Idea's and ?'s for a piston upper

[greywuuf]

Hey guys, I notice some of you have put together some piston uppers and some of you have even designed and created your own systems. I find it all very amusing as I resisted the Black rifle urge for a LONG time as I was a BIG FN/FAL fan and kept saying I really like the adjustable piston gas system.( most of my AR bud's kept telling me that the piston was just extra reciprocating weight and DI was sooo much better) I even went as far as making a short barreled short gas system .358 win version of an FN. My question is this, are any of you familiar with the FN's and if so what do you think about a gas block like that for the AR. with a nice easily adjustable gas vent, to allow tuning for individual loads ? If you are not familiar with the system I can try to dig up an exploded view to post. I am considering just threading a tube into the gas block and then outside threading the end of it, so that i can put the adjustment nut on. Also someone that is more familiar with the pressure curve of various rounds should comment on piston diameter and how it relates to port pressure and such ( in other words how do I decide how big my piston should be ? )

Thanks
Dan

[love2shoot]

with these cartridges, the port pressures are not high, so if you can get it running with a small to medium sized piston, the better.

With mine, it is a POF system, which is kind of a progression of the fn49/FN-fal system but instead of having the gas adjustment like you described, this system has a 3 position gas plug. kinda like how with the fn49, you turn it around so it is turned off for grenade launch mode....... the POF plug has a normal, suppressed and off position with the plug....not quite as tuneable as the 49/fal.

with the Tok, the port pressure levels in both my sub-sonic, super sonic and surplus are all low enough that a gas adjustment actualy is not needed what so ever. something like what you are describing might be better for a 300 whisper/blackout build whish will have more gas with the super sonic loads due to case capacity.

[greywuuf]

hmm, kind of a conundrum I guess. the lower the pressure the larger the piston area has to be but the larger the piston the faster the pressure drops with piston movement. I am sure I could think my self to death with this with pressure graphs and calculations or I can just start drilling holes and try to make it work.

Any one have any recommendations about piston size and port size/location ?

[HMC710]

I've got a couple of pistons from a Harley Twin Cam 88 you can try.
What if you took your BCG apart and measured the piston on the end of the bolt - the area for the firing pin?

[greywuuf]

Oh ! you mean the part of the BCG that acts as a piston to power the original action? Man this logic thing just might keep me from trying to invent the wheel. As far as the 88 twin cam offer.. I think I would have to to hog the cylinders out of the sporty just a bit to far to make them work.