OKB Grigorov


USA Self Propelled Anti-Tank Gun M50A1 Ontos

Kit #: 72023 Preview by Will Alcott - will_alcott(at)yahoo(dot)com
Edited by Al Magnus


The Multiple 106mm Self-propelled Rifle M50A1 Ontos was developed to provide a powerful self-propelled antitank weapon that was light enough to be airdropped. The original customer, the US Army, rejected the entire production run of the M50, and they were transferred to the Marine Corps, who had similar needs for an antitank vehicle that could be used in amphibious operations. Of the 297 M50s produced, 176 were re-engined and re-designated as M50A1s. The M50A1 was distinguished by the extensive engine louvers on the sloping upper hull.

The Marines took the Ontos into combat in the US intervention in the Dominican Republic, where it actually was used successfully as designed, as a tank killer. Soon afterwards, the Ontos was deployed to Vietnam, where there was little call for its antitank capabilities, but it proved itself a useful infantry support weapon, firing HEAT rounds against bunkers and beehive rounds against troops in the open. The M50 had many serious shortcomings, such as thin armour and the inability to reload the recoilless rifles without leaving the vehicle. The Marine M50A1s fought through the Tet offensive and the siege of Khe Sanh, but were retired by 1970.

The kit

The kit is packed in a sturdy box with an illustration of a sandbagged M50A1 on the lid. Inside are 76 grey resin parts (the box says 83), two photo-etched frets with about 107 parts (the box says 106), and the instruction sheet. The parts are bagged, and the hull is wrapped in bubble wrap. Some of the smaller parts had separated from their casting stubs in the bag, but none were broken. OKB's resin seems to be a little more flexible and less brittle than most.

As usual, OKB's moulding quality is first rate. I found no evidence of sink marks. There is a little resin flash on some parts such as the spoked roadwheels, but this is easily removed. Some long and thin parts (recoilless rifle barrels, spotting rifle barrels, barrel travel lock tube) suffer from warping.


The instructions consist of a single double-sided A4 sheet, with shaded colour illustrations of the kit components, and various detailed diagrams of the assemblies with components in place. The resin parts are shown in grey and PE parts in brown or yellow, with part numbers in small black test, often on top of grey shaded areas. This makes for a difficult time in deciphering what goes where, exacerbated by the parts provided not always matching what is shown in the diagrams (see below for details). I find the instructions to be the main area where OKB Grigorov's kits could stand improvement.

No painting suggestions or decals are provided. Though consistent with OKB Grigorov's other releases, the lack of decals is disappointing, given that the yellow registration numbers worn by Marine Ontos (Ontoses?) are difficult to find.


The largest part of the kit (which is still only 50 mm/2 inches long) is the hull casting. This features nice sharp detail on the engine louvers, filler caps, and hatches. The driver's hatch and rear crew hatch are both moulded closed. The underside of the hull has no detail.


The eight paired roadwheels and two sprockets are crisply cast. The sprockets have 8 spokes and 13 teeth - they should have 15 teeth, though the difference is unlikely to be noticeable. The roadwheels have nice deep details, but will largely be hidden by the suspension side channels.


For a small vehicle, the Ontos has a surprisingly complex suspension. Unfortunately, the shaded diagrams in the OKB instructions don't do a good job of sorting out the parts. The two pairs of 'plain' suspension mounts (parts R27 - left and R28 - right, red box in below photo) mount the second and third paired roadwheels. The pair of mounts with both inside and outside trailing arms (parts R26, blue box in below photo) carry the fourth paired roadwheel, which acts as a trailing idler. The remaining pair of mounts (parts R29 - left and R30 - right, green box in below photo) are for the first paired roadwheel. Note that the mounts for the first paired roadwheels should have a cutout in their leading edge, as they would interfere with the sprockets otherwise. Each mounting arm should have a pair of track skids (R23) on top. The kit provide two pairs of small skids (yellow box in below photo) and six pairs of larger skids (purple box in below photo), but the instructions only show one style. Looking at reference photos, I can't see any difference between the skids on any of the mounting arms, so I think they should all be the same size. In any case, these will largely be hidden behind the side channels and track.


OKB have recently built up a reputation for their finely-detailed replacement tracks, and the parts in this kit live up to that standard, with very finely moulded details inside and out. Four lengths of 50 links are provided, for a total of 200 links. Only 75 links are needed per side, so there should be plenty of spares. Spare track sections were sometimes carried on the glacis in Vietnam. The Ontos was equipped with T123 continuous rubber band tracks, each track composed of 5 sections with 15 crossbars each. The kit track width measures 8.7 mm and the crossbar pitch is 1.5 mm. This scales out at 23% too wide and 5% too long in pitch compared to the actual dimensions of 20 inch with and 4 inch pitch. While the width difference is surprisingly large, it's about 1.66 mm on the model, which is not likely to be noticed.

Recoilless rifles

The most prominent feature of the Ontos is the armament of 6 M40A1C 106-mm recoilless rifles, mounted externally. Each rifle is provided as a single resin part (R6), to which PE parts are added for the brackets for the resin spotting rifles, the vented breech, and the breech operating lever. The operating lever in particular is a bit two-dimensional as a PE part, and could stand to be replaced. Note that while only four of the recoilless rifles mounted the spotting rifle, all six of them had the brackets to carry it. All the barrels suffer from various degrees of warping. Most of the barrel is a simple tube, so the warped sections can be replaced with plastic or metal tube of the appropriate diameter. OKB also offer an upgrade set consisting of 6 turned brass barrels, which eliminates the warping issue. However the brass barrels do not include the flared breech section, which will need to cut from the resin barrels.

Gun mount parts

The kit provides six M8C 50-calibre spotting rifles (parts R19, red box in below photo), however on the Ontos only the upper 4 recoilless rifles were fitting with spotting rifles, so two can be consigned to the spares box. All six suffered from warped barrels; however these could be easily replaced with plastic rod or hypodermic tubing. The rifles are missing the gas cylinder running parallel to the barrel for about half its length. The M8C could accept a curved 20-round magazine, or a shorter (10 round?) magazine. The kit seems to depict the smaller magazine. The tube that carries the barrel travel locks (R7, blue box in below photo) is also warped. While the instructions clearly show an M2 50-calibre machine gun fitted to the turret, the kit provides a 30-cal M1919 (green box in below photo), which is the correct weapon, although the guns fitted to the Ontos often featured a flash hider on the barrel. Strangely, while the resin parts provide the correct gun, the photo-etched parts include the spade grips, barrel handle, ammo tray, ammo can and ammo belt for the 50-cal. As a result, there is no ammo can provided for the 30-cal. A PE shield for the machine gun is provided, which seems to have been a field modification on some vehicles. The rotating gun mount itself is moulded with the hatch closed.

Hull Details

The small casting stub shown in the red box (below photo) contains the headlights (parts R14 and R15), taillights (part R17) and another, unidentified part. Comparing to reference photos, the parts can be identified as (left to right on my photo) blackout marker light, blackout driving light, service headlight, taillight assembly. My kit provided only one set of these; however two each of the blackout marker lights, service headlights and taillight assemblies are required. Early in its career, the Ontos carried a horn in the right hand headlight group; however this does not appear in Vietnam-era photos. The two liquid containers (green box) are the same parts provided in other OKB kits, and to my eyes are too thin and poorly detailed (though the carrying handles, cap and mount and strap are all provided as PE parts). A nice improvement over other OKB kits of US vehicles I've seen is the inclusion of a set of pioneer tools (blue box). However these don't quite match the instructions - the mattock is moulded as one part, and shown as two separate parts (R13 and R16) in the instructions. To fit correctly in the pioneer tool rack, the mattock head should be separate from the handle. A PE rack is provided, however the close-up computer-generated image of the resin tools in the rack shows the blade of the axe interfering with the handle of the spade, so some adjustment may be necessary. Another nice inclusion is the provision of a separate door for the external ammo compartment (part R5, yellow box). Eight rounds were stowed underneath the floor of the Ontos, and accessed via this separate hatch. The remaining hull details include a group of four spare track crossbars, a stowed 30-cal tripod, the exhaust system (with hollowed-out tip) and the sprocket mounts. Strangely, despite the extensive PE parts provided, the expanded metal screen used as a protective cover for the muffler and exhaust pipe is not included. Some vehicles had a vertical extension at the rear of the exhaust (perhaps for deep wading?)

Photo-etched parts

Two frets of PE parts are provided. A brass-coloured fret 0.2 mm thick includes the barrel travel locks, and the various brackets and clamps for the recoilless rifles. In my kit, this part suffered from a registration issue, and the details on each side were slightly offset from one another. A silver-coloured fret 0.08 mm thick includes the fenders, vented breeches and breech operating levers for the recoilless rifles, headlight, taillight and periscope brush guards, pioneer tool rack, doorstops, and antenna bracket. Less useful details include towing clevises, to be bent from flat PE.

I reported the issue to OKB, and I was eventually provided replacement PE parts. The registration issue was no longer present on the thicker sheet. Interestingly, the thinner sheet, which was now silver in colour, featured fewer parts, and the part numbers were different. The part numbers on the new sheet appear to match the instructions.

Some of the parts such as the barrel travel locks and breech operating handles can definitely stand to be replaced with more three-dimensional parts. I'm also concerned with the fenders. As provided these stick out a little over 1 mm from the hull at the rear (due to the tapered hull shape of the Ontos, the fenders decrease in width towards the rear). Looking at reference photos, the fenders should taper to almost nothing at the rear of the hull. I suspect this may be due to the overly wide track that would otherwise protrude beyond the hull, but it's difficult to tell without building the model.


Without assembling the model, I can't easily judge scale accuracy against published dimensions. The main points of accuracy that concern me are the overly wide tracks and fenders. Having seen a built-up model, the finished product does look very wide to me.

Out-of-the-box, this is one of OKB Grigorov's most complete offerings, with no need for the builder to provide pioneer tools or other standard stowage.

For an interesting diorama idea to show off your tiny Ontos, there are pictures of Marine Ontos hitting the beach at Da Nang, rolling off LCMs (probably LCM-6s, which would require a conversion to the Dragon or Trumpeter LCM-3 kits).


[1] R.P. Hunnicutt, Sheridan: A History of the American Light Tank, volume 2
[2] David Doyle, M50A1 Ontos - Detail In Action
[3] afvdb

Preview sample purchased by the author.

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Article Last Updated: 13 September 2017