my profile | register | search | faq | forum home
  next oldest topic   next newest topic
»  COMBATSIM.COM Forum Archive   » Real Military Discussions   » Air Defense, Weapons, Platforms   » FA-MAS question

   
Author Topic: FA-MAS question
Rosco
Member
Member # 1779

posted 12-10-2000 03:09 PM     Profile for Rosco   Author's Homepage     Send New Private Message   Edit/Delete Post   Reply With Quote
On another website someone stated that the French FA-MAS assault rifle has a noticably higher velocity when used in semiauto mode than when used in full. Can anyone tell me why that may be? Does the FA-MAS use some kind of modified blowback action?
Posts: 984 | From: Hazzard County | Registered: Jan 2000  |  IP: Logged
Rick.50cal
Member
Member # 172

posted 12-10-2000 11:47 PM     Profile for Rick.50cal     Send New Private Message   Edit/Delete Post   Reply With Quote

Hmm, interesting. I had not heard anything like that before.

The FAMAS is a delayed blow-back action that does not use a rotating bolt head, nor does it use gas taped from the barrel either. In fact, it does use features that are a bit similar to the HK roller-delay family.

I understand that the barrel chamber is fluted, like the HK rifles (but not the G36), to aid extraction (since the bolt head does not tilt or rotate, which would help unseat a sticky case in the chamber). When the cartrige is fired, the breech head is forced rearward. Because the pressures are too high at this time, a method of slowing the opening is needed, and a lever system acts from the receiver lugs, breech/bolt head, against the very heavy breech carrier, which also has the recoil spring to work against.

Slowly, the carrier moves backward, and eventually the breech is opened and cartrige ejected.

Now, in semi auto, everything would be normal, but I know from experience, that straight blowback SMG's in 9mm actually ignite the cartrige slightly BEFORE they are fully chambered, as this reduces recoil, reduces bolt weight (further reducing recoil). I also know that the FAMAS is considered by designers to have an overly heavy bolt carrier assembly compared to all other 556 rifles, and it also has a higher bolt velocity than normal, too.

It's likely that during a full auto burst, after the first round fired, the auto-sear on the FAMAS would trip the hammer before the breech is completely sealed in battery, and with the flutes, this would account for significant gas loss, and thus velocity loss.

Keep in mind that I am not an expert about the FAMAS, but I have given a plausible explanation.

I DO know that the FAMAS is not widely considered to be a great rifle design, though. You canot use ANY ammo other than French 5.56 that was intended for it, or you risk reliability problems. The standard magazine is only 25 rounds, and a non-standard type, which explains why the export version uses the common M-16 STANAG type. There is no simple way to mount a scope other than the factory model, and the trigger is pretty heavy. Having said all that, it is still a much better rifle than: the L85/SA80, Cetme 5.56 (I don't remember the name), and a few other new pieces of junk.

BTW: what kind of average velocity loss are we talking about?? 50 fps? 300 fps? or is it listed in metric Meters per second?

------------------
Rick.50cal


Posts: 520 | From: Vancouver, BC, Canada | Registered: Sep 1999  |  IP: Logged
Rosco
Member
Member # 1779

posted 12-11-2000 12:23 AM     Profile for Rosco   Author's Homepage     Send New Private Message   Edit/Delete Post   Reply With Quote
As I understand, the FAMAS is a solid but not spectacular weapon, with its only real weakness being that it's somewhat heavy. It's certainly not of C-7 quality but it seems to be fairly well built. The barrel has a respectable life of 15,000 full auto rounds and is appearantly known for being reliable and resistant to overheating for an assault rifle, that's if I remember correctly here.

As for the velocity drop question, I don't know the amount, I'd guess it would be no more than 150 fps. I was originally told this about 4 months ago by a Frog who's brother was in the Armee de Terre. He said that semiauto shots dropped "noticably" less at distance.


Posts: 984 | From: Hazzard County | Registered: Jan 2000  |  IP: Logged
Tony Williams
Member
Member # 8202

posted 12-11-2000 01:18 AM     Profile for Tony Williams   Author's Homepage     Send New Private Message   Edit/Delete Post   Reply With Quote
I think it is unlikely that the gun would fire before the bolt is firmly back against the breechface. Guns which do this (known as API blowbacks) are designed to do it with every shot (both full and semi-auto) and there is a careful balance struck between the instant of ignition, bolt weight and bolt forward velocity.

If the FAMAS fired on full-auto as you suggest, then it would fire before the delay mechanism was fully in place, so the outcome would probably be violent recoil and a face full of gun gas

Tony Williams
New book: "Rapid Fire: The development of automatic cannon, heavy machine guns and their ammunition for armies, navies and air forces"
Details on my military gun and ammunition website: http://www.users.globalnet.co.uk/~autogun/index.htm


Posts: 24 | From: | Registered: Nov 2000  |  IP: Logged
Rick.50cal
Member
Member # 172

posted 12-11-2000 06:27 PM     Profile for Rick.50cal     Send New Private Message   Edit/Delete Post   Reply With Quote
Tony, I had to think about this a while, and ordinarily I would agree with you, but in this case I disagree that it is "unlikely that the gun would fire before the bolt is firmly back against the breechface."

"then it would fire before the delay mechanism was fully in place," - not true, since the locking lever is engaged with the receiver already, and the forward momentum of the bolt carrier will ensure the bolt will continue forward even with the begining of recoil. Remember, with rotating bolt systems, the auto sear is tripped before the carrier is fully forward (were talking only 1 mm or so, though), yet the bolt has been fully locked already for several MM's, and round fully seated. Remember that a typical rotating bolt starts locking as much as .5 inch/ 12mm before the carrier is fully forward, but the same is probably not the case for the lever-locking FAMAS.

Also, while the FAMAS shares a lot in concept with the HK roller series, the HK has the boltface gets seated BEFORE the locking even starts. The same does not seem to be true for the FAMAS, due to the layout of the lever system.

The recoil of the FAMAS is considered to be a little bit more violent than normally found for 5.56 rifles, but we would not be talking about a straight blowback either. Also, the fluted chambers would prevent any problems with the cases rupturing. The exiting gases would not be flying into the shooters' face, as the ejection port is already opposite his cheek, but instead would pepper his shoulder.

It would seem that this rifle seems to provide good service for the French armed forces, as all your ammo would come only from normal procurement chanels. Having said that, it must be noted that just like most specops/commando/hostage rescue units, the French do not use their regular issue rifle, but instead use the Sig551 and M-203(M-16A2).

The ammount of velocity drop may be a clue to just what is happening. If the trajectory is indeed 'visible' as your friend states, then I would venture that it may be as much as 300-400 fps loss or more. I will have to check out the trajectory tables for 5.56, 5.45 (slower), and 7.62Nato as well as 7.62x39mm (way slower at only 2200 fps, versus 2700-3100 fps for 5.56)

------------------
Rick.50cal


Posts: 520 | From: Vancouver, BC, Canada | Registered: Sep 1999  |  IP: Logged
Rick.50cal
Member
Member # 172

posted 12-11-2000 07:45 PM     Profile for Rick.50cal     Send New Private Message   Edit/Delete Post   Reply With Quote

I seem to recall that the French Army ammo uses a steel case, and while that is not in itself unsusual, it is unusual for 5.56 ammo (until recent Russian 5.56 cheap ammo using laquered steel cases, and mild steel bullets, under the company marketing name "Wolf"). I suspect that the steel case would prevent the casing from deforming under pressures while being not being completely chambered during a full auto burst. This would also explain why the FAMAS is considered not able to reliably utilise normal western brass cased ammo.

BTW: I like your site! While I did not find too much I did not recognise, you have a number of good pictures, and obviously understand the subject materials very well.

Question for you about the GM-93: is this in service? Who makes it, and is there any pictures of it? I had not heard anything at all about it, but it sounds interesting!

Also, the Cyparis looks like a copy or newer version of the Klin/Kedr SMG's. Is this a competing company or an evolved version? Who makes it?

------------------
Rick.50cal


Posts: 520 | From: Vancouver, BC, Canada | Registered: Sep 1999  |  IP: Logged
Tony Williams
Member
Member # 8202

posted 12-12-2000 01:32 AM     Profile for Tony Williams   Author's Homepage     Send New Private Message   Edit/Delete Post   Reply With Quote
I am not familiar with the specific details of the FAMAS, just with the general principles of gun operating systems, so I am willing to concede to your superior knowledge on this one.

My understanding of the retarded blowback system is that the maximum resistance to bolt movement occurs right at the start of the movement, then becomes progressively easier. Therefore if the gun were to fire before the bolt was fully home, much of the retarding effect would be lost. I accept that the continued forward momentum of the bolt would provide some additional resistance but if the French designers combined elements of retarded and API blowback mechanisms in the same gun then they had to do so much juggling with pressures, bolt weights, spring strengths and resistance locks that it's a miracle that the thing works at all.

Leaving SMGs aside (which use relatively low-pressure pistol ammo) high-powered guns which used API blowback mechanisms (Becker, Oerlikon, MK 108 etc) all had extended or hooded chambers to make sure that the cartridge was well inside the chamber, and fully supported by chamber and bolt, before it fired. This obviously isn't possible with the FAMAS as the cartridge doesn't have a rebated rim.

Incidentally, in the rotating lock system you mention there is a difference between the carrier being fully forward and the bolt being seated - in that case, the bolt is clearly firmly locked before ignition.

Tony Williams
New book: "Rapid Fire: The development of automatic cannon, heavy machine guns and their ammunition for armies, navies and air forces"
Details on my military gun and ammunition website: http://www.users.globalnet.co.uk/~autogun/index.htm


Posts: 24 | From: | Registered: Nov 2000  |  IP: Logged
Rosco
Member
Member # 1779

posted 12-12-2000 07:02 PM     Profile for Rosco   Author's Homepage     Send New Private Message   Edit/Delete Post   Reply With Quote
300-400 fps sounds at little drastic, it didn't sound like the autofire trajectories dropped dramatically, just enough to be noticable if you're familiar with the weapon. That's why I guessed at ~150 fps or so. Who knows, it may not have anything to do with chamber pressure at all and maybe it's due to mechanical differences between the firing modes, just as many semiautomatic pistols shoot to a different point of aim with the hand cycled first round than they do with the subsequent autoloaded ones.

BTW, check out this also excellent site on Russian ammunition, it spans small arms to light cannon and includes lots of pictures.
http://www.geocities.com/Pentagon/Base/1852/

[This message has been edited by Rosco (edited 12-12-2000).]


Posts: 984 | From: Hazzard County | Registered: Jan 2000  |  IP: Logged
Rick.50cal
Member
Member # 172

posted 12-13-2000 03:04 AM     Profile for Rick.50cal     Send New Private Message   Edit/Delete Post   Reply With Quote

Ok, so you are saying that the bullets were not tested for velocity using a chrony at all, but there was variation in point of impact?

Well, this might be simpler than we first thought. Is it possible that what he was refering to was normal full auto dispersion, instead of anything else? It's possible that the FAMAS may share a rare behaviour with the AK-74, in that in burst fire, it has a tendancy to fire rounds DOWNWARD, instead of upwards. In the '74 this is due to the muzzle break, while the FAMAS, despite higher recoil, might also do the same but due to the especially high mounted buttstock and low barrel axis, particularly if fired from the prone possition.

------------------
Rick.50cal


Posts: 520 | From: Vancouver, BC, Canada | Registered: Sep 1999  |  IP: Logged
Rosco
Member
Member # 1779

posted 12-13-2000 03:49 AM     Profile for Rosco   Author's Homepage     Send New Private Message   Edit/Delete Post   Reply With Quote
I don't know, I'm pretty sure there was some velocity drop but between the Frog's poor English and my poor memory I'm not really sure.
Posts: 984 | From: Hazzard County | Registered: Jan 2000  |  IP: Logged

All times are MST (US)  

   Close Topic    Move Topic    Delete Topic next oldest topic   next newest topic
Hop To:

Contact Us | COMBATSIM.COM Home

COMBATSIM.COM, INC. All Rights Reserved.

Powered by Infopop Corporation
Ultimate Bulletin Board 6.04b

Sponsor
2014 COMBATSIM.COM - All Rights Reserved