Comparison of 50mm Bushmaster III with the 30mm Bushmaster II. By comparison the 25mm’s length over all is 105.2 in (2.672 m) and its barrel length is 85.6 in (2.175 m)

SNAFU has a discussion about what up-gunning Infantry Fighting Vehicles might mean to land warfare and included the graphical information above.

The dimensions provided give us some clues about the characteristics of the gun that were not available before. Length of the barrel in calibers (bore diameter) tells us something about the gun. You usually see it written as caliber/length in calibers, e.g. 5″/38 where 5″ is bore diameter and the barrel is 38 time 5″ in length. Length in calibers suggests other characteristics including muzzle velocity, time of flight, flatness of trajectory, accuracy, and penetration ability relative to other weapons of the same caliber. Greater length in calibers usually translates into higher muzzle velocity which imparts a flatter trajectory to a given range, which usually translates into greater accuracy and better penetration ability relative to other weapons of the same caliber. Larger caliber weapons might, and usually do, exceed these characteristics even using relatively shorter length calibers.

The Coast Guard uses or has used 5″/38s, 3″/50s, 76mm/62s, 57mm/70s, and 25mm/87s. In each case, greater length in calibers translated to higher muzzle velocities.

The barrel length for the 50mm indicated in the diagram above, 117.6″, translates to a length of 60 calibers, so we should expect a muzzle velocity similar to that of the 76mm/62 Mk75 (3,000 – 3,024 fps (914 – 925 mps)), perhaps slightly lower.

There are not a lot of contemporary weapons of similar characteristics. Perhaps the closest was a Soviet 45mm/78 anti-aircraft gun with a maximum ballistic range (firing at an elevation of 45 degrees) of 12,140 yards (11,100 m). Certainly the 50mm’s performance will exceed that of the Soviet 43mm/46 which had a max ballistic range of 10,060 yards (9,200 m).

On a more practical basis this probably means that, while the Army claims an effective range of 4000 meters (probably against another Infantry Fighting Vehicle), even without guided projectiles, it would start scoring hits against larger maritime targets at 7,000 yards, which was the maximum range we used to train 3″/50 crews for, using local control. In any case it would be able to engage from beyond 4000 yards which I believe would be the maximum effective range of any improvised armament available for use on even a terrorist vessel.

Looking at the two rounds mentioned above, Programable Air Burst Munition-tracer (PABM-T) and Armor Piercing Fin Stabilized Discarding Sabot-tracer (APFSDS-T).

• The APFSDS-T is probably going to have a better chance of disabling a large marine diesel engine than any weapon we currently have in service.
• The PABM-T should be effective against personnel in small fast highly maneuverable vessels and the programable feature means misses will detonate before going any great distance beyond the target, minimizing the possibility of collateral damage. It might also be effective against drones.

Thanks to Lee for pointing me to this information. 

The 25mm guns (M242) in the Mk38 mounts that are the main armament on the Coast Guard’s 210 foot, Webber class, and Island class Cutters are part of a family of electrically powered guns marketed by Northrop Grumman, referred to collectively a “Bushmasters.” The 25mm is also used on the Bradley fighting vehicle and the Marine’s LAV 25 wheeled fighting vehicles. A slightly larger version, the Mk44 30mm is used on the San Antonio class LPDs, Littoral Combat Ships, and the DDG-1000 class.

Lately, the Army and Marines have become concerned that these guns are being out-ranged by adversary weapons. (I share this concern with the regard to a possible maritime terrorist attack.) This has lead to development of larger weapons of 40mm and most recently 50mm. The 50mm is claimed to have twice the effective range of the 25mm. These weapons were recently demonstrated at the “Bushmaster Users Conference,” 9-10 April, in Kingsman, AZ with a live fire demonstration at Big Sandy Range (video above). The Coast Guard may use one of these weapons in the future in place of the current 25mm.

Going from 25 to 30, 40 or 50mm may not sound like a big jump, but projectiles are three dimensional and the difference in weight and hitting power is striking.

Below are listed the maximum projectile weight in pounds associated with each caliber. I did not have this information for the 40 and 50mm chain guns, so I have used the weight of modern Bofors 40mm projectiles and an estimate based on a scaled up 30mm round for the 50mm to provide approximate values:

• 25mm–0.406
• 30mm–0.94
• 40mm–2.16 (Bofors)
• 50mm–4.38 (estimate)

As can be seen, the 40mm round is about five times as large as the 25mm and the 50mm is about ten times as large.

The larger caliber is not only more destructive, it also means it is easier to include smart fuses and possibly even guidance systems.