Soft touch for trim nailer
The &quot;mysteriously gigantic&quot; nailers are probably 15 gauge trim nailers similar to this one:

Sounds like the one that you have is a finish nailer, and is likely a smaller gauge and uses nails with smaller heads. Thus less driving power required and a smaller gun. A trim nailer will generally give you better holding power, but also higher possibility of splitting wood, especially thinner stock.
Assuming the nailer you have is working for what you're using it for and can use long enough nails, I don't see any reason to stop using it - I've trimmed with a finish nailer before without issues.
As far as your metal tip goes, I would doubt you'll be able to find one from the manufacturer for a cheap Harbor Freight gun.  One thing I've done in the past when I've lost mine or broken it is to lightly dip the tip in Plasti-Dip (the rubberized tool handle coating stuff). Just make sure that it doesn't interfere with the action of the gun at all 1/16 - 1/8 inch is plenty.