Typically speaking, is a gas action heavier or lighter than blowback?

[PercyShelley]

Just what the title says. I was reading that the CETME was largely based on a blowback based version of the STG-44.

Why would the nazidogs want to change a perfectly good gas operated design to something else? Was the delayed blowback lighter or cheaper, and if so, why?

I find gun design fascinating, but they never taught it to me in school!

 

[BigG]

Blowback

That is not exactly true. Blowback means held closed by the mass of the breechblock alone. The CETME uses a recoil operated action that forces the bolt head to move rollers out of locking recesses at considerable mechanical disadvantage.

To answer your question, a straight blowback type autoloader using rifle power ammunition like 30/06 would take a breechblock weighing some 16 pounds, according to Julian L. Hatcher (IIRC). You can see that gas operated is much mroe efficient for a rifle type mechanism.

Blowback is more typically limited to small pocket autoloaders whose cartridge power is carefully balanced to the size of the pistol.

 

[jerkface11]

To see how much heavier a blowback action is pick up a hipoint 9mm.

 

[PercyShelley]

Clearly, a straight blowback design would be impractical for larger calibers.

It would appear that the CETME and the FAL are almost exactly the same weight. I guess the weight difference between a gas and roller-delayed blowback isn't huge?

Again, how about costs? Is either system particularly more difficult to manufacture? What about ammunition sensitivity? Is either gas or blowback particularly choosy about which rounds it will and won't cycle?

Gas operation seems to be the norm, butthe CETME and FAMAS are otherwise designed. Do the French and the Spanish know something we don't?

 

[AZ Jeff]

In small calibers (rimfire and pistol centerfire), blowback long arms are usually lighter, and certainly cheaper to manufacture.

Once we get into centerfire bottlenecked rifle cartridges, true blowback arms become increasingly heavy, with breechblock weights going up exponentially, as noted by another poster in Julian Hatcher's famous book, "Hatcher's Notebook".

The Cetme/HK type rifles are not true blowback, and the roller locked breech they use is more correctly called "delayed blowback". The advantage of these systems is that they have less parts in them, and also are theoretically self-adjusting for ammunition of varying breech pressures.

The downside to them is that, to function well, they often need fluted chambers to allow the cartidge case to float on a film of gas during extraction, and they thus tend to blow residual gas fouling back into the breech area.

I also personally think that delayed blowback centerfire rifles have more recoil than their gas operated equivalents.

 

[gezzer]

Germany went with the G3 after having the G1 which was the gas action FAL made in Belguim. FN would not license Germany to make them as they had bad feelings for the Germans something to do with the invasion of WWII. THAT is the reason they went to the blowback system designed in Spain.

 

[PercyShelley]

The roller delayed blowback doesn't have any particular advantage then? Interesting.

Does anyone know why the FAMAS doesn't use a gas system? Does it have anything to do with its ridiculously high rate of fire?

 

[BigG]

I don't think the roller delayed or retarded blowback action as manufactured by H-K has any advantages, because of the fluted chamber.

To work properly, I think the roller locks should be mated to a moving barrel which would have to move a fraction of an inch. That would keep the breechblock locked to the bbl until the bullet left and pressure dropped. Then you wouldn't need a gimmick like flutes to keep the heads from being torn off of cases.

The moving bbl would introduce other objections like where to mount sights, etc. The roller lock works best in a machinegun action, imho. Look at MG42, for example.

 

[BigG]

Does anyone know why the FAMAS doesn't use a gas system?

If you study history, you will find a continuing theme in France was, "if it wasn't invented here, we don't want it." John Browning designed the High Power for their army and they rejected it, accepting a French design in place of what some call JMB's masterpiece.

 

[AZ Jeff]

Does anyone know why the FAMAS doesn't use a gas system? Does it have anything to do with its ridiculously high rate of fire?

The FAMAS is delayed blowback as well, but it uses a lever type arrangement to delay breech opening, as opposed to the rollers used by H&K and CETME.

 

[Tigerclaw_x]

It isn't the weight.

Delay Blowbacks are not much heavier then Gas operating systems.
The problem is that Gas operating systems we have more moving parts, and they are heavier, and they are connected to the barrel. Think of it:
in the gas tube, there is a gas piston moving, the gas tube is connected to the barrel, making it vibrate. The piston is connected to the bolt, making it vibrate. G3 is inherintly more accurate then anything gas operated. Of course, the G3 has a bad trigger (reliable but bad), which nullifies all the advantages that delayed blowback system is offering

 

[AZ Jeff]

The main virtue of delayed blowback operated centerfire arms is the use of LESS PARTS, and in turn, lower cost to manufacture.

The trade-off, as can be seen in this thread, is that weight and/or shooting characteristics can be inferior to gas operated arms in the same centerfire calibers

 

[Daniel T]

The downside to them is that, to function well, they often need fluted chambers to allow the cartidge case to float on a film of gas during extraction, and they thus tend to blow residual gas fouling back into the breech area.

That would only be a downside if it affected function and reliability in the weapon, which is highly debatable.

Though, it does chew up brass badly, which would be a concern to reloaders.

 

[BigG]

Drawbacks -

{HK}- it does chew up brass badly, which would be a concern to reloaders.

I think that is the concern. That, and you usually have to drive to the next zip code to look for your brass.

 

[Langenator]

the blowback system designed in Spain.

My understanding is that the roller-delayed blowback system used in the CETME and subsequent G3 rifles was, itself, descended from the famous MG-42 machinegun used by the Wehrmacht in WWII. The engineers who designed the CETME rifle were Germans who fled to Spain after the war.

The MG-42 continues in service as the MG-3, rechambered for the 7.62NATO round.

 

[Sylvan-Forge]

This thing really appeals to my mechanical nature;

A Kiraly-type retarded blowback. Looks alot like the AK .. but even more simple .. dang it's cool!

http://world.guns.ru/assault/as74-e.htm

 

[Scottmkiv]

I think felt recoil is a dis-advantage of the roller delayed blow-back system. If you fire a G3 and a FAL one after the other, the G3 recoils noticably harder even when shooting the exact same ammunition.

 

[PercyShelley]

I've heard that the delayed blowback has a stiffer "bite" that way a lot. Do the parts recoilling in the reciver just make it jump that much more?