You really just need to be aware of MG1 and 2. If I recall, the manual for MGS even included a plot sumarry, with a handy retcon for 2.
Also, I highly doubt MGS5 will be called MGS5 but we'll see. I think the series should have dropped numbers after two, as all series pretty much have to do it eventually, for their own good.
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^Poorly disguised anti-caps sig