The Halo franchise has built up a unique arsenal of weaponry over the years that loyal fans have kept up with and created their own replicas. Some of which are for sale and a great gift for mega Halo fans.
Etsy.com has a wide array of fan-made Halo gun replicas for sale that you could gift to a friend/loved one for them to display and collect. This foam 1:1 scale Covenant Plasma Rifle, for example, looks cool and is relatively inexpensive. There are much more detailed and lifelike choices, too, if you’re willing to step up your budget.
Even Walmart is getting in on the action. They are selling this BOOMco Halo Convenant Needler Blaster that actually fires Nerf style! It currently has 5 stars with 71 reviews, so it sounds like it might work pretty good, too. Alternatively, you could also pick this and other similar items up from the official Halo Waypoint store, but it might be a little more expensive.