In the one image you posted above your track Lee Anne has a 106mm RR mounted - how many of the ACAV's in a typical troop [hope that is the correct term] had the 106mm RR's?
Also, when bashing through the bush - was the barrel of the 106 depressed or kept horizontal?
No more than one RR per Company or Troop, if they had any at all. Ours, and as far as I know, all M113-mounted 106M RRs, were a field modification by some Mechanized Infantry Companies and Armored Cavalry Troops in-country.
The big gun was originally mounted on the M515A1C Jeep, but since the little Mutt could not follow everywhere the tracks went, it was decided to mount it on the Weapons Squad track, C14. I had the right MOS and an Expert Badge with the Recoilless, so I was assigned as the gunner as soon as I arrived in-country.
Ours was the only 106 track in the entire 2/47th Inf Battalion, and from what I have been able to determine, it was a fairly rare beast in-country. Many, probably most, Mech and Cav outfits had none at all. The rarity of Vietnam era photos of them is one indication of this.
On ours, the top of the original travel-lock from the Jeep was attached to a hinge arrangement welded just aft of the exhaust grill. This secured the muzzle end of the barrel in a depressed attitude, a few inches above the deck. That arrangement did a pretty good job of deflecting most branches and vines when we busted jungle. https://www.flickr.com/photos/132974504@N07/19667692526/
But the gunner (me, at first) stood in the open cargo hatch on the ammo box seat in the troop compartment, so he could move back and forth to prevent vines from snagging the .50 cal. spotting rifle, the elbow-sight housing, or the breech operating handle. That "wait-a-minute bush" was nasty!
The 106RR was a lightweight weapon - 462 lbs., less than a tenth of the weight of a towed 105 Howitzer. Consequently, its elevating mechanism was not terribly robust. Unless you were moving on a super-smooth surface (never happen, G.I.!) you would never leave the travel lock off. Slamming into a bump or ditch would shear teeth off the elevating gear. A sturdy branch could do the same thing. So the tube would never be horizontal going through the woods.
But whatever its faults, it was a very effective weapon against bunkers and structures using HE rounds, and absolutely devastating as an anti-personnel weapon using "Beehive" flechette rounds. Too bad there weren't any enemy tanks to take on - OK, not really!
If you have any other M40A1 questions, shoot. Just be sure to clear your back-blast area, first! MAJOR! ON THE WAY!