Rifles/shotguns and a motorcycle
A soft bass-guitar case with backpack style straps is the way to go. A rifle shaped bag may attract too much attention and a rectangular rifle bag with only one strap gets too fluttery at freeway speeds, unless you use a very short one for a carbine or shotgun with the barrel off.
I suggest a bass case, as they are a bit longer than guitar cases and built to carry a heavier load. Measure the over all length the longest rifle you will be travelling with, and go to your local Guitar Centre or such and check out their wares. I use a Roadrunner case; it is well padded, and has a few Velcro inserts that help to keep the rifle from shifting, or to keep two from banging into each other. A small lock will keep the zippers from opening unexpectedly, and keep it legal if you decide to bring your EBR to the range. I did one trip of about 150 miles each way with my case and it worked like a champ.
http://www.musiciansfriend.com/product/Guitar/Accessories?sku=541330
SHOTGUNS AND RIFLES
Nonconcealable firearms (rifles and shotguns) are not generally covered within the provisions of California Penal Code section 12025 and therefore are not required to be transported in a locked container. However, as with any firearm, nonconcealable firearms must be unloaded while they are being transported. A rifle or shotgun that is considered an assault weapon in California must be transported in accordance with Penal Code section 12026.1.
REGISTERED ASSAULT WEAPONS
California Penal Code section 12285(c)(7) requires that registered assault weapons may be transported only between specified locations and must be unloaded and in a locked container when transported.
The term "locked container" means a secure container which is fully enclosed and locked by a padlock, key lock, combination lock, or similar locking device. This includes the trunk of a motor vehicle, but does not include the utility or glove compartment. For more information, refer to California Penal Code Section 12026.1.