What do you think they are going to do?? Chuck the assembled upper in the lathe???
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I know the barrel has to come off, I'm no dolt... I was talking about the FSB folding mechanism itself being disassembled.
Depending on what the final needs would be, .076" can be fine for some, or too large for others. I really don't like over gassing, but it would need the required porting for function in needs in operation for its use. Will it ever see a can or anemic loads for serious use?
I am not and never will be using a gun for a living but if I did I would want it over gassed. Dependability would be worth more than a slightly reduced recoil impulse. That is why fighting gun manufacturers drill big ports.
MRBS/MRBF can be the attribute to look at. Sure, a slightly over gassed can lead to a descending level of increased function or an increase in use depending on how much. What gives the wider span in use of operation it's attributes? A level of function that runs at that or descends into that?
If the gassing is too small marginally, then the descending operation leads to higher MRBS. Over gassing can lead toward MRBF.
In operation, the preferred gas port would work well within the entire span of use for that port.
When I was at SAW and was looking at the tools they use, I could have sworn it was explained that way. I may be wrong and I do not work on 6940's at all.
Also, my port size was incorrect. I had midlength on the membrane. A .071 would be fine as that is Crane spec. For extra comfort you could go .073.
I wouldn't, and couldn't know now, but working with Crane had been an experiance onto itself then. They can have their own ideas on things that wasn't necessarily what I tried to explain. It was a hard crowd then, maybe better now, who knows?
I'm super familiar with 10.3" barrels and Crane spec (.070) but I wasn't aware they had a spec for 11" barrels. Surprised it's that close.
For 10.3" barrels, Crane spec will definitely give you issues with .223 powered ammo. You'll short stroke a bit unless you're using full 5.56 nato pressured. The shorties do get gas port erosion more quickly over time so it'll "work itself" in to reliability with .223 but you won't have it from the start. Most people who want reliability seem to start with something around .073/.074. But again, this is for 10.3", not 11".
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To the OP, where are you having this done? If you have it done at a reliable shop you shouldn't need to worry about any of this. Shops like ADCO maintain lists of standard port sizes they use for each size they chop/thread for with proven testing and reliability. And you also wouldn't need to worry about FSB issues as they deal with them all the time. I'd highly recommend sending it off to them, for $70 to get something you know will be reliable and something they do thousands of times a year in their sleep it's a pretty great price. I certainly wouldn't be playing with a smith who's requiring you to spec the port size and explain how to disassemble the FSB - especially with a work gun.