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Safe starting load with increased seating depth


Tom5587
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Understanding that there are a number of variables impacting pressure, would anyone have a have insight on how to evaluate seating depth vs charge?  The problem that I am trying to solve is:

 

Loading 115g 9mm hollow points with Tite Group.  Two load manuals show starting load of 4.5g, max load at 4.8g and tested at 1.125" OAL.  The problem is that anything longer than  about 1.080" leaves the bullet touching the lands in one pistol.  If I did the math correctly, this reduces the combustion volume by about 12%.  Any thoughts on how much the starting load should be reduced to ensure safe pressures with the reduced combustion volume?  If the charge is reduced by something more than 12%, would that ensure a safe load?

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So QL says a 115gr XTP at 1.125" with 4.5g of TG is 32.5k PSI. Shortening the OAL to 1.080" increases the pressure to 38.8k PSI (or a 20% increase). Lowering the powder charge to 4.1gr (reduction of 10%) lowers the pressure of the 1.080" round to 32k PSI.

 

115gr XTP is 0.545" long, for reference, so you're increasing the seating depth from 0.174" to 0.219" in this example, which yields an 11.7% reduction in useable case capacity which is pretty close to what you calculated for your bullet.

Edited by thormx538
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And finding the correct OACL with a .355" jacketed bullet is rather easy. Just barely start the bullet into a FIRED case, I use WIN cases, and place the overlength "dummy" into the chamber. Push lightly on the case-rim until it stops moving forward. The leade will seat the bullet. The barrel hood should extend about 1mm beyond the case-rim, certainly no more than flush. I would do this about 5 times to ensure that you're getting the same exact length every time. That will be the Max Possible OACL. Then simply make your loads .010" shorter. That way you'll know the exact distance your bullet is from the barrel's leade/throat.

 

I also want to commend you for seeking a mathematical solution for powder volume. One problem there is that as you go deeper into the case, the brass case-walls become thicker which kinda makes for a difficult calculation. Increasing OACL is different because you gain case volume as you lengthen due to the case walls thinning. Maybe the best way to know is by water volume for the 2 different seating depths and OACLs.

 

Another option for just finding a safe Start Charge is to divide the shorter OACL by the longer one given in the data. Using the numbers you posted that would be 1.08" / 1.125" for .96 then just multiply that by the Start Charge listed where 4.5 grs. becomes 4.32 or 4.3 grs.😉

 

 

Edited by K-Texas
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I learned this trick a few months ago, but use at your own risk. You want to shorten your Titegroup load to fit your barrel which has a short throat but you want to keep your same fps. Here’s what I do. Find all the different 115 grain bullet weights profiles that’s used  with Titegroup. This could be hollow points, jacketed, lead, plated, find the max load with slowest fps of all the different bullet profiles. Now you can shorten oal or increase or decrease powder charge as long as you don’t go over that max load and you should be safe. But you do need a chrono to use this method. If you start to go over that fps of that max load stop.. 

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