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M777 Light Towed Howitzer (Working)
Fully functional shooting model of 155mm L32 ultralight towed field howitzer in scale 1:10
About this creation
*Visit my Lego helicopters blog also
** See all pictures in better quality at Flickr
***See model and building instructions in Lego Digital Designer (LDD): Here

Figure 1: Overview of M777 light towed howitzer

Figure 2: Long range indirect fire: In February 2012, a US Marine Corps M777 howitzer in Helmand province, Afghanistan, was used to fire a single M982 Excalibur GPS-guided, folding fin, base-bleed round that killed a group of insurgents at a Marine record range of 36 kilometers (22 mi)

1. Introduction

Although TLG offers many specialized parts and sets for old historic guns and all kinds of futuristic sci-fi weapons, it refuses support of building actual weaponry, and it is totally banned from Cuusoo. This has the sad consequence that TLG parts are very weak in modeling large diameter barrels, which are used all parts of life, not only in weapons. Most of the alternative building solutions are stronger but pretty much unaesthetic:

Figure 3: Different barrel designs from TLG parts

See model in LDD: Here

For example, one of the classic top MOCs in artillery theme, Gyuta K抯 White Tiger fully functional RC Tank with PF electric driven gun and autoloader from 2010 was forced to use the nasty rectangular shaped barrel design:

Figure 4: Gyuta K抯 White Tiger

My purpose was to create working gun with more aesthetic barrel in roughly the same scale. I opted modeling BAE Systems M777 155m L39 ultralight towed field howitzer used by US Marines instead of a tank gun because of two reasons:
1.In a towed field howitzer, there is no space for bulky electric drive and batteries, so it has to be a relatively simple mechanic device.
2.In my best knowledge, nobody ever attempted a shooting field howitzer MOC yet, so the modeling challenge is on. Among numerous non-working MOCs of M777, Colonel Henry抯 M777 minifig-scaled diorama is the best:

Figure 5: Colonel Henry抯 M777 diorama

2. List of working functions of M777

Figure 6: M777 in mountain battery: Flat trajectory weapons are pretty useless if the terrain is a little bit bumpy. Therefore, never forget the good old M777 from your luggage preparing your holiday in Afghanistan.

*The following parts are technical and assume some basic knowledge of artillery terms. See corresponding Wikipedia article for explanations.

**Parts of M777 are referenced by numbers can be found on attached technical drawings

***Parts of M777 are color-coded by function:
- White: Basic structure
- Red: Handles of firing, trigger
- Green: Projectiles
- Orange: Bolt
- Gray: Fire controls and telescopes
- Yellow: Manual handles of telescopes

Figure 7: M777 cutaway view
See model in LDD: Here

My M777 model follows the basic layout of BAE Systems M777, but it is not an exact proportional copy of it, because I had to make compromises between styling and functionality: ABS has a little bit smaller tensile strength than forged titanium the real M777 is made of. List of working functions of my model:

2.1. Shooting mechanism

Figure 8: Flying bolt-type sling shooting mechanism
See model in LDD: Here

The (E2) flying bolt-type sling shooting mechanism is driven by (E6) four 1.6mm rubber chords fixed to bolt and running forward between inner wall of barrel and projectile. Then they turned backward and outside of barrel by four (B8) rubber chord rollers placed in the muzzle brake. (E7) outer end of four rubber chords are anchored at the breech. (E2) Bolt can be pulled back by (E8) rope lead through the central hole of breech. One has to pull bolt until (E3) arrester hook depressed by (E4) rubber dumper catches it, clicking into the circular valley in the middle of the bolt. One can shoot depressing (E5) trigger arm, which disengages arrester hook. (E8) rope has a small ball fixed at its rear end, which stops at the breech抯 hole, preventing bolt flying out from (B1) barrel after shooting.

2.2. Chamber and barrel

Figure 9: Barrel mount mechanics
See model in LDD: Here

The shooting mechanism described above provides high propellant force in a relatively short barrel, but requires the breech to be closed. Therefore (E1) projectile is loaded into (B5) chamber through a slot at its top.
(B1) barrel has aesthetic 4-stud circular outer cross-section, because it is made of parts 慍urved wall element 2󫶙, which have rectangular 22 studs inner bore. This allows inner section of rubber chords fit between barrel wall and the 2-stud (16mm/0.63, in real size 160mm/6.3) diameter circular cross section projectiles and bolt. Curved wall elements can tolerate high compressional stress of rubber chords pretty well, but tensile strength of barrel is weak. Therefore we cut back barrel length from L39 of the real M777 to L32. Moreover, barrel is reinforced by (B2) barrel girders. The shortcomings of this barrel solution are:
-Girders disturb aesthetics.
-Barrel wall cannot be tapered towards muzzle as in the reality, so it will look disproportionately thick.
-There are bays in the inner wall of barrel because of the awkward inner shape of 憆ounded wall element parts it is made of. If projectiles and bolt are stuck in these bays, they jam the gun.
The latter is prevented by the symmetric layout of (E6) inner sections of four propellant rubber chords around projectile and bolt, holding them tightly. But it means that one had to press projectile among tensioned rubber chords at loading. Chamber has valleys in the inner wall to accommodate rubber chords while they are pressed aside at loading.
As the barrel takes high forces at manual pulling back bolt by (E8) rope, (S16) elevator and (S5) traverse gear would not tolerate this. Also it would be not very ergonomic to pull back rope when barrel is elevated at high angle. Therefore barrel can be attached/detached to/from recoil dumper mechanism to facilitate easy and fast reloading. (B7) barrel arrest hooks clamp into (R4) barrel receivers to hold the barrel in its place against the force of recoil, and transmit force to recoil dumper. The tricky thing is that sling shooting mechanism has forward coil effect after the initial recoil, when the flying bolt is decelerated at the muzzle. (R5) receiver locks loaded by (R6) springs made of parts 慍orrugated pipe 2M hold the barrel in its place against the force of forward coil. Setting smaller tension of (R6) springs, barrel can be auto-ejected by forward coil after firing. This is not very realistic that barrel jumps out from cradle after each shot, but largely enhances playability facilitating fast reloading. When (R6) springs are set to high tension, barrel will stay in the cradle and has to be removed manually. It is more realistic but slower to handle.

2.3. Recoil dumper

Figure 10: Recoil dumper
See model in LDD: Here

Recoil dumper mechanism uses four (R2) 慚otorcycle shock absorber parts allowing barrel to slide back by recoil 4 studs (32mm/1.26, in real size 320mm/1 0.59) then recuperate. As sling mechanism has smaller recoil than chemical propellants, I omitted hydraulic system of the real M777, so the non-working (B3) hydraulic accumulator at the top of the barrel just serves as a grip to hold barrel at reloading.

2.4. Firing cycle

Figure 11: Firing cycle of M777
See model in LDD: Here

Summarizing the previous paragraphs, firing cycle of the M777 consists of the following steps:

STEP 1: Depending on tension setting of (R6) barrel receiver lock springs, barrel is auto ejected after shot from cradle/ or detached manually pulling it forward and up.

STEP 2: Grab barrel by (B3) hydraulic compressor lines.

STEP 3: Pull back (E8) rope until trigger catches bolt.

STEP 4: Load projectile into the top loading slot of chamber, pressing it among the 4 tensioned inner rubber chords.

STEP 5: Press down and backward barrel into its receiver until the lock clicks.

STEP 6: Press down trigger with your thumb to fire (you can hold the bottom of the cradle in the meantime with your index finger to prevent influencing elevation angle).

2.5. Cradle

Figure 12: Cradle
See model in LDD: Here

Cradle houses the recoil dumper assembly, which slides in that at (C6) numerous points. It connects ordnance to saddle through (C4) trunnion axles, moreover to elevator gear and balancing gears. Cradle was pretty easy to create from Technic. The biggest difference from real M777 cradle is that there is no forward yoke build into cradle, to enable barrel to be removed upward at reloading.

2.6. Saddle and elevation-, traverse-, balancing gears

Figure 13: Saddle and elevation-, traverse-, balancing gears
See model in LDD: Here

The saddle with (S2) trunnions, the (S1) traverse turntable made of part 慣urntable 7 studs diameter, and the (S5) traverse control wheels with (S9) traverse worm gear+Z8 gear combo were pretty straightforward to create.
But building elevator gear was a nightmare, as I wanted to follow the layout of the real one. Designers of BAE Systems M777 apparently wanted to save space under the cradle and used a hinged screw protruding forward from the forward edge of turntable + rotating nut. But it means that elevator driver gear is at different angle and distance from trunnion at different degrees of elevation. First I tried to use TLG part 慙inear actuator in elevator gear, but it was too long, too bulky, and not enough massive to handle such a lifting mass. Therefore I created a bigger linear actuator from technic parts: Rear end of (S13) elevator main shaft has a bearing in (S10) hinge attached to (S1) turntable. Main shaft has (S11) three consecutive worm gears creating a longer screw, which can be turned. It is rounded by (S12) fixed nut assembly created from girders and Z8 gears, whose teeth catch worm gears. (S13) main shaft wit screw can be rotated by (S14) distributor gear made of 3 Z12 half bevel gears (LDD cannot draw all 3 in correct alignment). Distributor gear receives drive from (S15) right and left half axles, which are rotated by left/right (S16) bevel gear pairs. As these can be at different angle and distance from trunnions at different elevations, we connected them with (S3) elevation control wheels placed at trunnions with the combination of (S8) telescopic axles + (S7) universal joints + (S6) springs. Springs has the role to press driving axle into hollow of telescopic axle at high elevations, to prevent their disconnection.
It was also interesting story to create (S17) balancing gears. These are to reduce stress of elevation gear at low elevations, when it supports lifting mass by just short arm of force. The barrel is more depressed, springs in balancing gear are more compressed to hold large proportion of lifting mass. The original balancing gears resemble to TLG 憀inear actuator part, but it has a pretty different function. So, I had to create balancing gears, which do not follow the original ones in styling, but they work correctly. Springs are made of 6 parts 慍orrugated pipe 2M.

2.7. Platform, spades and stabilizer arms

Figure 14: Platform assembly
See model in LDD: Here

Platform is a gridded lightweight structure supporting the turntable by (P6) aligner pins. At its rear edge, there are (P12) spades placed on liftable spade arms. Lifting is made by (P7) worm gear+Z8 gear combo and (P8) lifting wheels. Spades have the function to prevent platform sliding backward on the ground by recoil forces. (P10) rubber dampers in spade arms are to reduce recoil shock (in the reality, they are hydropneumatic cylinders). Moreover I added (P9) safety clamps, which lock spades arms in lowered position to prevent their accidental jacking. (In the real M777, the special geometry and weight distribution of the platform prevents it.). Further difference from the real one is that spades can be rotated around (P13) pivot axis in spade arms by 45 degrees inward, when they are lifted. This reduces the gauge my M777 can fit into (16 studs wide 15.5 studs tall). Rotation of spades are prevented by (P14) spade locking pins held by (P11) rubber friction grips, when they are deployed. Stabilizer arms protruding from (P4) hinges placed to the forward corners of the platform are pretty similar to real ones. They can be locked alongside platform in road transport mode with (P3) transport locks and (P5) locking pins. I added (P1) leveling screws to the end of stabilizer arms made of worm gear + two Z8 gears combo.

2.8. Road wheels, suspension, brakes

Figure 15: Road wheels, suspension, brakes
See model in LDD: Here

(W1) road wheels are retractable alongside cradle. Their (W5) swing arms can be lifted from horizontal to vertical position with the help of (W8) Z24 gear rotated around (W9) pivot axle by (W10) worm gear driven by (W11) crank. (W10) worm gear could slide easily on crank axis, but this is prevented by (W12) spring made of part 慍orrugated pipe 2M. This is how we created retractable spring suspension (in the reality it is made by hydraulics). The real M777 has pneumatic road brakes operated from the towing vehicle through a long cable. We solved braking with much more simple inertial brake: as M777 is towed by its barrel, when towing vehicle deploys its brakes, it presses back barrel in cradle, just like recoil. Therefore (R3) girders of recoil dumper press (W15) brake arm placed by the rear edge of cradle. Then it pulls (W14) and (W7) brake bowden cables held by (W13) and (W6) holders. Pulling cables press (W3) brake shoes to (W2) brake discs.

2.9. Fire control telescopes

Seiler Instruments M137A2/A3 Panoramic Telescope is a 4-power, 10-degree field of view instrument with mechanical counter devices to control indirect fire. It is used on the left trunnion of M777, M198 and M119 howitzers. It is a highly complicated mechanical device, so its modeling was a big challenge, even with strong simplification. (Nobody ever tried working fire director telescope MOC yet.) As my Anonym Legoholics Therapy Group was on a winter holiday, I tried it.

Figure 15: Seiler Instruments M171A1 Mount with M137 Panoramic Telescope
See model in LDD: Here

The M171A1 mount can be attached to left trunnion by (F1) hole. It enables vertical alignment of M137 Panoramic telescope even if M777 howitzer is placed on sloped terrain. It is solved by two worm gear + Z8 gear combo driven by knobs (F3) and (F5) for forward/rear and left/right alignment. Correct alignment can be checked by (F25) and (F26) fluid leveler glasses. Also the mount enables to raise/depress telescope to achieve correct alignment with barrel. It is done by (F10) vertical worm gear + Z8 gear combo. Alignment with the barrel can be checked by (F27) detachable probe. Azimuth of the target can be measured at (F15) azimuth worm gear and (F16) knob, relative to a direction showed by (F38) fluid compass. Elevation of the target can be measured at (F24) elevation gear and knob. The telescope mixes the picture from (F15) objective and (F23) movable mirror at (F17) 45-degree prism. It can be viewed by (F12) side ocular and (F19) rear ocular. (F23) mirror is rotated by (F21) knob and (F22) gear to measure distance of target. Firing solution is computed at (F6) instrument panel.
The M138A1 Elbow Telescope is an 8-power, 8-degree field of view instrument for controlling direct fire. It is used on the right trunnion of M777 and M198 howitzers.

Figure 16: Seiler Instruments M172A1 Mount with M138 Elbow Telescope
See model in LDD: Here

It works pretty similar to previous telescope except that M172A1 mount cannot lift/raise telescope, and telescope does not have movable mirror.

3. Dimensions of M777

Inner bore of barrel: 2.00 studs / 16.00 mm / 0.63 in, Real size: 0.16 m / 0 ft 6.30 in
Outer diameter of barrel: 4.00 studs / 32.00 mm / 1.26 in, Real size: 0.32 m / 1 ft 0.59 in
Length of barrel without chamber: 66.00 studs / 528.00 mm / 20.79 in, Real size: 5.28 m / 17 ft 3.74 in
Total length of barrel: 80.00 studs / 640.00 mm / 25.20 in, Real size: 6.40 m / 20 ft 11.81 in
Total length of howitzer deployed (with barrel at 0 degrees elevation): 106.00 studs / 848.00 mm / 33.39 in, Real size: 8.48 m / 27 ft 9.65 in
Width of carriage in road mode (without telescopes): 16.00 studs / 128.00 mm / 5.04 in, Real size: 1.28 m / 4 ft 2.36 in
Height of carriage in road mode (without telescopes, platform nearly sliding on the ground): 15.50 studs / 124.00 mm / 4.88 in, Real size: 1.24 m / 4 ft 0.79 in
Total length in road mode: 102.00 studs / 816.00 mm / 32.13 in, Real size: 8.16 m / 26 ft 9.06 in
Length of carriage in road mode without barrel: 36.00 studs / 288.00 mm / 11.34 in, Real size: 2.88 m / 9 ft 5.31 in
Elevation of barrel: -5..+45 degrees
Traverse of barrel: -20..+20 degrees

4. Gun crew of M777

Figure 17: Gun crew of M777
See model in LDD: Here

Realistic modeling of M777 assumes realistic modeling of gun crew also. Besides modeling small details, this was the primary reason I shifted towards modeling in scale 1:10, while I have to pay tremendous price in material requirement and costs. Minifigs in scale 1:38 have the proportions as toddlers would handle a howitzer: the more proportional and realistic the gun, the more ridiculus they look. Using the thin and feminine type of Technic figure in scale 1:20 just looks as howitzer would be a decoration in a fashion show. While Bionicle parts has oversized limbs and too small heads to give monster-like outlook of warriors. But clever combination of Technic and Bionicle parts enables us to model everyday GI Joes with their barbecue parties and aunt Polly from East Aurora, NY. Our fully poseable figures are scaled 1:10, have separate eyes, separate fingers, and can have differences by race, gender and age.

5. Action screenshots of M777

Figure 18: Airlift of M777 by medium helicopter
See model in LDD: Here

BAE Systems M777 is the only 155mm L39 field howitzer with the ultralight weight of 3,420 kg (7,540 lbs). It can be airlifted by wide range of medium helicopters. Just for comparison, its predecessor, M198 155mm howitzer weighs more than 11,000 kg for the same firepower and can be airlifted only by few heavy helicopter types. The ultralight weight was achieved by open cradle design using titanium tube girders, which also serve as tanks for hydraulic fluid of recoil dumper.

Figure 19: Reloading M777

Decoding what the loader yells in Marine抯 language: Charge 5 means that powder bag is cut back containing 5 powder increments. White bag has stronger type of powder for long ranges than green bag. Rear end of both bags are marked with red, and contains burning accelerator helping the primer to ignite charge. Reversed loading of bag leads to misfire, so one has to make sure that the powder bag is loaded in correct direction

Figure 20: M777 fires M982 Excalibur extended range projectile

Behind the projectile we can see gas stream from base bleeders. These are small nozzles at its base plate, emitting gas from a small burning charge. They do not provide serious thrust, but reduce the projectile抯 drag filling the vacuum behind that. One can see that forward control fins of M982 are already opening, while the bigger rear wings providing aerodynamic lift for flying longer ranges are just about to open. Forward fins spin together with the spin-stabilized projectile and they are GPS controlled. Rear fins are fixed to a bearing, so they do not rotate, otherwise they could not generate aerodynamic lift.

Figure 21: Night fighting

This picture is just for the show. In night fighting, less spectacular flashless propellant is used, to prevent disclosing the battery抯 position.

Figure 22: In the line of fire

In Hollywood movies, this is the point where all propellants detonate spectacularly ignited by the fire burning nearby. In the reality, propellant is pretty safe: if it is thrown into fire it will only burn developing large amount of heat, but will not explode. It can be exploded only by the highly concentrated, high temperature flash flame of the primer. Unused powder increment bags are routinely disposed at shooting ranges by burning them. (Sometimes infantile Marines make bonfire from that, which is totally against the rule.)

Figure 23: Road mode
See model in LDD: Here

慡AVE THE DRAMA FOR THE MAMMA, PRIVATE AND PULL! The picture is NOT about restoring slavery and segregation in southern states. The center of gravity of the 4-ton M777 is above the road wheels in low position. So if it is carefully balanced it can be moved at leveled paved road by single man. This would be totally impossible with its predecessor, the M198 Howitzer, which weighs 11 tons for the same firepower. This is the benefit of the deadly expensive but light titanium parts and open cradle design. In road mode wheels are lowered, stabilizers closed in, spade arms are lifted and spades are tilted inward. This way my M777 can fit into 17 studs (136mm/5.35, real size: 1.36m/4 5.51) wide 16 studs (128mm/5.04, real size: 1.28m/42.36) tall gauge (with fire control telescopes detached and its platform almost sitting on the ground). This is way smaller than the gauge of the real M777, but the purpose was to fit into the cargo hold of my SkyTank heavy battlefield helicopter WIP project.

6. Bonus: M249SAW (Working)

For those poor MOCers who read my lengthy and boring post until this point, there is a bonus: I thought that it is pretty bad to leave gun crew without any defense. As my therapist was on a holiday, I created a spring driven, belt-feed shooting model of M249 Squad Automatic Weapon in scale 1:10.

Figure 24: M249 SAW right view
See model in LDD: Here

We can see wide range of nicely detailed working handgun MOCs in scale 1:1 to 1:2, but they are rarely attempted in smaller scale, as technic parts became huge and bulky there. The key for the tricky solution is part (12) 揅orrugated pipe 2M, which can act as weaker type of ABS spring. Alternatively, we can get stronger steel spring disassembling part 揝hock absorber extra hard. Both parts are 2 studs (16mm) long with 1 stud (8mm) diameter, compressible around 1.5studs (12mm). However mechanical force of steel spring is too strong for technic parts, and requires gluing them together to prevent falling apart during shooting. Moreover, we have to fix springs in a very compact way. The solution is provided by part 揟echnic cross axle 4M/8M with end stop, which acts as a pullrod sliding in part 揃eam 21 with cross and hole compressing springs. Its rotation can hook a catch, holding springs in compressed position. In shooting cycle, one can pull back the rod against the force of spring by (10) bolt handle, then rotate it 45 degrees upward to lock it. The (1) non-disintegrating belt made of parts 慣echnic beam 1󪻒 and 慍onnector peg is advanced manually, to place next projectile (made of part 慡crewdriver) in line with barrel. (7) belt holding teeth take care about correct alignment. Then, downward rotation of (10) bolt handle will unlock spring, which press a bolt forward 1.5 studs (12mm) made of part 慡crew driver in the belt at high speed, launching the projectile through the barrel made of parts 慍onnector pegs. (As screwdriver projectile has strong friction in connector pegs, belt- and barrel pegs have to be drilled up to somewhat larger inner bore, but it will not affect their functionality). Girder and grip are made from part 慒unction element male. The weapon has working (8, 6) optical sight, utilizing part 慡haft 3M 3.2mm in transparent color.

Figure 25: M249 SAW left view

7. References

The helicopter at airlift action is my earlier Bad Guys Escape Helicopter model:

Figure 26: Overview of Bad Guys Escape Helicopter

Building instructions
Download building instructions (LEGO Digital Designer)


 I like it 
  April 20, 2015
You should try to make a self propelled gun with this!
 I made it 
  June 25, 2014
Quoting E S. This is incredible. I just found this and didn't know why our logo was on a curious-looking device I had never seen before. I wonder if you would want to make one for Seiler Instrument? This is so cool.
Thanks. First of all, sorry for I used your logo without permission. I currently work on the next version of M777 howitzer, which will use self-contained, spring driven propellant cartridge. So the funny looking rubber band propellants can be omitted, and the howitzer can look even more realistic. For this version, I would like to build advanced version of Seiler Instrument telescopes. The problem was that I did not really know their functional details, just whatever was available at your home page. I feel that I could build better model if I have some more detailed info. If your company are interested in that, please contact me at paulerWORMt-onlineDOThu address.
 I like it 
  June 24, 2014
This is incredible. I just found this and didn't know why our logo was on a curious-looking device I had never seen before. I wonder if you would want to make one for Seiler Instrument? This is so cool.
 I made it 
  February 14, 2014
Quoting Kurt MOC Excellent work! Truly amazing.
 I like it 
  February 13, 2014
Excellent work! Truly amazing.
 I made it 
  February 11, 2014
Quoting killswitch95 [DEMON] your creations are amazing, and im honored that you put me as one of your favorites, i did the same to you because, well, your possible functionality you put in your builds is astounding!
You are welcome. I hope soon I have the money to buy enuogh material to try out those functions in practice. This is the price of modeling in scale 1:10.
 I like it 
  February 11, 2014
your creations are amazing, and im honored that you put me as one of your favorites, i did the same to you because, well, your possible functionality you put in your builds is astounding!
 I made it 
  February 11, 2014
Quoting Deus "Big D." Otiosus Most of all I like the effort and the level of professionalism.
 I made it 
  February 11, 2014
Quoting Corellian Engineering Amazing! Simply Amazing.
 I like it 
  February 10, 2014
Most of all I like the effort and the level of professionalism.
 I like it 
  February 10, 2014
Amazing! Simply Amazing.
By Gabor Pauler
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LEGO models my own creation MOCpages toys shop M777 Light Towed Howitzer (Working)Technic

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