Like I said...
Gravity is not needed. Some early experimenters probably did not know how to convert grains(mass) to slugs(mass). They may have been more familiar with pounds. So they converted grains to lbf (pounds force) rather than lbm (pound mass). At which point they realized they needed gravity to get the units to jive. That is where the error came in. They used a roundabout way via gravity. With all it's pitfalls.
Your muzzle energy will be the same, even in the absence of gravity (deep space). So, no. Gravity is not needed to determine muzzle energy
Also: most all scales are calibrated to a known mass. So even though some scale may be using the resulting force of gravity as a measurement, they are calibrated to readout mass. The exception would be something like a trigger "weight" scale, which is calibrated to readout force.
A scale, such as a balance beam scale, always compares the mass being weighed to an existing mass. So the value of local gravity is irrelevant. That type of scale reads the same on the moon or on earth. It does not matter what value of gravity is used.
A spring scale measures a deflection distance to determine a force. If that force is the results of a mass under acceleration, and we know the acceleration (local gravity is a convenient means of acceleration), we can determine the mass. The value for the local acceleration of gravity is already built into a calibrated scale.
The conversion of grains to slugs:
https://www.google.com/search?q=grains+slugs&ie=utf-8&oe=utf-8#q=slug+grain That's where the 450436 comes from. (2 x 225218).
You can use gravity in the equation if you really want to. Just make sure you are using the correct value. But it's not needed. So why complicate things?
