Thank god...and I hope nobody actually listened to him since he clearly didn't know anything about acro...never seen a zero G barrell roll.
'Ballistic' roll maybe, but those are a good way to dump oil all over the belly if you don't have inverted oil. Truth be told, this airplane (like almost any airplane) could be successfully (aileron) rolled. But if you're going to take it upon yourself to perform acro in an airplane not approved for aerobatics, you better be experienced and knowledgeable about exactly what you're doing. I have a feeling his "zero G barrel roll" would have ended up an "accidentally very high speed, high G barrell roll".
The "non aerobatic aircraft barrel roll" comes up quite often. Much of the interest in this area comes from Bob Hoover's displays in the Shrike and a few other display pilots who routinely do display acro in non aerobatic airplanes.
The way I've always handled this when it comes up is very much as you have done; to first of all explain that the pilots who do this are extremely qualified, on waivers, and operating these aircraft within their respective envelopes whether it be normal or utility. I next explain that these airplanes in the hands of a non professional attempting aerobatics are VERY likely to have an end result of stress either achieved in the maneuver itself or recovering from the maneuver that are outside the envelope limits.
In other words, SOME maneuvers can aerodynamically be done in these airplanes and some can not; NONE legally without special waivers.
So where does this leave us in attempting to barrel roll an Archer?
The problem we always have when dealing with issues like this one with a newbie or even in a flight safety seminar is that we have not one but 2 issues to address here; first is that it is most certainly possible (but as I've said, not advisable) to barrel roll an Archer or a like aircraft for that matter.
Issue number 2 involves explaining to a newbie the "facts" that would indicate the danger involved, if the barrel roll, or any aerobatic maneuver for that matter, is attempted by a NOVICE. In some cases this means convincing a pilot with extremely good flying skills but 0 aerobatic skills that HE or SHE is NOT safe in attempting these maneuvers in these airplanes, or in fact attempting ANY aerobatic maneuver in ANY airplane without proper training in a proper aircraft.
Addressing the roll issue per se, strictly from an aerobatic standpoint, and addressing the issue from the viewpoint of a PROFESSIONAL DISPLAY PILOT; barrel rolls are actually the safest of all the rolls, including aileron rolls, to perform in a non aerobatic aircraft. The reason is POSITIVE CONTROL of the g completely through the roll.
In an aileron roll in a non aerobatic airplane, the roll rate is usually so slow that unless an extremely high nose attitude is used for the entry setup, a HUGE dish out will occur on the backside resulting in excessive altitude loss, not to mention the added g and asymmetrical angle of attack issues inherent to a rolling pullout.
In a barrel roll, positive g can be varied, but it's positive all the way through the roll including the recovery; a MUCH safer scenario in a non aerobatic aircraft.
So the bottom line on this issue is that yes, it can be done, and yes, it MUST be done correctly or, YES, the results can be anywhere from a mild to a massive over g in a botched recovery. Yes, it's illegal, and definitely NO, no competent aerobatic pilot or aerobatic instructor would advocate that it be attempted at any time by any pilot outside the extremely small world of highly trained professionals who do these maneuvers on waivers.