The change of a single gun should not overtly affect the performance of any ship. If it's arc would be reduced, it depends on how drastic the change is. Right now it has 65/65 for left and right, and 10/35 for up and down (angles in degrees). The field gun has 15/15 for Left/right and 15/5 for up/down. That's a pretty big difference between the two.
Even if they cut the left/right in half, you'd still have a greater turning arc than the field gun.
For the mobula, the far right gun appears to be about 35 degrees off forward facing. Which means you'd need at least that much to point it forward, although you'd need a little more than that to hit things directly ahead of the ship (since the gun is off center). The bottom deck guns are angled just a tad less, maybe around 30 or so? So you'll have an easier time shooting forward and even scoring hits at enemies directly ahead.
Personally I think chopping it to somewhere around 45-35 for left and right, along with reducing the downward to 25-30. This still allows for the spread out mobula guns to hit a center forward target (and thus not really affecting it on this ship), but makes it harder on the junker. The junker side guns are 90 degrees from facing forward, and not centered with each other. While the front gun points directly forward. Around 35 it would be difficult to impossible to score a trifecta, but nearing 45 it's still difficult, but you generally hit the same ship. (You need something with more than 45 left/right degrees to get something approaching an effective trifecta)
At the very least getting 3 on a single target would be difficult on anything except a mobula, and that's probably fair considering the mobility weaknesses of the mobula (it actually has the best vertical acceleration of any ship, one of the best forward accelerations (average top speed), with an average turn speed and slow turning acceleration) and massive blind spot.
I'm sure there's a whole 'nother discussion about the weaknesses and strengths about each ship, but a single gun change shouldn't overtly reduce the effectiveness of any ship. Of course this idea is to reduce the ability to get this gun into a trifecta on most ships, making it easier to approach the ship / requiring more teamwork to use. There's plenty of other viable options that don't include carrying only artemis.
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Overall changing any gun is going to affect both the public games and the competitive scene, but allowing a weapon choice or combo to absolutely dominate in -either- of those is a bad thing. Making a little more difficult to use doesn't greatly affect the competitive play (I've done some 'silly' matches with high skilled clans and they've shown ability to use seemingly poor weapon choices to great affect), but should have a greater effect on the pub matches, where the greatest concern for this weapons abuse is.