M249 Rail Fitting

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To experienced M249 Owners and Experts:

I saw the following rails on EbAirsoft:
http://www.ebairsoft.com/eb-cnc-m249-to ... -2429.html
http://www.ebairsoft.com/aabb-aluminum- ... -3869.html

Both the upper and lower; I only want (can only have) one on there to keep the weight down. Does anybody know if they need to be mounted without the standard handguards, or can the standard heat shield be left on top/bottom while the rail (of either upper or lower) is mounted?

Bottom line: I want one rail on my M249. Do I need to take the plastic handguards/heat shields off or can I leave them on (besides the one I am replacing with rails)?

Thank You for your responses.

[league 4]

If you can only use one, you have to get the bottom RIS. You need to take the handguard off but the heat shield can stay on.

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That's what I figured. Thanks!