Is an AR-15 upper receiver authorized while on deployment?

No,. not even a little bit allowed.
Done, sure Im sure it has been.

I know of a SGT that took a side arm to a little conflict in Panama once...but authorized it was not...and having to ask your Regimental commander to hand carry it back as he was the only one that was not searched by customs was not a fun request to be made....it was later taken in stride and understood why, with no lasting negative ramifications,,,but that was luck of circumstances...said SGT very well could have been Court Marshaled, demoted and received a BCD instead of having a long and fulfilling service .

What part of no isn't understood? You're issued a weapon, got a problem with it, you got an armor, you know that dud who knows the ins and outs of said weapon. Deal with it.

Is the upper a brand that the government contracts with already? Where are you going to secure your issued upper? Have you spoken to your PS or PL about it. Would it improve your mission capabilities? Or is this just an attempt to be an operator operating operationally?

I am not familiar with the exact regulation on this subject, however, I would not recommend bringing and using such items during a deployment.

But, as a gun-guy myself, I do completely understand the desire to do so. As a leader, my biggest concern would be that opening such a door would lead to every Joe Snuffy bringing in wild modifications that could possibly be unsafe or unreliable. I am sure that most everyone here knows enough about basic rifle functions where this wouldn't be a problem, but there is always that one guy who would ruin it for everyone.

I recall an issue in Iraq where one of our Soldiers tried to replace the pistol grip to his rifle, not realizing that the safety selector spring and detent were held in place by it. Luckily our armorer was able to fix his modification for him.

I don't know how the Army does it but I know Marines and Navy have what is called a weapons card that you turn in every time you check weapons out. The Card states you will not do any modifications too your weapons with out the Armors or weapons officers permission.

I will also say MISPEC is the big thing.

It violates AR 750-10 "No modification is authorized for an application unless it has an approved MWO number. Commanders will not allow their equipment to be modified unless there is an official MWO.".

It does meet the definition under the regulation as a modification which is defined as "Modification is any alteration, conversion, or modernization of an end item or a component of end item (COEI), which in any way changes or improves the original purpose or operational capacity in relation to effectiveness, efficiency, reliability, or safety of that item. This includes, but is not limited to: conversions; field fixes; retrofits; rebuilds; redesigns; upgrades; extended service programs; engineering changes; software revisions; system enhancement programs; service life extension programs; system improvement programs; product improvement programs; preplanned product improvements; modifications developed and applied by contractors as part of prime vendor support
or contractor logistics support (CLS) agreements; horizontal technology integration; continuous technology refreshments (CTRs); technology insertions; and all other terms used to describe modifications as defined above."

At the ranges in which you are able to positively identify a legitimate target, the accuracy of the M16/M4 series is fine. So there isn't a need for greater accuracy for the average Soldier. A Designated Marksman is a different story, but they have different rifles for that role.

Like everyone and the regs are saying, no is the answer. Soldiers who are not well supervised do it, and there is no upside when it goes wrong. I've seen 100 rd circular drum magazines in M4's that were full of dirt from a lack of cleaning. My head exploded so violently that there were chunks of Soldier ass parts strewn everywhere. The weapon(s) looked cool, but were going to fail miserably when they really, really needed it to work. We all know what happens next.

If there is ANY piece of the weapon that is personally owned and not government issued, that is not authorized. I had a briefing on this when I deployed, but I can't remember the regulation or ALARACT, so I would contact your OPS NCO at your unit or at your BN. They can certainly clear this up and provide the document

As an additional question; if I have an ACOG (identical to the army issued one) but a unit only has the aimpoint cco available, am I able to use that instead? Seeing as all marksmanship principles lead to "shooter's preference"?

I thought that this discussion was about the AR-15 5.56mm rifle SGT (Join to see)
In times past certain units or individuals were given amphetamines for certain type of operations.
Of course individuals have illegally used amphetamines at times. This individual type of illegal use would never be "allowed."