IMR (Improved Military Rifle)-4227 was a smokeless propellant made by DuPont prior to the acquisition of their propellant operations by Hodgdon in 2003.
H (Hodgdon) 4227 was a smokeless propellant made by Hodgdon with similar characteristics to IMR-4227 intended to be a competitor with IMR-4227 in cartridges where the DuPont powder was used. Hodgdon used the 4227 number to suggest the powder could be used in cartridges where IMR-4227 was suitable; not to say they were identical powders.
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Mr. Flintstone wrote:
Why would load data for 125 and 150 gr bullets be the same for carbines, but for 140 gr in pistols there is such a difference?
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IMR-4227 is a "fast" powder for compact rifle cartridges or a "slow" powder for pistol cartridges. I suspect in the longer barrel IMR-4227 has time to burn to maximum pressure before the bullet exits the barrel, thus explaining why the load for two bullets would be the same.
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Also, why do some loads only list H4227 data?
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Not to sound flippant, but where H4227 is the only powder listed, it was the only powder tested for that cartridge and bullet weight. Because of considerations involving cost, time, access to test facilities, reported user demand and the like, manufacturers do not test every powder and bullet combination for every cartridge.
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Is it safe to use H4227 data in all situations?
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No.
They are
not the same powder.
I know you can find posts by people saying they used H4227 and IMR-4227 interchangeably and never had a problem. I liken them to the people who run red lights and justify themselves by saying they haven't had an accident,
yet.