YogiSC
03-26-2019, 06:12 PM
Novel Approach to Seating Depth (OAL), Pressure, and Lowering Charges
(DISCLAIMER: Just a suggested approach. Use at your own risk.)
As I get into boolit casting for 9mm, I see a lot of discussion of OAL changes worrying people about over-pressuring their pistols. I also wanted to try to insure my own safety as I make significant OAL changes (from published load data) in order to chamber my boolit rounds. I do have a chrono but still like a good feel for starting point on a charge where I’m making 0.05” or more OAL changes..
So one way to approach this is mathematically. Using Hodgdon’s website we can get start and max loads at certain OALs with CUP pressures for each load (the Lyman cast bullet handbook provides this as well). We know as OAL goes down, seating depth (SD) goes up, and pressure goes up LINEARLY (Google “Ramshot ZIP oal pressure chart”). We know as the charge of certain powder (CHG) goes up, max pressure goes up pretty linearly.
Using the data from Start and Max Loads, we can create equations that helps us stay safe and choose good starting points for changing OAL charges.
The equation can be [(SD) * (CHG) * (Xfactor) ] = Pressure … OR … [ (CHG)*(Xfactor) / (OAL) ] = Pressure. Xfactor is calculated from the published data.
Personal Example: Hodgdon TITEGroup data lists a 115 Gr LRN at COL 1.10” with start 3.9 gr – 25800 CUP AND max 4.3 gr – 30,500.
To find this “Xfactor” = 25800*1.1” / (3.9gr) and 30500*1.1” / (4.3)…giving 7276 and 7802 respectively. If you wish to be very safe use the 7802 as the Xfactor. Can probably use the average of the 2 as well.
So now let’s say I must lower OAL to 1.04” to chamber the round, what would be a good starting charge? We can pick a “midway pressure” (27500) for our equation.
(CHG) * (7802) / (1.04) = 27500… 27500*1.04 then divided by 7802 = 3.66 grains. Using the start load pressure (25800) give 3.44 grains. So you will likely be safe from 3.4 – 3.6 as a start load with the 1.04 OAL..and still be within published pressures.. Obviously, always chrono if you can..
This came in helpful when I started loading Lee 358-125-RF for 9mm.. Really no load data for 9mm with this boolit, but I used some seat depths for published loads in Lyman CB handbook for similar and worked the equation - shot where I expected FPS wise.
(DISCLAIMER: Just a suggested approach. Use at your own risk.)
As I get into boolit casting for 9mm, I see a lot of discussion of OAL changes worrying people about over-pressuring their pistols. I also wanted to try to insure my own safety as I make significant OAL changes (from published load data) in order to chamber my boolit rounds. I do have a chrono but still like a good feel for starting point on a charge where I’m making 0.05” or more OAL changes..
So one way to approach this is mathematically. Using Hodgdon’s website we can get start and max loads at certain OALs with CUP pressures for each load (the Lyman cast bullet handbook provides this as well). We know as OAL goes down, seating depth (SD) goes up, and pressure goes up LINEARLY (Google “Ramshot ZIP oal pressure chart”). We know as the charge of certain powder (CHG) goes up, max pressure goes up pretty linearly.
Using the data from Start and Max Loads, we can create equations that helps us stay safe and choose good starting points for changing OAL charges.
The equation can be [(SD) * (CHG) * (Xfactor) ] = Pressure … OR … [ (CHG)*(Xfactor) / (OAL) ] = Pressure. Xfactor is calculated from the published data.
Personal Example: Hodgdon TITEGroup data lists a 115 Gr LRN at COL 1.10” with start 3.9 gr – 25800 CUP AND max 4.3 gr – 30,500.
To find this “Xfactor” = 25800*1.1” / (3.9gr) and 30500*1.1” / (4.3)…giving 7276 and 7802 respectively. If you wish to be very safe use the 7802 as the Xfactor. Can probably use the average of the 2 as well.
So now let’s say I must lower OAL to 1.04” to chamber the round, what would be a good starting charge? We can pick a “midway pressure” (27500) for our equation.
(CHG) * (7802) / (1.04) = 27500… 27500*1.04 then divided by 7802 = 3.66 grains. Using the start load pressure (25800) give 3.44 grains. So you will likely be safe from 3.4 – 3.6 as a start load with the 1.04 OAL..and still be within published pressures.. Obviously, always chrono if you can..
This came in helpful when I started loading Lee 358-125-RF for 9mm.. Really no load data for 9mm with this boolit, but I used some seat depths for published loads in Lyman CB handbook for similar and worked the equation - shot where I expected FPS wise.