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Not sure on the weight of the bottom metal nor a 5rd AI.
Judging off your questions based on weight I assume this is a weight sensitive build??
The DBM system is hardly anymore weight than a BDL. Don't think it will make a huge difference in the end...
The weight of our 3rd mag .308 is on the website.
We have plans to make SAUM 3rd mags but it will probably be after our LA Extended.
Our current 3rd SA .308 mag will hold 2 SAUM cases.