米軍は敵艦攻撃や対地攻撃を想定していたが、自衛隊のそれは対空ミサイルなどが目標だ。
大きさが異なるのはそのため。
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https://en.wikipedia.org/wiki/Railgun

As of 2020 the Navy had spent $500m on rail gun development over 17 years. The Navy
was focusing on firing hypersonic projectiles from existing conventional guns already
available in numbers.[98] On 1 June 2021 The Drive reported that the US navy's proposed
2022 fiscal year budget had no funding for railgun research and development.[99]
Technical challenges could not be overcome, such as the massive forces of firing wearing
out the barrel after only one or two dozen shots, and a rate of fire too low to be useful
for missile defense. Priorities had also changed since railgun development started,
with the Navy putting more focus on longer range hypersonic missiles compared to
comparatively shorter range railgun projectiles

■２０２０年現在米海軍は５億ドルを１７年間に渡りこの電磁誘導砲に費やしてきた。
　２０２２年度にはこの予算はゼロになった。

■いくつかの技術的課題が克服できない。例えば１０発から２０発発射した後にかかる巨大な力や
　ミサイル防衛で実用化するにはあまりにも少なすぎる時間当たりの発射数などだ。

■現在では優先順位が変わり、電磁誘導砲よりも極超音速ミサイルの方が高い。