BigDork Posted November 4, 2003 Share Posted November 4, 2003 I'm sure someone here can help me out, you are all so knowledgable. What's the L** number after the gun size? ie 75/L42 I'd assume the bigger the number after the L the better but I'm not sure what it stands for. Thanks. BigD 0 Quote Link to comment Share on other sites More sharing options...
Ant Posted November 4, 2003 Share Posted November 4, 2003 L is the length of the barrel expressed as a multiple of the calibre...if that makes sense So a 75/L42 would have a barrel length of 75X42 = 3150 mm Or just over 3 meters long. 0 Quote Link to comment Share on other sites More sharing options...
flamingknives Posted November 4, 2003 Share Posted November 4, 2003 ... and the longer the barrel, the higher the muzzle velocity from the same charge. The higher the MV, the greater the energy in the shell and therefore the greater the armour penetration ability (assuming a similar shell) 0 Quote Link to comment Share on other sites More sharing options...
Redwolf Posted November 4, 2003 Share Posted November 4, 2003 The longer barrel is not what causes it to be faster. A bigger explosive charge is what causes it to be faster, and to make full use of the air pressure from the bigger charge you need a longer barrel. Almost always the bigger charge will also require a stronger breech and a stronger recoil mechanism, followed by more space in the turret and a stronger turret ring. 0 Quote Link to comment Share on other sites More sharing options...
flamingknives Posted November 4, 2003 Share Posted November 4, 2003 Actually, a longer barrel does cause a given projectile propelled by a larger charge to travel faster. A real world example would be to compare modern assault rifles using 5.56mm x45mm NATO standard rounds. The LSW, used by the British army has a longer barrel, but it otherwise the is same as the SA80 assault rifle, and uses the same ammunition. It has a higher MV than the AR varient. The important thing to remember is that the propellant explosion is not a point event - it isn't even a high explosive, which comes close. When it explodes, the propellant generates a large amount of hot gases, which force the projectile in the only way it can go - out of the barrel. If the barrel is short, the projectile is subject to this acceleration force for only a short time before the barrel ends and the gases escape to the sides. In a longer barrel, the force, and hence the acceleration is maintained for a longer time, resulting in a higher resultant velocity. 0 Quote Link to comment Share on other sites More sharing options...
Eddy Posted November 4, 2003 Share Posted November 4, 2003 Does the barrel length affect the accuracy of the shot? I'd always assumed that it did but this is an ... erm ... assumption. 0 Quote Link to comment Share on other sites More sharing options...
Redwolf Posted November 4, 2003 Share Posted November 4, 2003 Well, the long barrel guns are more accurate because they have faster shells and sometimes more rifling (faster in-flight turning of the projectile). Other than that I think it takes a very short barrel to affect accuracy in any way noticable when compared to errors from range estimation, wind, bad ammo etc. Also, long barrels can actually degrade accuracy when they deform under temperature changes. That is why a lot of post-ww2 tanks have thermal sleeves around the barrel. 0 Quote Link to comment Share on other sites More sharing options...
Guest Mike Posted November 4, 2003 Share Posted November 4, 2003 Barrel length increases velocity up to a point - that point being where the pressure of the gas behind the bullet = the pressure on the front of it - ie it stops propelling it. If the barrel is longer than this then the bullet is effectively being decelerated by a lower gas pressure behind it than in front of it! Not that I know of any guns that had this problem, but it remains a theoretical possibility!! 0 Quote Link to comment Share on other sites More sharing options...
Michael Emrys Posted November 5, 2003 Share Posted November 5, 2003 Originally posted by Mike: Barrel length increases velocity up to a point - that point being where the pressure of the gas behind the bullet = the pressure on the front of it - ie it stops propelling it.Don't forget to factor in the friction of the rifling against the driving bands. That also increases with increasing velocity. Michael 0 Quote Link to comment Share on other sites More sharing options...
BigDork Posted November 5, 2003 Author Share Posted November 5, 2003 Wow guys, thanks. I really appreciate it. 0 Quote Link to comment Share on other sites More sharing options...
flamingknives Posted November 5, 2003 Share Posted November 5, 2003 I would have thought rifling friction to be more of a factor than pressure differential in slowing a shell fired from a long barrel. Thinks.... Unless you get a significant pressure wave built up in front of the shell, as the transmission of pressure in air is restricted to the speed of sound (330m/s) and HV tank shells travel in the region of Mach 3 (~1000m/s) 0 Quote Link to comment Share on other sites More sharing options...
rexford Posted November 6, 2003 Share Posted November 6, 2003 The "L" represents the length from the breech to the end of the barrel, and does not include muzzle brake length. The point on the breech block where the measurement started varied from country to country, I think the Sherman 75mm L40 gun designation actually works out to 75mm L37 in the British system, but I could be wrong. I'll look for the really good explanation that showed up on the Russian Battlefield site some time ago. The 75mm L46 German anti-tank gun fired 6.8 kg APCBC rounds at 792 m/s while the 75mm L48 tank gun fired the same round at 750 m/s. The 75mm L46 APCBC shell casing was much longer than the L48 due to more powder charge in the L46 ammo. The German 75mm L70 fired the 6.8 kg APCBC ammo at 935 m/s. Higher muzzle velocity means flatter trajectory which results in less of an error (measured from aim point) for a given range estimation error. If the 75mm L48 and 75mm L70 fire APCBC at a 2m x 2m target and both use the correct range setting (target at 1000m, both guns set for 1000m shot), more 75mm L70 shots will hit since it had a much smaller constant aim random scatter than 75mm L48, as shown in firing tests. 0 Quote Link to comment Share on other sites More sharing options...
Guest Mike Posted November 6, 2003 Share Posted November 6, 2003 Originally posted by Michael Emrys: </font><blockquote>quote:</font><hr />Originally posted by Mike: Barrel length increases velocity up to a point - that point being where the pressure of the gas behind the bullet = the pressure on the front of it - ie it stops propelling it.Don't forget to factor in the friction of the rifling against the driving bands. That also increases with increasing velocity. </font> 0 Quote Link to comment Share on other sites More sharing options...
Specs Posted November 6, 2003 Share Posted November 6, 2003 Wow -- kudos to BD, as well, for asking such a good question. That's one of those things I long ago stopped wondering about, like why the AI isn't hardcoded to avoid purchasing that flak truck with the backwards-facing gun, or wave/particle duality. Does anyone know the origin of the custom? I went scouting around on Google to see if I could learn anything, and it was noted in a site about the German navy that L allows one to express the length of the barrel in multiples of the diameter of the projectile. This seems like something that might be useful if you're shooting in indirect mode -- perhaps it somehow feeds into the math that the gunnery officers had to do to hit their targets? Or does it go waaaaay back to some long-lost Teutonic cannonsmith? 0 Quote Link to comment Share on other sites More sharing options...
Guest Mike Posted November 6, 2003 Share Posted November 6, 2003 I don't know about the custom of using "L", but in 1905 Janes Fighting Ships the notes for the ships note the different lengths of gun in terms of calibres, and that "new" ships with guns 5 calibres longer than old ships are "x" amount better - naval guns seem to go in multiples of 5 calibres, ie L30, L35, L40, etc. Janes rates guns and armour by a letter system, with 7 x A being about the most htey got up to I think - in 1905 I think they'd made 12" 35 calibre an "A" gun, then someone bought out a 40 calibre one so they had torate it "AA".....this went on until WW2 with 16" and 18.1" guns!! 0 Quote Link to comment Share on other sites More sharing options...
flamingknives Posted November 7, 2003 Share Posted November 7, 2003 An interesting thing that I learnt recently is probably relevant here, to an extent. More modern guns are able to fire shells at higher velocity by having a stronger barrel towards the muzzle - this way, if you get the charge right, you can maintain a pressure all the way down the barrel, giving a much higher acceleration. So beefing up the breech area isn't the only way to use a larger charge. 0 Quote Link to comment Share on other sites More sharing options...
Mr. Tittles Posted November 7, 2003 Share Posted November 7, 2003 Ive read that they use a plug to overstretch the barrel. They force a plug down a less than 105mm barrel and stretch it out from the inside till it becomes 105mm. This gives it incredible strength. 0 Quote Link to comment Share on other sites More sharing options...
Redwolf Posted November 7, 2003 Share Posted November 7, 2003 Originally posted by flamingknives: An interesting thing that I learnt recently is probably relevant here, to an extent. More modern guns are able to fire shells at higher velocity by having a stronger barrel towards the muzzle - this way, if you get the charge right, you can maintain a pressure all the way down the barrel, giving a much higher acceleration. So beefing up the breech area isn't the only way to use a larger charge. Do you have more info on this? This mechanism would require that you have a charge that burns in a controlled, non-instant matter. That's pretty fancy. I wonder who actually fields such a technology. 0 Quote Link to comment Share on other sites More sharing options...
flamingknives Posted November 7, 2003 Share Posted November 7, 2003 I don't have any written info on this, sorry. This mechanism would require that you have a charge that burns in a controlled, non-instant matter. That's pretty fancy. I wonder who actually fields such a technology. That would be any firearm ever used. Explosives can be separated into two broad categories - High and 'Low' (not sure if that's correct nomenclature) High Explosive is TNT, Comp B, C4 etc. This stuff detonates, i.e. the reaction propogates through the explosive by means of the shockwave 'Low' Explosive is gunpowder, cordite, gun cotton etc. This burns, very quickly, but the reaction is not supersonic, IIRC. Not sure how you control the burn rate in modern stuff, but in gunpowder it depends on the size of the grains. If you think that's clever, track down the Cased Telescopic Weapons System (CTWS) on google the main piece based on this tech. at the moment is 40mm, so adding that into the search term might help. In CTWS, the APDS round is embedded into the charge, shortening the overall round length by something like 30% and also the gun length by a considerable amount. 0 Quote Link to comment Share on other sites More sharing options...
Mr. Tittles Posted November 7, 2003 Share Posted November 7, 2003 It would seem a smoothbore weapon would naturally have a different pressure vs shell position in the tube than a rifled weapon. In a rifled weapon, there is a spikey surge in pressure when the band bites into the rifling. the projectile must be accelerated both translationally and rotationally. When the rifled weapon fires, the band makes a pretty good seal. I wonder how sealing is accomplished in a smoothbore high velocity weapon? 0 Quote Link to comment Share on other sites More sharing options...
flamingknives Posted November 7, 2003 Share Posted November 7, 2003 No reason that you can't have a driving band in a smoothbore 0 Quote Link to comment Share on other sites More sharing options...
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