So I picked up a used H&R 44 mag with 16 inch barrel ...
.....the frame which is stamped SB2 357 magnum 44 magnum and shotgun barrels only. So when the 44 magnum operates at 36,000 cup why is it not safe to add other low pressure load barrels to this receiver, one example would be 45-70 which operates at 28,000 cup....
Next has to do with the survivor barrel, is it safe to shoot core bon and buffalo bore P 45 colt loads from the barrel or because it is a shotgun barrel should I stick with standard loads at 15,000 cup.
some clarification is needed please.
does the RECEIVER have the stamp SB2? or the barrel?
you can pull the buttstock off and check the frame, as per FAQ to make sure.
You need to KNOW which receiver you have!!
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when considering pressures and breech thrust, I use the maximum diameter of the case head, NOT THE RIM, which builds in some safety margin.
then it is a matter of Math. I made up an excel spreadsheet to assist me. I just punch in the numbers.
another important thing is the Firing Pin diameter. if it is the larger pin, you may encounter difficulty with higher pressure rounds.
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I think some custom reloaded rounds, such as CorBon, are operating on the ragged edge of max pressure. Even some 45/70 loads are HOT! not all factory 45/70's are at the lower SAAMI pressures. (although they should be labeled, it is important to remember)
I think the custom loaders even tell you WHICH GUNS they recommend to shoot their stuff, and recommend you do NOT shoot it out of anything else.
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Because of your questions, and they direction they are leading, I'd suggest you stick to H&R specs, don't shoot the high pressure stuff, and don't fit a barrel that can shoot any high pressure factory stuff. ( I don't include CorBon et al in this as they are specialty, and no one should shoot them unless they understand their guns)
IF H&R says you are good to go, then fine.. BUT another gun in proper caliber is not that expensive and surely better than wrecking one you have because you didn't fully understand what was happening.
read LOTS on the subject, there is literally thousands of hours worth on the internet from REPUTABLE sources, and not hearsay like what I am telling you. After all I too am still learning!
once you grasp the basics and intermediates, you will find the complicated stuff not so bad.
in the mean time, stay safe and shoot often!
and remember, very large animals were nearly killed off to extinction with some low pressure rounds throwing middle of the weight range lead.