L1A1 gas block and system

[Peter Laidler]

Remember some months back we saw parts of a Canadian experimental gas system and I threw in a poser and commented '.....think Bren gas plug'? Well, some of you did and there has been a couple of to and fro PM's. Brian at BDL has finally got me to do a bit of reading up the old student notes followed up by some writing. So here goes.......

If you already knew this - and some experts out there will claim to, believe me - , please bear with me and read on.

Here’s a little thing that you didn’t know about the L1A1 rifle..... But first, a bit of history. While India was looking at the Vickers Berthier and later, the Bren at Ishapore, factory engineers noticed that when the propellant gas was vented from the barrel to operate the working parts and cycle the gun (called the working gas or WG), the ‘gas’ part of the mix did what professor Venturi and Dr Boyle told us it would do. In short, they always take the line of least resistance and are easily diverted because...... anyway. But any solid particles in the mix, such as brass and copper particles and the elements that create and bond the extremely hard carbon together and make it stick are NOT easily diverted and where possible, will always remain on the same trajectory – outwards! Outwards that is, until they can't get out, so remain, building up, festering and getting harder and harder, more stubborn as the shooting continues

They also noticed that it was this element that caused gas fouling problems. You’ve all been there...., you’ve all had to clean a Bren or other MG gas cylinder and piston....... Fouling pliers, reamers, plug cutters and other special tools (hey....., have you ever had to clean a .30 Browning muzzle cone?). So they set about investigating further. For their version of the VB gun, they devised a gas plug system that while it diverted the GAS, it didn’t divert the solid particulates within the WG. And it worked. It wasn’t as good as had been hoped but it actually worked. It was known as the Ishapore gas system. They tried something similar with the Bren. This time the trials got as far as Enfield but alas, so did the War! The whole of the Bren gun design was subject to strict and highly controlled permission while manufactured at Enfield. The actual design of the Ishapore gas plug system was designated the REGULATOR, Gas, IP Mk2. There......., does that answer why you have a wide Mk1 gas plug, a narrow Mk3 type but no Mk2 regulator?

This brings us to the L1A1 rifle. I’d like you to get yours and put it across your lap. Keep it fully assembled with a 6” or so length of thin soft or copper wire. Fully close the gas regulator and just tweak it open just sufficient to insert the length of soft wire down the angled vent hole just behind the foresight block. Push it in.... And guess what? You’d be thinking that the internal radiussed curvature that should divert the gas rearwards or the face of the piston would prevent the wire from going straight down and into the barrel. But that’s where the wire goes......, straight into the bore.

The vent from the barrel is totally unobstructed from the bore to atmosphere, regardless whether the gas plug or piston is in place. Can you see where the designers were coming from now?

I don’t know whether this is the same as the FN variants but it’s value on the Commonwealth pattern L1 rifles is a feature much discussed on the scientific design courses.

You can take a look at the gas plug and note the small radissed curvature. That is simply to aid the gas flow, diverting the WG rearwards to operate the piston. It’s been said that it would do that anyway but the MOST import feature of this design is that each time a shot is fired the not-so-easily diverted, solid particles within the WG mix just carry on going in a straight line, straight upwards and STRAIGHT OUT OF THE GUN! If you don’t believe this there’s a small laboratory test you can try but safety tells me not to say any more. But closer to home, have you ever wondered why it was such a doddle to clean.....and why you didn’t spend ages chiseling out diamond hard brass and copper infested carbon and why the cleaning kit was soooooo basic!

Now for something else..... Look at the small ‘C’ spanner attached to the combination tool. This is specifically to free a seized or stuck gas regulator. If the Ishapore gas system was so good, it would seem to be a paradox as to why there is a need for the C spanner at all to free a stuck gas regulator! Indeed, but because the system CAN be regulated, there MUST be a way of shutting down or restricting some gas loss. And so it is. But the pro’s outweigh the con’s.

I wonder whether the inventor of this new fangled gas system in the Forum article ‘Experimental C1 gas cylinder system’ dated 2 Dec ’15 has something like this within the new gas system. I bet it didn’t and simply on that basis alone, it was bound to fail the toughest test of all. That of the soldiers feed-back. Next time you spend an afternoon shooting your L1A1 on the range you’ll realise how well thought out the gas system is and how it is so easy to clean afterwards. And don’t tell me it’s because the ammunition is better or cleaner – Or I’ll throw another spanner into your works........

Information

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[mrclark303]

As ever Peter, a fascinating read, not something I have ever really considered. It just shows yet again what a brilliant design the L1A1 was, I don't know if it makes a difference in function (or how clean it stayed), but I think the Fal gas bleed vented straight up as opposed to the angled back inch pattern design.

[Peter Laidler]

That is VERY interesting Clarkie..... My experience with the FN type only extends to captured Argentinian rifles and the early trials FN's. If others confirm the FN 90 degree as opposed to the L1 30 degree approx gas track then I will add something else that I'd made ref to in my old notes. Are you there with the answer Skippy, Tankie?

[Brian Dick]

Yep, the gas vents straight out on an FAL and not angled as in the L1A1.

[Peter Laidler]

One of the designers (who was too late for the L1A1 programme) mentioned this Ishapore idea to the course, as did an old RSAF apprentice much later. They both said that Enfield were keen to try this 'open exit' gas system at the first opportunity. And that came with the experimental TADEN machine gun (the T was from that man TURPIN again.....). The next opportunity was the L1A1 rifle. The rest is history...........

[nzl1a1collector]

To help illustrate Peters description.

L1A1 Gas Block arrangement (Red is the action of the gasses)




FN X8E5 Gas Block arrangement (Red is the area that has been machined away)

[mrclark303]

You beat me to it Kevin, I was going to photograph the gas block on my BSA cutaway. Is that one of the very last production Lithgow L1A1 factory cutaways you have there? Very nice indeed.

[Peter Laidler]

It hasn't got the 'winged ears' on the sling lop band. But it does illustrate the idea perfectly doesn't it.

We had some large batches of those wrapped/formed ply handguards where the captive nut on the left handguard didn't remain captive and would rotate when you tried to undo the screw. There wasn't sufficient 'nut' to hold with pliers while you tried to unscrew either so plenty were ruined. The nut part was, so said, fixed with adhesive prior to being rivetted in place. The adhesive would break down with heat. There was a H1 (hardware) item replacement that you could use to replace a rotating nut but the best repair - if it all came apart easily - was to support the outer end of the now non captive nut and '.....gently swage'* the inner end with gentle blows with the 1 pound ball end of the hammer! Clean out with 5mm(?) plug tap. Worked a treat!

* Gently swage is a highly technical term used by Armourers when things are not going the way you would like. A similar phrase would be 'knock seven bells out of it'. Not used in polite company or once you are Commissioned. Are phrases such as swage and seven bells used on the bench today Skippy, Tankie, Son?

[Brian Dick]

I'd guess it's a Lithgow cutaway. I've only seen the winged front swivels on British rifles. I love those cutaway rifles! My BSA L1A1 is away finally getting the body lock lug welded up and remachined to bring it back to non "ZF" status. Looking forward to getting it back up to snuff.

[Peter Laidler]

Because 'up country' we had pooled Ordnance stores at 221 Base Ordnance Depot in Singapore, we could demand these winged sling loop bands from UK stocks which were better than keep repairing the lug type (shown). Most of the L1A1's that went through us had the band replaced as and when. Mind you, the screw still stripped the thread

In reality, the 'winged' bands were a 'non-modification' in real terms. But presumably someone got a good monetary reward for it. The wings only prevented the sling loop rotating SO far and even if you made the band parallel all around the circumference by a) filing the little original stop off completely or cutting the two wings off, the band and sling loop could STILL only rotate until it was stopped by the gas plug..............! So who cared? What did it achieve? A point much discussed as an example when the worthiness of a suggestion was on the table.

There was another such idea of a 'non-modification' mooted regarding why must the bayonet always be replaced button-up, ring to the left?