100g xtp in 303, using h322

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Hey all,
I am looking for input in how low I can safely start with h322 powder behind a 100 grain hp, in 303.
Hogden landing has load data for 30-40 krag, which I know is a safe allegory for 303.
My research has found, from reading a bunch of recipies published for both shells; typicaly the same charge will result in lower pressure and faster bullet in 303, which could be because of the couple of thou difrence in bore size.
I would like input in how to know how low I can go from published maximum, as the ‘starting load’ is higher than I want, if I assume I am going to pick up 100-200 fps in 303.
What I have read:
39 grain, 2542 fps
43 grain, 2898 fps
Same powder has loads down to 30 grains, under heavier bullets, is it safe to drop down that far under the light bullet?
A company that makes a 100 grain brass bullet sent me a calculated max load for 303, with their 100 grain hp:
41 grains, 100% case fill(because of boat tail) 3135 fps, 40791 psi, calculated from quick load.
Clearly faster than 30-40, and probably too fast for the xtp hp bullet to be effective on deer, but it is interesting to see the 303 loaded like a 243, at least out to where ballistic coefficient becomes something to consider. I am a 100y shooter at best.

Thank you all for your wisdom, and for reading my rambling.
 
This kind of thing is usually done with much lower velocities and more suitable powders in mind. H322 may work downloaded but it isn't great to begin with for your project. The bullet won't be good for much other than Varmints at 2000+fps and even then accuracy will probably be too poor for reliable hits.
 
For slower, trail boss works pretty good, clipping along at cast/gas check velocity, but I read that when these xtp bullets first came out, someone found they were a very good deer round at about 2500 fps, above that they were likely too destructive.
 
Some XTP may do ok but the OP is using a bullet designed for .32 pistol cartridges which aren't particularly high pressure or velocity. It is used in the .327 though so with some development there is no reason a powerful and useful load can't be made. I think accuracy will be the biggest hurdle but it could work out fine.
 
I have read somewhere in the past, that if you can find a load that lists imr4895 as one of the suggested powders (Hodgdon reloading lists it as a powder for 125gr bullets), then you can download that powder for a reduced load. However, I don’t remember where I read that and so I can’t confirm that. If you decide to go that route, put your desired charge of powder in the case, and then fill the rest of the case with cream-of-wheat uncooked cereal before seating your bullet. That way the powder stays against the primer and the cream of wheat acts as a ‘harmless’ filler.
 
I download h4895 in the 303 with cast, don’t think the same is recommended for imr 4895. I use unique or titegroup with the 32 xtp in my lee enfield. On another note when downloading I use a magnum primer as well. Mag primer is more reliable in the cold weather.
 
The xtp is a pistol bullet? Just for gets and shingles pushing that too 3100...sure I guess. For what purpose, can't find the typical 150-180 303 bullets, explode two litre pop bottles, or ?? Certainly, a pistol bullet isn't going to be used at long range??

I use the 180 xtp in my Rossi 44mag carbine @ about 1800 ft/sec. Really emphatic short range deer bullet; however, it's marginal to stay together. I'm thinking of loading up some 240gr for a truck gun load. Basically take it with me traveling out of here; with the likelihood of weather/cancellation of ferries, I do a fair bit of boondocking in bear and cat country.
 
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