You'd have needed MUCH better optics with an extended-range Ontos; we considered it pretty good shooting with a 106mm M40 to get a first-round hit on a moving target tank at a thousand meters, and that from a fixed position using the M8c .50 spotting rifle. Todays missile-squirters are good out to 3 klicks, even 5; and some are *fire and forget.* And they're getting better, and some have thermal night sighting units.
The real advantages of the 106 was that with the gun mounted on a Jeep towing a trailer full or ammunition, it could be kept in service with the minimum number of rounds close to the crew where an enemy hit or near hit would detonate them all. Also there was a *beehive* canister round available for the 106, which made a pretty good countersniper response.
The better way to go nowadays would be an inexpensive *dumb* nonguided projectile for the TOW II and Javelin launchers now in service, with their more expensive guided rounds available for high-value targets like enemy tanks.
Neither would the shoulder-fired 40-pound M67 90mm recoilless be the greatest weapon to bring back to service; after firing five rounds a half-hour cooldown period is required lest backblast gasses erode the breechblock venturi beyond use. Our Rangers and other lightfighters still using man-portable recoillesses now use the Swedish Karl Gustav 84mm M2 for that reason and since US ammunition for the 90mm hasn't been produced since 1973.
You mean the Army is still not equipped the .45/70????? I must be getting old.