Like others my information is based on a US model. When traveling by air with weapons the easiest thing to do is have your bag pre-inspected. This is actually required for firearms but that's another issue. Once that is done you get your bag pre-tagged and you don't have to use a TSA lock which is worthless as nearly every one that works with a ramp badge at the airport has a key along with every thief. This will allow you to use a real lock.
While airline carriers can not really offer a lot of help with this as it is government regulated you can buy insurance direct from your carrier for your checked bag so no matter what happens to your bag you will be insured. Our domestic carriers often cap that insurance at 5K which is usually around $50 one way. Pay for the up charge with AMX or another credit card that may offer you additional protection as well if you go that route. When you do this your bags are often tagged and hand delivered at the carousel.
In the US it is also wise to fill out a customs form before you leave and bring back items so you can show proof of ownership. There is nothing worse than paying tax on items you already own. I have no idea how that would translate to Germany.
I will tell you that I spent a few years working for a legacy airline and Frankfurt International had more problems with theft on the ground than any other International airport I dealt with. As phatch suggested shipping may be a very good option to consider but be sure to look into customs rules for Germany and shipper regulations first.