What aircraft gun round had the flattest trajectory?

[DAVIDICUS]

I understand that the .50 BMG was extremely flat shooting. Were there other cartridges that shot flatter?

 

[delcyros]

How about MG 131, MK 103 and MG 213B/20mm? They should come close if not better...

 

[DAVIDICUS]

I don't know. It depends on their muzzle velocity and ballistic coefficient (how easily they can slip through the air).

 

[delcyros]

Muzzle velocity was excellent on these guns but I think, I don´t have enoug datas to post a solution. Maybe Lunatic can help with that. (My books are still packed)

 

[The Jug Rules!]

I've never heard of an mg 213? Any info on it???

 

[CharlesBronson]

The MG-213 o Mk-213 is a revolver cannon wich was developed in the very late stage of war

It was available in 2 calibers one was the 20x135mm derived from 20mm flak, and 30x85B

The rate of fire was 1100-1300, the modern series of Aden, Defa, and Mauser BK-27 was derivated from it.

 

[The Jug Rules!]

If that had made it into the war, things could have really gotten messy...

 

[Anonymous]

The MK213 did not make WWII, it was still a good ways off when the war ended.

The MG131 13mm had poor ballistics, its sectional density of 0.256 (grams/sq. mm) is one of the poorest of all HMG's/cannon of WWII, and its velocity less than 750 m/s was not great either. The API round would have a little better SD, but lower veloctiy. This gun did not have good ballistics, but they were well matched to the MG151/20.

In my opinion, the BK5 firing AP ammo probably had the flattest trajectory. At 50mm and weighing 1540 grams, the sectional density of this round was a wopping 0.785 (grams/square mm), over twice that of .50 BMG AP round. Combine with its initial velocity of 920 m/s and it was probably the flattest shooting, longest ranged round of WWII aircraft.

The Soviet NS37 was a close contender, with a sectional density of 0.696 and an initial velocity of 890 m/s. The US M9 was similar (though very slightly less velocity and SD).

Generally speaking, it is hard for a small round to compare to a large one, as the sectional density tends to be relatively low. The .50 BMG had a high sectional density (0.383) because the round is quite long for its caliber. The .50 had the best ballistic shape of any round in WWII, but this is overcome by the huge SD advantage of some of the larger rounds (but none in the 20mm class).

=S=

Lunatic

 

[DAVIDICUS]

Good info. Thanks.

I always suspected that the .50 BMG was a flatter shooter than the Hispano 20mm's.

 

[Anonymous]

http://members.cox.net/rg_lunatic/gunpage/

The page is damaged (part of it is missing) but the table is still good.

=S=

Lunatic

 

[DAVIDICUS]

Outstanding collection of data.

 

[delcyros]

absolutely.
Excellent work, Lunatic. I only miss the soviet UKB and the german MG 151 with 15 mm AP cartridge. I expect both to have a somehow similar and/or better trajectory compared with the 0.50 cal BMG. And both are nearly around the same weight class.

Can You tell me more about the Baltraj software?
Keep up Your good work!

 

[lesofprimus]

Dude, u do know that the post above yours was from last year right???

 

[delcyros]

correct. But I had a refreshed interest in the matter. Esspecially for the interesting part of projectiles decelerating and the maths behind. The Ballitsic-link from Lun was a quantum leap for me...

 

[book1182]

Generally speaking, it is hard for a small round to compare to a large one, as the sectional density tends to be relatively low. The .50 BMG had a high sectional density (0.383) because the round is quite long for its caliber. The .50 had the best ballistic shape of any round in WWII, but this is overcome by the huge SD advantage of some of the larger rounds (but none in the 20mm class).

=S=

Lunatic

Didn't they base the X-1 off of the shape of a .50cal bullet???

 

[delcyros]

Yes for so far. The hull diameter is based somehow but not directly on the 0.50 bullet.
The sectional density of the 0.50 BMG is LOWER than both, soviet Berezin univerzal 0.50 (which also was the base for the US API bullet in some aspects) and MG 151 with 15 mm cartridge firing AP rounds. All three are exceptionally good for their calibre class. The 0.50´s both have excellent projectile shapes but the 151/15 isn´t that bad either (except for blund shaped HEI rounds), especcially the lighter APR round.

 

[Lunatic]

How about MG 131, MK 103 and MG 213B/20mm? They should come close if not better...

The MG131 had poor ballistics. In fact, they were almost identical to those of the MG151/20, which made it a good gun to pair with that 20mm. The 13mm's sectional density is poor and mv is releatively low.

The MK103 did indeed have very good ballistics, especially when firing AP rounds which were pointed. The BC of the round was not as good as that of the .50 BMG, but the sectional density (weight vs. horizontal section) was very high, more than overcomming the difference in ogive shape. Genrally speaking, larger caliber rounds tend to have better overall ballistics because the volume is increasing as a cube function, where the frontal area is increasing as a square function so doubling the size of a round increases its frontal cross section by 4x but its weight by 8x. The .50 has an unusually high BC because the projectile is quite long for its daimeter, leading to a high sectional density.

=S=

 

[delcyros]

Well said. I understand now the shortcomings of MG 131 and MG 151/20.
But what about the UKB and 151/15 in direct comparison to the 0.50 cal. BMG? The UKB had the same calibre, a comparable shaped projectile, when firing AP or API rounds but more weight (and therefore a higher sectional density). The 151/15 firing AP rounds isn´t that bad either but has more muzzle velocity. When firing the lighter APR round it has substantially more muzzle velocity but I expect the shell to loose it´s velocity advantage quite soon due to a lower sectional density. Corretc?

 

[Lunatic]

Well said. I understand now the shortcomings of MG 131 and MG 151/20.
But what about the UKB and 151/15 in direct comparison to the 0.50 cal. BMG? The UKB had the same calibre, a comparable shaped projectile, when firing AP or API rounds but more weight (and therefore a higher sectional density). The 151/15 firing AP rounds isn´t that bad either but has more muzzle velocity. When firing the lighter APR round it has substantially more muzzle velocity but I expect the shell to loose it´s velocity advantage quite soon due to a lower sectional density. Corretc?

The Brezin UBK was an excellent gun. The .50 BMG had the advantage of firing M8 API rounds, in almost every other respect the UBK (or UBS or UBT) was the better gun.

The MG151/15's only drawback was a lower RoF, something around 600 650 real world RoF IIRC, about 100-150 rpms less than the 20mm version. Other than that it was a pretty powerful gun for its caliber. The MG151/15 also had good sectional density figures - better than those of the MG151/20. I believe the round and projectiles were the same length, so that means higher SD. Personally I feel the Germans abandon this caliber too early, but their objective was to destroy 4 engine bombers, and for that the HE of the 20mm gives a clear advantage.

 

[delcyros]

By the time the Luftwaffe replaced the 151/15 in mid 1942, there wasn´t a serious problem with four engined heavies. Not intending to say there weren´t but it seems they preferred the higher HE ordenance generally. I read once that pilots with good marksmanship preferred the 151/15 while the average pilot found it more simple to get kills with 20mm HE rounds.
I do personally agree that the 151/15 was rejected too early. 600-650 rpm with average gunwear is ok And I never understood why they replaced the 15mm variant in fighter bombers. The penetration figures of the 151/15 are way superior to those for the 151/20.