Old guns are something of an odd thing when it comes to buying ammunition.
.30-06 is commonly available that will be very hard on an M1 Garand's op-rod. No warnings on the box or anything!
Some ammo is deliberately down-loaded from full capacity because lots of guns are still out there which would be dangerous otherwise; like .45-70 and Trapdoors.
So you start doing research to decide what's safe in your old gun.
In 1892 with a 30" barrel, .30-40 was a 220 grain bullet at 2,000 feet per second muzzle velocity. This was fine.
In 1898 they tried to up the velocity to 2,200 fps and encountered problems with the receivers stretching. This was not fine.
Today, Remington and Hornady both load .30-40 with 180 grain hunting bullets that do 2,430 out of 24" test barrels. They are noted as being fine for the old Krags too.
How does one be sure?
Momentum could hold the answer.
220 gr at 2,000 has a momentum of 8.69 kg m/s.
220 gr at 2,200 has 9.56 kg m/s.
180 gr at 2,430 has a mere 8.64 kg m/s.
This contrasts the muzzle energy numbers some.
220 gr at 2,000 sports 1953.7 ft/lb or 2,648.9J
220 gr at 2,200 sends 2,364.0 ft/lb or 3,205.1J
180 gr at 2,430 delivers 2,359.7 ft/lb or 3,199.3J
Going by the energy contained in the bullet, it would appear that modern ammo should damage the receiver. But going on momentum, modern ammo is nearly identical to the safe 1892 load.
Pretty neat, huh?
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