If you're looking to upgrade your airsoft rifle, why not start with an underslung grenade launcher? The live-round versions are considered "destructive devices" under the National Firearms Act because of their "non-sporting capacity." It is possible to find them on the NFA market, but you would be hard-pressed to find a seller. The actual 40mm grenade is $400 to $500, and they are subject to a $200 federal tax. The rounds must also be stored in a safe location, since they are considered high-explosives. Between the expense and the regulation, it's just not worth getting the real version, but that's not the case with airsoft replicas.
Depending on the quality and type of airsoft rifle, the price range on an attachable launcher could be anywhere from $15 to $200.
GP-25 Grenade Launcher for AK-47
With the recent death of Mikhail Kalashnikov, inventor of the original AK-47, this is an opportune time to mention the attached underslung airsoft replica that attaches to the world's most popular assault rifle. The GP-25 is modeled after the real version underslung that was used during the Soviet campaign in Afghanistan. It also comes with an airsoft BB VOG-25 40mm shower grenades, and it has the look and feel of the real thing. It is made from a quality metal, with a polymer grip, and an adjustable grenade sight. It is replicated with a double action trigger and has a safety.
The DBoys version of this grenade launcher is $99.95, and it can be used as a stand-alone projectile.
M203 Gas Grenade Launcher for M-4 Rifle
This is another product by DBoys, and it comes a little cheaper than the AK-47 version. It will set you back $56.95 with shipping, and it is made with real metal and a durable ABS. It also comes with a M4 barrel mount and a universal 20mm QD mount for RIS rails. This version comes without 40mm grenade slugs, but the following shell brands are compatible with this version: CA, DBoys, Mad Bull, King Arms, etc.
The actual grenade shells can vary in the same price range as the launchers. You can find the the 24-round BB shower grenade on Amazon for $27.00, and it is compatible for most launchers. It is one of the cheaper grenade rounds to have, and it smacks of realism for those looking for an authentic feel. However, this replica has been known to break, and you may want to upgrade to a higher quality weapon if that scares you.
One of the higher-end grenade shower cartridges you can get is also a DBoys version. You can find this one for $199.95. It includes (6) 18-round capacity 40mm grenades for the DBoys version, along with other brand rifles as well. This is a gas-powered version that is comprised of metal composite alloys for a real-world feeling.
203 Spray Advanced for MK-16 Rifle
This is one of the more versatile launchers to have. It swivels to the left and right for easy reloading. This is also a great launcher for the MK-16 rifle or certain M-4 versions; however, one customer reviewer said that the gun does not fit M4 versions, so be on the lookout for that warning if you have the M-4. You can find this puppy for $77.40 plus shipping. This is a launcher that can be mounted on the SCAR heavy or light model, and it comes with two forward sling points. It is a high-quality launcher that will provide you with authenticity and efficiency during mock battle sessions.
Note: Launchers and airsoft grenades of any kind are prohibited by Canadian customs.