@Aideb204 I understand where you are coming from. Been there, done that on other things.
All the preceding advice is true but you now have a Sabatti double rifle in 450/400. Great cartridge and many people have great luck with their Sabatti’s. Some do not. Hopefully, yours will be a good rifle.
Do you have a copy of Shooting the British Double, 4th edition? If not, buy one and sturdy it!
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Next, you need a chronograph. I recommend the Garmin Zero since they are so easy to use.
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Then a logbook to record each and every shot, its velocity, and point of impact. After shooting, transfer the data into a spreadsheet such as Excel. This will provide you the means to analyze your data. I use the sort function in Excel to group by different variables such as powder, bullet weight, velocity, POI / regulation group size and so on.
Range log book
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A short pencil makes for a long memory...
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Gathering data
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Don’t spend any money on the rifle until you find out if it will regulate with your loads. Start with something close to the Hornady factory ammo that was probably used to regulate these rifles. Go up a grain for four shots, and down a grain. This is a ladder and within a few groups you will discover which direction in velocity to proceed.
Note: Swapping triggers from right to left is a simple task for a competent gunsmith. I doubt a Sabatti has any cast in the stock either.
Changing anything a double such as length of pull may change the regulation. I tried this on a double that I had the stock shortened by over an inch. For this I added length temporarily and shot groups for analysis. My double did not change the regulation size. Yours may.
Once you figure how to change regulation group size by velocity, you will have the knowledge you need to “own” your Sabatti.
Now, all the above stated, $4,000 will buy a very nice magazine rifle.
But, that would not be a double rifle! I love my bolt guns. Simple and effective! I can make them whistle Dixie or shoot pin holes at long range. For my upcoming Tuskless hunt, I am taking one rifle, my Trusty Heym double. Left behind will be at least three superb bolt actions in .375 H&H, .416 Rem, and .450 Rigby. Those and another three or four dangerous game rifles will weep as my Trusty Heym goes afield once again. It’s a double rifle after all!