Pressure vs velocity

[Tioal]

I have a chrony that I use sometimes. Today I began to wonder this. Is it possible to reach too high pressure before you reach a normal velocity. I have no pressure measuring device so rely on velocity data and load manuels. I got to wondering if I was loading above the listed max but not really crazy about it, and the velocity was still below the ave. for the caliber ,am I ok? As an example 38 spl loads run up to say 900 fps. So if I am trying a new powder and get about 850 fps can I safely assume I am in a safe pressure range.Mostly I stick to load data from a reliable source but I do wonder and was wondering what you guys think. Tioal

[dtknowles]

You might be OK but how could you tell. You can be over pressure and under velocity. Not so much with a .38 spl revolver but clearly with a rifle. It could have a short throat, tight chamber, neck or bore. These would all drive up pressures. These would have less effect on a low pressure revolver round. You should see higher than expected velocity for a given powder charge in a gun that produces higher pressures. I have a .243 handi-rifle that is like that. It shoots faster than it should for a given powder charge and primers are flattened with factory loads and with less than max handbook powder charges. In the revolver you could have high pressures and low velocity if you have a large barrel to cylinder gap and are using heavier than max. charges.

Tim

[williamwaco]

No.

Low velocity does not assure low pressure.

DO NOT load above listed maximum.

.

[btroj]

William is right.

A light load of slow powder may give a lower velocity of a large charge of fast powder yet the fast powder is much higher in pressure.

Making assumptions like this can get you hurt.

[wch]

Get a copy of "QuickLoad" and you will see that velocity is often independent of pressure.

[300savage]

loads are like women..

[frnkeore]

Revolvers reloading info can be deceiving. You first need to find out if the load was produced in a pressure chamber or a actual revolver. If done in a pressure barrel the velocity will be much higher and unattainable in a revolver for similar barrel length.

If done in a revolver, the cylinder gap and chamber diameter has to be the same. I can't remember cylinder gaps ever listed in loading manuals.

So, you can say that revolver cartridge load info is not a reliable indicater to match velocity.

Frank

[leftiye]

I really don't like the "My chrono says my velocity isn't too high, Therefore my pressures are okay" thing we see a lot nowadays. Quickload might save you, your chrony won't. The term "powder too fast for cartridge" is all about low velocities and high pressures.

[WILCO]

wondering what you guys think. Tioal

Stick to established load data.

[fecmech]

There is no way other than published data or pressure testing equipment that one can determine safe operating pressures in low pressure cartridges such as the .38 spl and .45 acp. Primer signs and sticky cases are not good indicators. The first is totally meaningless in low pressure cartridges and if you get the second you are so far past safe it isn't funny. You also need to look at barrel length used for manual data when using a chrono. Many manuals list data for .38 spl using 8 3/8" or 10" barrels and a load that does 900 fps out of a 10" barrel is going to do about 750 fps out of a 4" .38 barrel. Also some of the published load data is somewhat "optimistic" in regards to velocity. If you stick with published data you shouldn't get in too much trouble.

[Tioal]

First I want to thank all who replied. Next so you do not think me insane or a dare devil,I almost never even go to the high side of valid load data.I value my guns ,many of which are old like me.I value me more. This was more of a I wonder what type of question. There was a number of points I had not cocidered,and some I had. I always like to get anothers point of view.In my opion safty trumps all other concerns Thanks agian for more to ponder on the cold winter nights. Tioal

[leftiye]

Nobody's downing you. Loading always starts with finding pressure data or at least data published as safe. I.E. a place to start. Should include reduction of loads for testing to start loads to insure safe pressures, and then testing to find max pressure as you desire. Reading pressure signs IMO trumps reading a chronograph. You need to know how. You won't know what the exact psi. is, but you'll know it is safe in that gun. Then chronograph to satisfy your curiosity. Only way that is better is to lab test pressures.

[like it all]

I may be a little old fashioned, but the old manuals used to mention, inspecting your spent cases. Flattened primers, cratered primers, difficult extraction, and so on, are all dead giveaways to over pressure. Common sense usually prevails, and the load data in most manuals will keep you out of trouble. Most of us can't compete with HP White Labs for equipment. Chronographs are great little tools for measuring projectile speed and projecting exterior ballistics, they can't help you with chamber pressures. I don't believe propellants are all equal in their moles of gas produced per mole of propellant; their speed of gas production is also non typical.

[fecmech]

I may be a little old fashioned, but the old manuals used to mention, inspecting your spent cases. Flattened primers, cratered primers, difficult extraction, and so on, are all dead giveaways to over pressure.

The problem with all of those signs is they are mostly related to rifles (55KPSI)and magnum handguns (40+KPSI). For the low pressure cartridges such as .38 spl and .44 spl, .45 Colt etc by the time any of those signs manifest themselves it's pretty close to too late.

[country gent]

There are several issues to related chrographs that make this hard to accept also. How do you calibrae a chrongraph so you know 1000 fps is actually 1000 fps and not 950? I suspet with sky screens ambient light will affect the readings also though slightly. we have shot over 2 skyscreens at the same time 1 shot passes over both screens several times and there is always a discrepency between the units. reverse screens and the discreation is the basicaly the same. Also some barrels are faster than others due to finish chamber dimensions throats and internal dimensions, these also affect pressures. Most manuals recomend state to start low looking for flattened primers, sticky extraction, hard bolt lift and case head expansion. They also advise you to drop back when ay occor not at max loadings but when they occur. Case head expansion on rifle rounds is fairly accurate but requires a blade michrometer to measure. Fire several factory loads and measure ahead of the extraction groove. Fire handloads and measure ahead of extraction groove more than .0005 more expansion is getting over pressure. You have to be able to measure the narrow part of the solid head. Loads in oading manuals are recomendations you still need to watch for the warning sighns and work accordingly. I have several that wont get to a max load from my manuals and others that would go past it by a bit.

[Larry Gibson]

The chronographs are "calibrated" at the factory. Screens of different chronographs set in line with each other will always give slightly different readings. Different screen spacing determines the accuracy a 2' spacing will give larger ES and SDs that a 4' spacing which will not be as "accurate" as a 10' spacing. Using the same ammunition and the same test firearm; 2 test strings of 3 shot will show a wide variation of ES, SD and MV. Test strings of 5 shot will be a little better but reality won't set in with "assurance" unless 10 shot test strings are used.

The myth of "faster barrels" with modern commercial rifles built to minimum/maximum SAAMI specs is over rated and over stated.

Larry Gibson

[SDGarrick]

something I noticed... according to Lyman's 49th a 30-06 loaded with a 311299 and 17.2gr of unique will give 1675fps and 38,000+PSI, 21gr of 4227 yields 1600fps and 19,100 PSI. Almost the same velocity, but unique has double the pressure.

[leftiye]

The problem with all of those signs is they are mostly related to rifles (55KPSI)and magnum handguns (40+KPSI). For the low pressure cartridges such as .38 spl and .44 spl, .45 Colt etc by the time any of those signs manifest themselves it's pretty close to too late.

Actually this is only true in rare cases. Most .38s (not just the specials) will not blow up before some pressure signs are seen. More than likely they won't bulge cylinders or barrels either. But you can't just throw in a load and let fly. If you work up from a known load (or a safe load arrived at by whatever method) in something like say a Colt Police Positive, things will happen to the brass, there will be expansion before anything bad happens. Cases will stick in cylinders before cylinders bulge. Cylinders will bulge before they blow. Scuffing and etc. may only come later, but you might see some too. Used to be there were gun rag writers sticking cases all of the time (though they didn't publish those loads).

[leftiye]

There are several issues to related chrographs that make this hard to accept also. How do you calibrae a chrongraph so you know 1000 fps is actually 1000 fps and not 950? I suspet with sky screens ambient light will affect the readings also though slightly. we have shot over 2 skyscreens at the same time 1 shot passes over both screens several times and there is always a discrepency between the units. reverse screens and the discreation is the basicaly the same. Also some barrels are faster than others due to finish chamber dimensions throats and internal dimensions, these also affect pressures.

Most manuals recomend state to start low looking for flattened primers, sticky extraction, hard bolt lift and case head expansion. They also advise you to drop back when ay occor not at max loadings but when they occur. Case head expansion on rifle rounds is fairly accurate but requires a blade michrometer to measure. Fire several factory loads and measure ahead of the extraction groove. Fire handloads and measure ahead of extraction groove more than .0005 more expansion is getting over pressure. You have to be able to measure the narrow part of the solid head. Loads in oading manuals are recomendations you still need to watch for the warning sighns and work accordingly. I have several that wont get to a max load from my manuals and others that would go past it by a bit.

Don't measure for case head expansion! That can be way too late to be getting warned. If any actual case head expansion occurs, the case is either junk, or one step away from being junk. Measure the expansion ring in front of the head. Correct loads will give you a reading, use that as a base line. When the barrel flexes (and it does at max pressures and above), you will see about half a thousandth more expansion. This is quite a bit before case sticking occurs in anything but minimum taper cases. Then's when to stop adding powder. Blade micrometer not needed, plain ole' el cheapo micrometer will work fine (but I still use a Starret).

[fecmech]

Cases will stick in cylinders before cylinders bulge. Cylinders will bulge before they blow. Scuffing and etc. may only come later, but you might see
When cases stick in cylinders you have expanded the steel cylinder itself and it is contracting around the brass. When you expand steel revolver cylinders in low pressure cartridges like the .38 spl, .44 spl, .45 colt you are as I said WAY past safe operating or even +P pressures. The same SP primers are used in .38 spl and 9MM, same goes for LP's in specials and mags. One operates at a max of aboute 18KPSI and the other over 30KPSI so primers will tell you nothing in regards to pressure. As I said in my original post there is no reliable way for the reloader to determine pressure in low pressure cartridges except published data or pressure equipment.

[paul h]

My phylosophy on using a chrono to work up loads is that lower than book velocities aren't necessarily due to lower pressure, but higher than book velocities are due to high pressure.

There are a host of dimensional differences between your gun and what the laboratory used to work up loads that can lead to slightly lower velocities in your gun at the same pressure as the test gun. Unless something is mechanically wrong with your gun, staying with published max loads should keep your pressures within the design limits of the gun.

As mentioned with lower pressure rounds, the traditional "pressure signs" only show up at dramatically over pressure levels. The other thing with the 38 sp is that while a high pressure load likely won't blow up the gun, it will beat the gun up and cause it to wear out prematurely.

Use the chrono to make sure you're not going to fast, but strive for accuracy. If you get an accurate load at 800 fps call it good.

[leftiye]

When cases stick in cylinders you have expanded the steel cylinder itself and it is contracting around the brass. When you expand steel revolver cylinders in low pressure cartridges like the .38 spl, .44 spl, .45 colt you are as I said WAY past safe operating or even +P pressures. The same SP primers are used in .38 spl and 9MM, same goes for LP's in specials and mags. One operates at a max of aboute 18KPSI and the other over 30KPSI so primers will tell you nothing in regards to pressure. As I said in my original post there is no reliable way for the reloader to determine pressure in low pressure cartridges except published data or pressure equipment.

Argument for the sake of what? Both statements are true. Neither of them contradict the other. Each concerns a different thing. There ARE some indications even in low pressure cartridges. Determine what pressure is? No. Determine that pressure has advanced? Yes. As for when steel expands, that varies with construction of the arm and steel used. Test everything to sticking? NO. The point is that you CAN still test safely, until it does stick there is no danger. And prior to this there are still indications. Actual precise pressures? Not really important. Your perspective in producing loads, very important. Being able to function without being prostrated by the unknown. Priceless.