1. You were wrong to not remove the detachable magazine from your shotgun during the cease fire. (Your post failed to mention if the bolt was open to the rear, so the RSO had every right to be concerned).

2. The RSO was wrong to put his paws on your shotgun without first verbally asking you to comply with the ranges safety rules.

Two wrongs do not make a right, however, you committed the first wrong and therefore bear responsibility for all actions thereafter.

BLUF: You should be pissed at yourself...but hopefully will learn a lesson from this encounter.

I have seen good RO’s and some real dirt bags.

One of the worst things I have seen was at Quail Creek.
A guy had his family, wife and two teen aged kids, with him. He had a couple of pistols and was just there for some family fun. I spoke to him a couple of times during cease fires. He was not super experienced but was being safe and teaching his family how to shoot safely.
All was good until the range nazi came over while the husband was showing his wife how to load and shoot. She was the typical nervous and new shooter. After she shot about 5 rounds, the RO came over, picked up a loaded magazine, loaded the gun and began telling her how to “properly” stand and steady her shots. He then shot the 5 rounds in the magazine to show her “How it’s done”. He didn’t ask if they needed help or pointers. He didn’t address any safety violations. He just took it upon himself to load and shoot the guys pistol.
The guy was pissed but, what you going to do?

I have not been back there in years.

When it's all said and done Your lucky he didn't throw it in the Trash.

I believe I covered that in the last sentence of my OP when I noted how the RO must have thought the bolt (that's not under spring tension) would somehow close and lock itself on a live round.

It's because I own rifles with an internal magazine and floor plate (Winchester Model 70 and Weatherby Vanguard) that I've always left my bolt-actions on the rack with the bolt open and the detachable magazine in the rifle. After all, a closed floor plate conceals any rounds that may have been left in an internal magazine, as would be the case with a detachable magazine left in the rifle. IMO, it's the bolt left resting fully open that makes the rifle safe, if left alone of course.

I always lock the bolt open and remove the magazine on semi-auto rifles due to the spring tension and self-locking mechanism of the bolt.

Go find someplace else to shoot

i's got a single shot 12ga pump...
shot me 16ga Rem. 1100 for years dove hunting.
Whin son wanted do some duck hunting. Bought the 12ga (price of 16ga shells way to expensive).
Took forever ta remember ta pump...

& now back to the story...
Take it away Dan...

RO has full authority on a range. You subscribed to the rules of the range when you decided to shoot there and gave the RO authority to handle issues on the range to stay within his interpretation of those rules (not yours). So no, you had no right to be pissed. You want nobody to touch your gun? Buy 100 acres and shoot on your own.

99 times out of 100, the question "so was I right to be pissed" can be answered with, "no, because being pissed only effects you and not anyone else. You may as well choose to be happy and deal with the situation with grace and kindness."

question - was the range already 'declared safe'? were there any people already downrange?

My take away. RO handled a potential loaded weapon behind firing line. If he truly suspect a dangerous situation, a cease fire should have been called, line cleared etc.