inclined plane on which the bullet has to rise. Lubricant, finish of the steel, ... everything matters. The best we can do is guesstimate according to the most important ones, primer, powder, seating depth, case capacity and chamber volume. Reloaders try to minimize the variation in pressures by controlling the things they can control as best they can.

 

[dedogs]

This is one of the best answers I've ever read on this forum. dedogs

 

[coues52]

great answer pogson

 

[DRNewcomb]

Within the limits of normal chamber variations, not counting free-run to the rifling, I believe this would be one of the lesser impacts. I'm talking about just a "tight" target chamber vs "loose" military chamber. The volume difference is hundredths or thousandths of an inch. Unless I messed up my math, increasing the diameter of a .308 chamber by four one thousandths of an inch would make roughly the same volume change increasing the free run by 0.07".

So yes, a tighter chamber would create higher pressures but so would a change in rifling, free run, firing pin impact and a host of other factors, most of which might be more significant.

 

[Sheldon N]

I'm curious about this as well... specifically in regards to case shape.

I know QL has a field where you can adjust the "Weighting Factor" of a case as it relates to whether a cartridge is an overbore bottleneck, a traditional shape or a purely cylindrical shape. The little dialogue box says it's to adjust for how much the unburned powder moves forward with the bullet, and playing with the numbers says that a more cylindrical shape produces more pressure, as compared to a bottleneck cartridge.

I always read people saying things like that the short/fat cartridges are more inherently accurate, is there a specific reason why? Would it relate to rifle pressure due to cartridge shape?

My apologies if this is off topic from what the OP was asking.

 

[pogson]

So yes, a tighter chamber would create higher pressures but so would a change in rifling, free run, firing pin impact and a host of other factors, most of which might be more significant.

For reloaders the biggest factors are likely case capacity and seating depth, and charge/primer, of course. I use a lot of "found" brass so I have about six batches of .308 brass. There are two definite groups, military brass to NATO specs and commercial brass such as Winchester. The variation within the groups is significant but between the groups it is huge. When I want best accuracy, I weigh and sort the cases. Most reloading manuals suggest 3-10% below max for starting loads but if all the factors go against the reloader, 10% is the way to go. In my experience 5% is enough but then I mostly use the same brands of components for decades. Different brands or batches can easily tip the balance. Some recommend against going lower than 5% below maximum particularly with the really slow powders. I haven't seen a really good explanation of that but it could be as simple as bridging of the granules to a bullet left in the barrel. Too much case capacity can cause bad things to happen as well as too little. In my experience with several cartridges the best loads are nearly full cases and I weigh almost every charge.

 

[pogson]

The little dialogue box says it's to adjust for how much the unburned powder moves forward with the bullet, and playing with the numbers says that a more cylindrical shape produces more pressure, as compared to a bottleneck cartridge.

I always read people saying things like that the short/fat cartridges are more inherently accurate, is there a specific reason why? Would it relate to rifle pressure due to cartridge shape?

A bottleneck definitely slows the movement of propellant out of the case. It's the size of the pipe and the difficulty of finding the exit. OTOH, we love bottlenecks because that overbore capacity helps us keep a higher pressure on the bullet longer for higher velocities. It's not just the peak pressure that matters to the bullet. The average pressure and how rapidly the pressure builds are very important. There is also an effect on accuracy. When propellant burns/decomposes, the gas produced is very hot and in a bottlenecked cartridge hammers the unburned grains more effectively. The bottleneck helps the pressure in the case build faster and stay hotter longer until all the grains are consumed. That's a recipe for better/faster/more uniform ignition. That's a recipe for getting the bullet out of the barrel faster and more precisely timed from the release of the firing pin. This helps accuracy. Making whatever happens happen faster helps accuracy because it's more uniform. You could get most of the benefits of a bottleneck by making a really long cylindrical cartridge but that might be difficult to feed/handle. Bottlenecking was the right thing to do so Mauser and a bunch of others did it. Straight cases are still popular for large calibres and artillery where bottlenecks might be impractical.

A long time ago in a land far away when I was a kid and TV was new, Walt Disney was trying to explain to kids how nuclear bombs worked... He laid out a large room with a bunch of mouse-traps loaded with ping-pong (or was it golf?) balls. A ball was tossed in setting off a trap which then triggered others until the room was full of flying balls (Oh,thank Goodness for YouTube https://www.youtube.com/watch?v=vjqIJW_Qr3c). That was supposedly modelling nuclear fission with the balls being fast neutrons causing radioactive nuclei to go off on cue. A similar model could explain what I was trying to say in the previous paragraph. If the room were long and narrow, it would take longer to set off all the traps and there is room for more variation in the timing of the triggers. In a square or circular room, all the traps are as close as they can be so whatever propellant gases are released get to more of the charge faster. So, the charge goes off sooner and more likely closer to the way previous charges went off so we get better/more uniform propulsion. In the extreme case of a long line of traps, it is possible that one of the balls would go off to the side and not trigger a trap at all and you get a squib load. Geometry matters. I think, if any of you are gunsmiths wanting a challenge, the most accurate cartridge would be a sphere with a primer at the centre... ;-) Good luck feeding the rounds. I guess you'd need a dished bolt-face of rather large diameter or some matching plug that followed the round into the chamber. What shall we call this round? 308-grenade? .45-ball ammo? Oh, that last one's been taken already. I'm hopeless at naming things.

 

[madderg]

If you were so inclined...you could find a cartridge and bullet combination that was what you consider "the best". If the case is not full up, and most are not, you could see where the best load sits in the round and shorten it until the neck is to that level. Use a 30 degree angle and lengthen the neck. Use minimum body taper and have the lead into the rifling at 1 to 1 1/2 degrees. If you want to ease the round into the barrel to keep peak pressure delayed wet-moly you bullets. You will find that you can achive the same speed, your accuracy node, with less powder because you just made a better "boiler".
Just my own experience over the years. It take a lot of work, but that's what winters are for. Good shooting, Gary

 

[DRNewcomb]

... Some recommend against going lower than 5% below maximum particularly with the really slow powders. I haven't seen a really good explanation of that but it could be as simple as bridging of the granules to a bullet left in the barrel.....

Filling a case to near capacity helps assure uniform ignition. The worst case would be a case filled to less-than 50% capacity where the primer showers sparks across the top of the powder. This provides very unreliable ignition which is also highly dependent on how the bullet is handled, uphill, downhill, etc. I've actually used dacron filler when making some down-loaded cartridges, such as when duplicating .30-30 or .30 carbine ballistics in a .30-'06.