Armory Sponsor
Posted: 8/7/2013 11:47:08 AM EDT
|
I've got a friends French MAS rifle (don't know the model, the semi-auto one) that was part of the batch that Century converted from the original obscure French military chambering to .308. In the little discussion I could find online about these conversions, it's apparent that it was common for them to fail to extract, and it is believed that the higher pressure .308 round was causing the bolt carrier to accelerate so fast that the extractor just cannot hang on to the rim of the cartridge.
Has anyone had experience with this rifle and problem? Is there a fix for it? My initial idea is to thread the gas port that impinges on the carrier, and plug it with a set screw that I could drill out to a diameter that allows proper function. |
|
Quoted:
I'm unfamiliar with the design, any chance of changing/adding spring pressure to delay extraction until the pressure has lowered some? I don't know, but will investigate that. To me, it seems like the gas pressure required to cycle the action is being made prior to the residual pressure in the barrel being reduced to the point that the brass is no longer being forced against the chamber walls. The ideal solution (if this is the case) would be to move the gas port in the barrel closer to the muzzle. IE why a Garand can function with the gas port as close to the muzzle as it is, but a gas operated pistol caliber AR needs the gas port somewhere around 3-4" from the chamber. It hand cycles just fine. |
|
I have dealt with a couple of them in 308 that had issues like described.
I modified the gas tubes with a small restrictor plug crimped into the tube, I would have to dig up my drawings to get the size of the orifice I used but if memory serves me correctly it was about 2/3-1/2 the diameter of the gas tube. Check the chamber, on both I dealt with the chambers looked like they were machined with a rock. I used a fired casing from each rifle and lightly lapped the chamber, keeping the lapping compound away from the shoulder so headspace would not be affected. |
|
Changing the gas port might be a more ideal solution for a system that is perhaps overgassed by a higher pressure round than it was originally designed for, but changing spring pressure to retard the opening of the bolt might be more practical, or at least worth a try.
I'm basing this on the fact that overgassed AR's can often be "fixed" by increasing buffer mass and spring tension. No particular experience or expertise with the MaS platform. But just tossing it out there...... |
|
Quoted:
Changing the gas port might be a more ideal solution for a system that is perhaps overgassed by a higher pressure round than it was originally designed for, but changing spring pressure to retard the opening of the bolt might be more practical, or at least worth a try. I'm basing this on the fact that overgassed AR's can often be "fixed" by increasing buffer mass and spring tension. No particular experience or expertise with the MaS platform. But just tossing it out there...... My only problem with increasing spring pressure is that generally speaking, springs won't retard the opening of the bolt, or they will retard it only by tiny amounts compared to your second statement, adding mass. For example, most blowback guns have very heavy bolts with relatively light springs. Altering the spring pressure in these does very little to retard the bolt, but has a very great impact on return slide velocity. Now that's blowback, and apples to oranges, but basically I think it has more to do with the residual pressure in the barrel at the time of unlocking than it does with the acceleration of the bolt carrier. But that's just my farmhand physics and I'm more than willing to try springs. Also, the chamber in this gun looked like it was carved by an angry beaver as well, prior to polishing. |
|
I agree with you on all points, and polishing the chamber sounds like a good idea. Of all the things you can do, adding spring is the least expensive, and least labor intensive, and could buy you a millisecond of time unlocking to bring you off the pressure spike.
Or it may do nothing. 
But it's easy to undo if it doesn't work.... |
|
Quoted:
can you take out the chamber insert and convert it back to 7.5 french? No. The barrel was set back and rechambered. There is a writeup out there from the smith who designed the conversion for Century. Most of his recommendations were not followed, and he and Century had a falling out of sorts. If you can find that writeup, it should give you a guide to fixing it. |
Armory Sponsor