Hello Frederik, and congrats on your Luger. Hopefully you can find an aftermarket magazine or two soon and stop using your wood-bottom magazine, which may crack and fail (and be ruined). This is an example of what you want to buy, or its European equivalent.
http://www.midwayusa.com/product/137...-8-round-steel
I strongly advise you NOT to look for heavier bullets. They may go a bit slower but they will put MORE stress on your gun. You seem to be doing well with 123 gr. bullets, so I would stay in that 115 to 124 gr. range.