M305 Gas system issue.

J

Jaytwo

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In some past threads I mentioned some ammo not cycling and the brass expanding too much. It would seem it is all related to the gas system, specifically the combined length of the piston and the threaded plug.

Something in the system is too long. Which means the bolt isn't in the proper position during firing, which I think explains the brass stretched too the point where I can't get it back into the chamber.

The piston is the correct length from the op rod end to the gas hole in the piston. That lines up when the piston is in the correct position. I suppose it is possible it is too long from that point to the plug end of the piston, but I imagine that isn't it.

The plug might be too long, when you thread it all the way in, it pushes the piston out almost an 1'8th inch against the op rod. Possible fix, a washer of some form so it doesn't thread in as far. Of course in this position the gas hole in the piston is not lining up with the port, so it won't cycle. The washer should fix this. I could also mill some length off the end of the plug. Advantage there is the plug is still threaded into more threads.

The part that locks the gas system to the barrel might be to thin, causing all of this. It is just under 8mm thick. Combined length of piston and plug up to the mating surface of the plug is about 11cm total.

In order to cycle correctly this is where the op rod and piston need to be. Notice the plug isn't threaded all the way down. Any more and it pushes the piston out into the op rod.
20130411_143030.jpg


Here you can see the piston and op rod when the plug is fully tightened.
20130411_143153.jpg
 
check to see that the bolt is fully closed while a round is chambered. If the right side lug is up a bit, you might have something there. ...might...

Mine does the same thing (gas plug advances against the op rod, shifting it); my brass is fiiine to reload.

Your fret is related to the brass' lifespan and suitability for reloading, yes?
 
Jaytwo said:
The piston is the correct length from the op rod end to the gas hole in the piston. That lines up when the piston is in the correct position. I suppose it is possible it is too long from that point to the plug end of the piston, but I imagine that isn't it.

The plug might be too long, when you thread it all the way in, it pushes the piston out almost an 1'8th inch against the op rod.

Here you can see the piston and op rod when the plug is fully tightened.
20130411_143153.jpg
Click to expand...

Thanks for making me go tear apart my M305 to look... :p

Everything you are stating about lengths is correct on yours. My gas plug pushes the piston out by that exact same amount. If you very seriously think that the gas port in the barrel and the gas acceptance hole in the piston don't line up MY OPINION OF the solution is to elongate the piston gas acceptance hole until it matches.

Do I think that is the issue: NO. I would seriously look at something 'jamming or sticking' or a 'headspace issue' long before I tinker with the gas system... and I have tinkered with my gas system (utinize, shim, clean gas port in barrel, replace spring guide, fix plug threads...). All it could take is a weaker brass in a sloppy 7.62 military chamber and/or a poor headspace and all the problems you talk about arise. If you have the crappy Norc spring guide rod, get the F/n rid of that to start: I was getting feeding problems until a new solid round guide rod solved it (my pick as the only manditory Norc 305 upgrade).

You have an interesting situation, and my opinions may not have helped, but I am interested in the solution.

Maybe one of the greeat and wise M14/M305 members can wiegh in on this. :canadaFlag:
 
Go to M14forum.com There is lots of talk on that board about "timing" the gas system. There are guys that will shorten the gas plug, and there are others that will shorten the piston on the op rod side. I believe (don't hold me to this) they set the gas piston up so that as you tighten the gas plug down, the gas piston contacts the op rod about 1/2-3/4 of a turn before the plug is tight. This ensures the op rod will NOT contact the gas system, only the gas piston.
The thought process is that the op rod may be in rearward motion before the bullet clears the barrel.

By "timing" the gas system, it allows the op rod to start to travel backwards without affecting the bolt lock up until "after" the bullet has left the barrel.
It is supposed to be an "accurizing" mod.

I agree with John ^^ Id try turning the gas system off and fire some rounds through it and then check the brass before I started modding the gas system.
 
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The reason that the op-rod moves back a few mm before camming the bolt out of battery is a safety feature to allow chamber pressures to drop before unlocking the bolt. This reading between the lines means that the op rod is supposed to move before the bullet clears the muzzle. Thats the way shes designed to run. Timing the gas on a semi-auto is a sign maybe you should go use your bolt action instead ;)

CanuckR said:
Go to M14forum.com There is lots of talk on that board about "timing" the gas system. There are guys that will shorten the gas plug, and there are others that will shorten the piston on the op rod side. I believe (don't hold me to this) they set the gas piston up so that as you tighten the gas plug down, the gas piston contacts the op rod about 1/2-3/4 of a turn before the plug is tight. This ensures the op rod will NOT contact the gas system, only the gas piston.
The thought process is that the op rod may be in rearward motion before the bullet clears the barrel.

By "timing" the gas system, it allows the op rod to start to travel backwards without affecting the bolt lock up until "after" the bullet has left the barrel.
It is supposed to be an "accurizing" mod.
Click to expand...
 
Sorry got off topic. My M305 gas cylinder lock tightens against the piston too. I'll have to check my M1A and see if it does as well. I dont think thats your problem, but the brass stretch is not good either. Sorry to not be a lot of help.
 
Jimenez, thats what I am talking about.
By allowing the op rod to move farther forward a few mm's it allows the op rod to move farther rearward BEFORE it starts to cam the bolt open.
 
Everything you all are saying makes sense. I went through my brass, found Remington brass was ok, winchester ok, Both Federal and their American Eagle brass, too tight.

When it doesn't cycle, the op rod doesn't even wiggle. Otherwise it cycles fine. But it is definitely not reliable. The gas piston doesn't just drop in, it doesn't take a lot of force, but it has to be pushed through lightly until it comes out about half inch on the op rod side then it moves freely. It has gotten better since I have been polishing it a bit. Will keep working on it. If that doesn't fix it, I might have to resort to elongating the hole in the piston a bit to ensure the gas is getting in.
 
Don't mess with piston , you might need to shim the gas system a bit . Remove the piston and slide a drill bit through the gas relief hole it should go into the barrel. If not you may need to shim.
 
I polished slowly until I could slide piston in with little force. The hole into the barrel seems fine. Going to go try it out after lunch hopefully
 
I've got a 2009 M305 and have noticed the brass needs trimming when I reload it.

That said, it IS a semi-auto rifle, built with military chamber specs (which are known for being loose to accomodate dirt and debris in combat without blowing up the rifle), and having brass stretch isn't unusual.

I'm wondering what position the gas port screw is in....

If the screw isn't completely vertical, it MAY, I say MAY, cause feeding issues. Take a look and let us know.
 
Put 60 rounds through it and it ran flawlessly. Other than one mag sometimes wasn't picking up a round. Other mag was good. One problem at a time.
 
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