Hodgdon Data vs. GRT Data (.357 Mag)
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Orr89rocz
Silver $$ Contributor
In my experience it can be off and is dependent on alot of variables that need to be correct, such as case volume and bullet dimensions and seating dimensions. Powder stats such as burn rate and densities also sometimes dont match reality. You need to calibrate it by starting with min loads of the same book components and record your actual velocity. If all the case volume and dimensions are set right, then you play with initial shot pressure, powder burn rate, etc to get velocity to match your measured results to “true” it. I havent used it for handguns yet but have in a variety of rifle cartridges from straight wall to overbore bottlenecks. It leaves alot to be desired as an initial guestimate but better than nothing
Remember the manual data is based on real data based on the components used. I don't like computer guestimates , especially since they give incorrect loading data that has to be corrected by fudging numbers to get it to match chrono numbers. Try to lookup data in several manuals and compare. Some companies put loading data free on line. Never try to push the limit. A medium accurate load is the safest.
I pay close attention to the location where burnout occurs. I look for a powder that has a nice range of charges between max pressure and the smaller charge at which burnout happens right at the muzzle. If burnout occurs after that, you are wasting money. I suspect accuracy may also suffer, but I don't know that for a fact. For my 9mm loads in a 4 inch barrel, this interval is often around 1.0 or 1.5 grains of powder.
Grt can be a bit lacking with handgun and straightwall cartridges. Revolvers will have to have the cylinder gap added into grt. Its in the grt manual. It does help. I find grt quite accurate when the input is accurate.you will need to calibrate the powder model with actual velocities.
There is quite a difference in saami pmax and cip pmax in 357 as i recall. I used cip pmax and barnes thought i was nuts but no pressure signs
There is quite a difference in saami pmax and cip pmax in 357 as i recall. I used cip pmax and barnes thought i was nuts but no pressure signs
Can't speak for handgun calibers, but I see data in manuals as being very conservative. I've loaded Hornady data to max in 6Br and still had very rounded primers. As stated , with Gordon's, it is imperative you enter all possible parameters, INCLUDING bullet jump. Published data for rifles is always based on considerable jump since everything is mag length. Cyl gap would play the same role as bullet jump where initial pressure spike is concerned.
charlie b
Gold $$ Contributor
I use GRT a lot.....but, it is not a reliable source by itself. The powder information is based on experimentation by the user base (they show how reliable they think the data is). The originator is no longer working on it so it is what it is.
For that matter, most manuals are not a great source by themselves. They will give you a starting point and they do list max loads for the conditions they tested. Look at data from different mfgs for the same loads and you will get a variety of answers.
I ended up with the same model rifle Sierra used in some of their data (Savage BVSS in .308). I got the same components and loaded with their specified OAL. The vel was over 100fps different. I have no idea what pressure was but at max I still had no signs of high pressure.
OTOH, I've loaded some Hodgdon recommended loads and had severe pressure signs a full grain less than their max (and much higher vel).
Compare loads from different manuals. Start low and work up.
What you can explore with Quickload or GRT are the effects of things like seating depth or optimal barrel time. It gives you load densities for different powders, you can get 'recommended' powders based on parameters you enter.
I like to start with Sierra or Vihtavouri data. They have historically been more conservative for me than other manuals. For cast bullets I go to Lyman.
For that matter, most manuals are not a great source by themselves. They will give you a starting point and they do list max loads for the conditions they tested. Look at data from different mfgs for the same loads and you will get a variety of answers.
I ended up with the same model rifle Sierra used in some of their data (Savage BVSS in .308). I got the same components and loaded with their specified OAL. The vel was over 100fps different. I have no idea what pressure was but at max I still had no signs of high pressure.
OTOH, I've loaded some Hodgdon recommended loads and had severe pressure signs a full grain less than their max (and much higher vel).
Compare loads from different manuals. Start low and work up.
What you can explore with Quickload or GRT are the effects of things like seating depth or optimal barrel time. It gives you load densities for different powders, you can get 'recommended' powders based on parameters you enter.
I like to start with Sierra or Vihtavouri data. They have historically been more conservative for me than other manuals. For cast bullets I go to Lyman.
GRT and QuickLoad are simulation tools that can be used to model real world performance of internal ballistics. BUT they both include certain simplifications and and as someone has already mentioned need to be "calibrated" to real world data. They also require specific data on the propellent of interest. In the case of GRT most of this data is obtained from actual user info as well as some test data that will always be lot specific. Hence some powders are based on more real world data than others. Also new powders when introduced will likely be based on the manufacturers data only.
As for data in manuals, in today's world that data is usually obtained from SAAMI testing methods (in the US) and is very specific in the chamber and barrel definitions. What most see as conservative loads is actually the result of differences between the testing equipment and real world factory chambers and barrels or specific chamber designs and match barrels. Two items of note, Sierra Edition V mostly used actual test rifles and in some cases their data changed substantially in Edition VI which in most cases used the SAAMI testing protocol. As for SAAMI, the specific cartridge design is defined based on minimum chamber dimensions which defines volume to be occupied by the powder and case. It also defines certain dimensions not directly related to the pressure such as rim thickness and extraction groove dimensions. It also defines the maximum case dimensions allowed for manufactured ammunition such that all cartridge specific types will chamber in rifles/pistols chambered for that cartridge. SAAMI as often referred to is an ammunition specification not a firearm specification.
As for firearms, when chambered for and marked as a 308 Winchester it means that the rifle and chamber is designed to accept 308 Winchester ammunition and safely fire that ammunition while not exceeding the SAAMI pressure limits. It does not necessarily mean that the chamber meets all SAAMI dimensional requirements in terms of dimensions. In particular, the throat length and freebore are often exceed for various reasons. One of the more notorious chambers is the one used in the 308 Winchester Remington 700 which has an extremely long throat/freebore.
As for data in manuals, in today's world that data is usually obtained from SAAMI testing methods (in the US) and is very specific in the chamber and barrel definitions. What most see as conservative loads is actually the result of differences between the testing equipment and real world factory chambers and barrels or specific chamber designs and match barrels. Two items of note, Sierra Edition V mostly used actual test rifles and in some cases their data changed substantially in Edition VI which in most cases used the SAAMI testing protocol. As for SAAMI, the specific cartridge design is defined based on minimum chamber dimensions which defines volume to be occupied by the powder and case. It also defines certain dimensions not directly related to the pressure such as rim thickness and extraction groove dimensions. It also defines the maximum case dimensions allowed for manufactured ammunition such that all cartridge specific types will chamber in rifles/pistols chambered for that cartridge. SAAMI as often referred to is an ammunition specification not a firearm specification.
As for firearms, when chambered for and marked as a 308 Winchester it means that the rifle and chamber is designed to accept 308 Winchester ammunition and safely fire that ammunition while not exceeding the SAAMI pressure limits. It does not necessarily mean that the chamber meets all SAAMI dimensional requirements in terms of dimensions. In particular, the throat length and freebore are often exceed for various reasons. One of the more notorious chambers is the one used in the 308 Winchester Remington 700 which has an extremely long throat/freebore.
My post above raises the question as to why would a firearm manufacturer deviate from the SAMMI dimensions? One of the main considerations is that SAAMI is looking for ammunition influenced pressure not to be exceeded and is tested in a clean chamber with a maximum of 10 rounds fired. In a firearm a limitation of ten rounds would be impractical and most things that happen as a firearm is fired is that the pressure will likely increase due to chamber/barrel fouling. While this may not be the only consideration it is one of the most obvious reasons for deviation.
Always wondered how the manufacturers determined pressure for the loads they publish. I don't feel like looking it up in the manuals since I don't shoot much anymore. It would cost a huge amount of money to use the copper crusher pressure measurement method for a hundred different barrels. Do they use strain gauges glued to the barrels or are they just guessing. I read that the different methods don't give the save results.
How do you get pressure data for a handgun?
You need to read SAAMI standards:
SAAMI Standards - SAAMI
NOTE: Dimensional information for cartridges and firearm chambers, along with performance characteristics, is contained within the American National Standards published by SAAMI, listed below. As these standards are only periodically updated, drawings for cartridges and chambers introduced and...
saami.org
Suffice it to say that they use high speed transducers and recording equipment in specifically designed test chambers and barrels when testing ammunition. Similar/same method is mostly referenced in loading manuals published today.
RegionRat
Gold $$ Contributor
Yes. The test barrels are not cheap, the staff isn't cheap, the facility isn't cheap,.... well you get the picture.
Even within this statement, there are two big buckets of issues.
One bucket is the concept of the pressure measurement.
The other bucket is based on factor of safety, or in other words assume we have a pressure value and now the debate turns towards whether or not we agree on how high that should be for a given context.
Far too many instances of folks who publish loads without regards to calibration standards, and far too many who publish without the reference frame of safety factor or industry standards.
It has been indirectly reported that some outfits who publish load data will even publish simulation predictions for instances where they don't have test results because they don't have the facility.
IIRC, The bullet jump was only intended to be used as a last resort
Gordon passed a couple years ago but there is a team working on GRT NexGen but dont know when it will be released/completed. Also phone apps.
While the powder models are getting older with each lot of powders, the same is true of reloading manual data as they come out less often these days.
Once your data is input into GRT, the powder model needs to be calibrated via the OBT tool and you'll find it much more reliable.
When I was working on the powder development team, Charlie and Gordon would take my information and use a pressure trace 2 system to test the powder models I submitted.
American powder manufacturers wont share their data with GRT or QL. They treat it all as top secret info. European makers however did and do share their powder information much more freely making the European powder models more reliable. Again, these are aging as more and more new lots are produced.
The cylinder gap is critical for revolvers as is the gas port measurements for gas operated rifles.
As I understand it, sierra took data then extrapolated down to max by adjusting for temp, powder lot variation, and likely some other proprietary fudge factors to put out a number they were safe with.
Long been known to publish conservative data, but the fps delta could be lot variation?
And yes, the sme's on grt know the simulation has difficulty with start pressure of straight wall cartridges.
charlie b
Gold $$ Contributor
And that's one of the problems with published data, and why mfg's insist on starting low and working up. Even if you get the same components the lot variation can throw you off by a bunch. Especially if you get 'stacked' errors.
Published data is a decent starting point, but, isn't something to bank on.
Published data is a decent starting point, but, isn't something to bank on.
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