I only have 1 that is not factory. (Actually have 2 but the 2nd one is not installed yet).
It is a PT&G bottom metal with Accuracy International magazine.
I don't care to deal with PT&G as a company and the AI mags are expensive. I have been happy with how they both work together so far though and would buy again if needed.
Depending on your rifle platform, you may have better and/or less expensive options.
What are you trying to accomplish with a new mag system?
Edit - I don't want to turn this into a PT&G bashing thread. But I did think it might benefit Sully2 to explain why I made the comment above. I have ordered 3 things from PT&G. They have mostly been good at responding to phone calls and emails (they get a B+ there). However 2 of the 3 things I ordered that showed "In Stock" on their website were in fact not In Stock and took many months more than the 6-8 weeks estimation listed on their website. I think the last bottom metal I ordered from them took 7 months before I got it. 1 of the 3 orders I put in with them I did receive quickly. (So I would have to say C- there).