I can't argue for keeping this one.

https://www.navy-net.co.uk/community/threads/four-gun-broadsides.36940/

Someone's personal experience with half-broadsides and full-broadsides.  It's possible that someone a long time ago may have used 半舷砲 to indicate firing *half* the guns on one side of the ship?

半舷砲	0	0.0% <-- This term, no book results (no nothing)
舷側砲火	0	0.0% real book results (example: 舷側砲火が発揮可能な範囲内で敵艦隊に対して斜め方向に進撃し)
片舷齊發	0	0.0% antiquated kanji, all book results are military vocab references 片舷齊發交換す to exchange broadsides
片舷斉発	0	0.0% small handful of non-reference book results
片舷斉射	213	100.0% many book results
したがって、六発のロケット弾を一斉に発射する一機のヘルキャットは、駆逐艦の片舷斉射に匹敵するパンチ力を持っていた。

Usually a google book search returns matches for even our most obscure terms.  This search returns exactly one match:

The Supplementary Japanese-English Dictionary
By United States. War Department · 1945
https://www.google.co.jp/books/edition/The_Supplementary_Japanese_English_Dicti/s0hFZK2MMlUC?hl=en&gbpv=1&dq="半舷砲"&pg=PA72&printsec=frontcover
半舷砲 -> broadside fire

(The resource itself is incredible to look at(you can download the pdf).  The English is typewriter-typed, the Japanese is hand-written in Kanji for every single term. This would also appear to be the source of the "half watch ashore" gloss for 半舷上陸, which might have been an invention of the translator even then)

Otherwise, no book results, no news results, no ngrams, no Japanese references (except a single example sentence from sources unknown), basically nothing.

(three pages of standard search results are all J->E dictionary entries)

We already have 舷側
1. ship's side; broadside

We don't have 舷側砲, which is in wikipedia, and doesn't mean "fire" from the broadside, but "the artillery battery" on the broadside. (or, a single canon on the broadside)
https://ja.wikipedia.org/wiki/舷側砲
舷側砲	202	1.1%
舷側	17385	98.9%

The War Department reference actually has another term for broadside fire the matches the above:
舷側砲火 GENSOKU HŌKA Broadside Fire.
This also gets 0 ngrams, but *does* appear in book results on war/naval activities etc.

Perhaps more promising is this entry:
"片舷齊發" HENGEN SEIHATSU Broadside, in naval gunnery. 

The kanji seem obsolete, I would expect this to equate to:
片舷斉発 <-- no ngrams, but has some book results

But an ngram search on 片舷 gives... 
片舷斉射	213 *This* would seam to be the rough equivalent of "broadside fire". Quite a number of google book results.

We don't have any of these other terms.  Might be worth adding any of them with appropriate [rare] entries, except maybe for 片舷斉射.

The simplest explanation for 半舷砲 is that this was a mistake when it was catalogued in the first place. Or, it was a rare one-off usage that someone happened to capture and catalogue, but it would seem no one has used it in writing since.