I have to admit, I'm a bit a newb when it come to replacing the handguard with a quad rail because of the gas tube block placement. My new Model M&P10 Compliant SKU#811310 has the original plastic handguard and I want to replace it with a rail w/ a 45 degree angle I've measured the original one and it seems to be 8", but I can't find a 8" replacement. I really didn't want to have to remove and replace the gas tube block with one that would fit in the new rail, any ideas or ways around this or am I measuring the handguard wrong? (could it be 8.25" long?) I watched videos, but still a bit unsure about the correct method to measure it.

I believe the hand guard is 9". As far as the quad rail, if you can find one that will accept accessories and rails, I think you would be better served. Like an m-lock or key mod compatible hand guard. Quad rails tend to be heavy compared to other hand guards and are nick named cheese graters for what they do to you hands.

There should be a few options out there for you that will use the existing delta ring and gas block you have. Check out Brownell's and Midway USA to start.

Thanks for the heads up, you're right, I forgot about the weight issue, I added a Midway Quad to my SAMR7 Arsenal Milled AK with accessories and now it almost needs to be MG mounted to the Jeep it's so heavy...lol
I'll check those out today and see where it leads.