What are the relative advantages and disadvantages of semi-fixed artillery rounds vs separate loading ones.
I know that semi-fixed can enable higher ROFs but what others considerations are at play?

The cases for semi-fixed rounds allow the use of sliding breech blocks, because the provide obturation. Otherwise the cannon has to have an integral obturation system, and an interrupted screw breech.

The cases also take away a considerable amount of waste heat when they are ejected.

Also, no separate loading of charge and primer.

I guess those all could be considered contributory to higher ROF, but there's at least a little more detail there.

Semi-fixed is actually a tad unusual, the US 1935 pattern 105mm arty ammo being one of the few. Most other QF (using UK terminology for sliding block breach) guns are seperate loading, ie the shell and metal cart case are loaded seperately. Obviously bagged charges (ie no metal case with obturation provided by the breach block) are also loaded seperately.

The advantage of semi fixed is that the complete round is loaded as a single item and this includes ramming action. This may or may not be faster is practical terms, it depends a bit on the gun and the detachment's duties and training. However, it does probably mean that one less detachment member is needed (providing the loader also closes the breach as was the case with the Pack How) and this has obvious manpower benefits. Of course traditionally the US has squandered this benefit by using two man laying!

There are downsides to semi fixed, first it means that all charge bags have to fit within the cart case and leave a bit of room for the shell aft of the driving band to seat in the top of the case. Seperate loading can have charge bags protruding beyond the case, which reduces the amount of brass (or whatever), and slightly reduces weight ofr the same amount of propellant. Another potential problem with semi fixed is that as the barrel wears and the commencement of rifling moves towards muzzle, then there may be a problem with 'shot seating', ie the driving band (rotating band in USspeak) engaging with the rifling, obvously this doesn't occur if the shell is loaded and rammed seperately.

At the risk of being "Captain Obvious", I thought that the main delimiter was the weight of the complete round - beyond a certain weight (say, 150mm and up), ammunition has to be separate loading, as there is a limit to what the average human loader can carry. (Heh, if you want to have semi-fixed 203mm ammunition in an auto-loader, go for it, just don't expect me to wrestle the shells into the breech if the autoloader breaks. OF course, at 200-ish pounds, I would not be wrestling 203mm shells in any case.)

IIRC, "ramming" the semi-fixed US 105mm ammunition just meant shoving it in, whereas you have to use a manual or flick rammer for separate loading, to ensure that the rotating/driving band has engage the forcing cone/rifling, otherwise the round could "set back" into the chamber (if the tube is elevated at all, the usual case for artillery) and when the propellant ignites, some will go past the front of the shell...

Hopefully my memory hasn't gone too soft over the years...

Ha, I always get a chuckle at that quote, Tony:
In Soviet Russia, cat puts Schrödinger in box.

<MrPicky>
Seperate loading projectiles and cased powder charges for the US 5"/38 DP gun were classed as "semi-fixed". Your definition would seem to be indiosynchratic to, ummm, you.
</MrPicky>

My understanding was that semi-fixed are rounds made up of a projectile and a case holding the incremental charges and an integral primer. Separate loading involves a projectile, bagged charges and a non integral primer.
Correct me if I am wrong on the above. Can a gun using cased incremental charges and an integral primer still be dubbed as separate loading based upon the loading sequence?
I knew that cased charges with integral primers enabled the use of sliding breechblocks and others things that sped up the loading cycle but since its adoption has been far from universal (isn't the M777 still using an interrupted screw breech block with bagged charges?) I was wondering if there was more than that, other than compatibility with existing ammo stocks I suppose.

Well, in the case of the previously mentioned 5"/38, the powders and projectiles were as seperate as seperate could be, but they were still officially labeled "semi-fixed". So I figure semi-fixed as a classification means powder in a metal case seperate from the projectile, whether or not the projectile is mounted on the case before loading.



See CrossedSabres comment on ammo component size and weight. Above about 120 mm, whole rounds just get too heavy.

Thing is, the russians have used cased charges and sliding breechblocks on their 152mm guns since the D-20
http://svsm.org/gallery/152mm_D-20/PA220123
http://warfare.ru/thumb.aspx?img=0702ey70/...n2005/3/d20.jpg
http://www.globalsecurity.org/military/world/russia/d-20.htm


The US had cased 8 inch guns in actual service
http://www.navweaps.com/Weapons/WNUS_8-55_mk16.htm

But the ammo was loaded in two components -- the projectiles and powder, not as a unitary round.

10 rounds of 203mm per minute, reliably by all accounts. Amazing.

I like the hot case chutes too -- take all the waste heat and put it outside. Very classy.

Ordnance pamphlet on Salem class 8" turret. Amazing piece of technology.

Naval guns are a bit different because there's more scope for mechanical handling and cased charges are generally better for this, at least up to a fairly large calibre (there's also better safely with cased charges (danger of flash ignition or something of bagged charges?), which I understand is a factor on ships).

Generally, about 45 kgs/100 lbs is considered the upper weight limit for a complete round able to be handled by one man (or woman). Hence larger calibre shells, even when they have metal cart cases have to be loaded seperately if it's done manually.

Germany, Russia, Sweden have traditionally used metal cases and seperate loading with almost all calibres of artillery (obviously Germany changed post WW2). It's not clear why this is, perhaps they had trouble developing an obturating breach a century ago and just stuck with metal cases. As I said the US 105mm 1935 pattern was unusual in being semi fixed. Fixed rounds, of course, mean that the charges can't be varied, fine for AA and anti-tank (and naval), and used with 'pure' field guns, which mostly lost favour after WW1 although they lingered on (eg 18-pr).

I tend to doubt it. The germans were at the forefront of breech loading artillery technology in the 19th century. The prussian artillery of 1870-1871 was made up of 4 and 6 pdr breech loaders using bagged charges. Metallic cases were introduced later.
The interesting question IMHO is not why those countries used metallic cases but why the US is still using bagged charges for its 155mm guns. Weight could be an issue, but the russians seem to manage with their 152mm.

Weight and historically cost. In late WW2 Germany started development of a sliding block breach for use with bagged charges because of the cost and availability of raw material for metal cases. The tricky bit is achieving obturation with a sliding block breach and no metal cart case.

The first to manage it was UK with the 120mm Chieftain tank gun, then Germany with FH70 (also used in M109G and PzH2000), AS90 also has a sliding block but a different design. US tried to develop one for M198 but this failed. Clearly the engineering is a bit tricky, the French haven't managed it either, in fact I'm not sure that anybody outside Germany and UK has although there is some licenced production, eg Japan, possibly Korea with K9. That said primer magazines are becoming quite common and used with screw breaches.

Sliding block, bagged charge and a primer magazine gives the best of both worlds - fast operation, minimising space (important in SP turrets) and none of the weight, etc issues of metal carts. Actually as far as the US is concerned they haven't really got the weight benefit because charge bags are delivered in individual steel containers, however, other nations such as UK and Germany use plastic ones that are lighter and work fine.

On the whole separate, semi-fixed, fixed thing. My understanding of the definition is that it comes down to this.

Separate - components (shell and charge/s), handled separately and rammed individually.

Semi-Fixed - components delivered to the gun separately but assembled as part of the loading process and rammed as a fixed round

Fixed - round exists as a single entity from issue to expenditure.

It gets a little blurry at sea if Mr. Picky takes charge, mostly due to mechanical subtleties and individual service definitions. Take the 5"/38, shell and case are delivered to the gun house separately, with the shell having its fuse set in the hoist, but the two are united in the loading tray and rammed as one piece. The RN did much the same thing with their 4.7" although in a more manual process, but considered the ammunition to be separate (IIRC). Where some of the post war Bofors 120's stored shell and charge separately, united them on the way to the breech, rammed them together, but had the shell separate from the case and ram itself (by inertia) inside the chamber.

I think if there is a hard and fast rule to be established here between semi-fixed and separate ammunition that is just rammed together, it comes down to how far/tightly the shell and case become connected once bought into contact. If the case just pushes the shell ahead of it, then its still separate, but if the shell enters the case with more than just a lose fit it could be considered semi-fixed.

On the other hand, to me trying to find watertight legalese type definitions for grouping weapons at this level is a joke, each one is an individual aiming to fill broad roles in its own way, so it ends up being pornography, hard to define but you know it when you see it.

shane

Close enough, 'delivered separately' could be argued over semantically.

Fixed means the shell and case are crimped together and loaded as a single item.
Separate means shell and charge are loaded separately, and in the case of charges without metal cases the primer also has to be loaded separately. Charges may or may not be variable.
Semi fixed is loaded as a single item but the the shell and case are not crimped together and charges are usually variable (possibly always variable but there may be an exception somewhere).

Rather than starting up a separate thread for a simple question, I will ask here.

Does anyone know whether in the 105mm howitzer armed version of the Sherman the ammunition was fixed or semi-fixed, or did the crew have a selection of ammunition available with the varied propelling charges. That is, did the howitzer function as a howitzer, or as a low velocity 105mm gun?

Semifixed it seems; the ammo was the same as the 105 How.
The tankers of the French 2eDB complained about lack of space in the tank to manipulate the charges when on firing missions.
HTH

Thanks for the answer

Regards

Doug