I'm more familiar with 3-gun but want to start competing in Service Rifle matches, so I'm not familiar with how the CMP and NRA sway with their interpretations of the rules.

I have an Aero M4E1 receiver set which is a nonstandard forging; originally I wanted to use it for the CMP High Power Service Rifle matches but never got clarification since it was a nonstandard forging.  I checked today and the rules appear to have changed to allow billeted receivers that don't have an integral trigger guard; which the Aero receiver does have.  I also wanted to use the rifle for the CMP Games Modern Military matches as well as the NRA High Power Service Rifle matches.  

Both of those rulebooks don't make mention of the nonstandard forging or the integral trigger guard but instead just mention that the rifle has to me similar in appearance/function to the M16 pattern.  Should I have an issue using this receiver set for Modern Military or NRA High Power Service Rifle?  I haven't finished assembly of the rifle yet, but would prefer not to switch to a different receiver unless I have to.

Any info helps, thanks.

If it has an integral Trigger guard, then no, not legal. NRA rules and CMP rules and CMP modern military rules are all different. NRA rules are more restrictive and I would follow those as they will keep you good for CMP as well. Best bet, just build it with a regular mil spec upper and lower and avoid any problems. It’s Just too easy to do it and be well within the rules, instead of trying to interpret your way around the fringes, and create potential problems later on.

I'll likely finish this build on a milspec receiver set, I just happened to have this set already then decided that I would build it out for SR.  Trying to make do with what I have before I sink anymore money into this and have another set laying around.