I would suggest a different approach--
and it is one I have used this summer to build a good 'replica load' for the Speer 38+P / GDSB135-gr. PD round. Don't break out the chrono--simply try some reloads will feel the same way the factory round shoots. The point is, you are trying to clone the shooting experience. It won't even matter if the POA is off a bit; this is for practice. So, concentrate on getting the same subjective feel (type of recoil: 'sharp', 'hard,' 'full,' and duration ('quick,' 'extended,' etc.) of the shot.
So, start with checking the Speer site to get the ballistics (fps) of the round you prefer to shoot.
Now, dig out the recipes--from the Speer manual, particularly. Try a development load with, for example, the HS-6 / HS-7 recipe. This needn't be an extensive workup--double-check the velocity ratings and the barrel lengths, and choose a range within the recipe. If your gear allows it, I would definitely use one-tenth-grain increments for the development ammo--so, say you load fifty rounds, ten each with .1 gr. jumps.
Shoot five of each on separate targets--and what you are concentrating on is how it feels, not how accurate you or the load is. "Think" about the recoil. I started out by shooting five of the factory round to begin with, then shot the reloads. Usually I would "remember" the recoil so I could check it again later.
On the component side of things, I would definitely get some new brass--it's the only way to really have consistent shooting, and since the factory round is probably near max pressure, there's a safety consideration. And, to start with, I would buy a couple hundred (or more) of the Speer bullets. Don't try to jump to lead or to "something cheaper" right away; you want to clone the factory load.
My guess is that you can home in on a powder and powder charge fairly quickly. If you need to try a different powder to fine-tune that feel, look at the powder burn charts and select a faster / slower one. repeat the drill. With the 38+P / GDSB135 work I did, I found the first clone load fairly quickly: It was Power Pistol--and after the lead-bullet testing, I had sorted out "version 0.9" of the clone load, for Power Pistol's recoil was not really similar in impulse to the factory load; it was harder and sharper. As it turned out, AA#5 is the powder for that particular load of the powders I have tested so far.
Once you get the clone load for ANY powder sorted out for the Gold Dot bullet, then you can find the cast bullet load you want to shoot. Back up a few tenths and do a development set, repeat as needed. Once I got the feel sorted out, I could shoot "clone loads" for about 12 cents a round (which includes brass amortization spread over ten loads).
Obviously, I was dealing with a revolver load, and the clone load recoil for a semi-auto has a different component for operation of the pistol.
Good luck; any questions, feel free to ask.
Jim H.
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