Never used them for weapons retention, but have used them in other tool applications. The source is KJ Magnetics and you can obtain sample packages. Even the tiny magnets are powerful and can draw themselves into places you don't want them to go.
http://www.kjmagnetics.com/
They have a FAQ page that answers some questions.
KJ notes standard neodymium magnets will lose strength if heated over 178°F.
I will note that you do not need to roll in the dirt to attract iron bearing soil. Take that into consideration when using for waepons.
If the magnets are mounted on the weapon itself, will it affect the reliability to chamber or cycle rounds? Definite no-go with a semi-auto shotgun, I would think. They will affect a standard compass and possibly comms in general, so you'll have to inlcude your Echo to test the use. KJ indicates their magnets are RoHS compliant, but then they also can not ship amounts over a specific power through aircraft. This has to do with airborne electronics or navigation equipment.