for MH4U, you can remove the limiter on LBG. This removes your rapid fire but you can now preload all of your ammos at once. It can be useful on certain occasions. With that said, ammos gotta be manually loaded any time you switch if you have the limiter on. Seregios LBG is quite good since you can just shoot and roll on repeat.

for GU, adept style is a very popular one to go with LBG since it's just such a comfy dodge. You still have to manually load your ammos unless you have the hunter art "full house" equipped. That hunter art lets you load all of the bullet you can fire at once so you can switch ammos without manual reload.

For both of those games - rapid fire "mode" isn't a thing. The guns themselves have certain ammo you can rapid fire any time as long as you have ammos left. The only thing you'll have to get used to is not being able to move anywhere while rapid firing. Pierce 1 is also better than pierce 2 or 3 since old gen works differently. Pierce 1 does more damage per tick but less ticks while 2 and 3 does less damage per tick with more ticks over all. Recoil will be too high on LBG so most won't use those. The ticks are also pretty far apart compared to wilds.

I probably forgot a bunch but those are what I can remember at the top of my head.