I thought about trying o-rings for a single test firing and I may still try it tonight. It might demonstrate to me if the tighter piston I am making will do the trick or I need a bigger gas port. The o-ring ought to hold up for one shot at least!
I am working on the tighter gas piston because I would need to buy more aircraft extension drill bits (12" long) at $3.00 each to enlarge the gas port. The gas block is so close to the rear sight block that the drill chuck hits the rear sight block even with 6" long drill bits.
I used 4130 tubing from wicks for a gas tube. When you put your finger over one end and pull the piston out it "pops" yet slides smoothly and freely. Used it on the 45wm build and IMHO really boosts the piston movement.
Since my gas piston is so short (1") I couldn't plan on a "long stroke" gas operation. I can only get the standard short push of the piston inside the gas block. My gas tube is 4130 tubing that is sized big enough in back for the bolt carrier OD and then it tapers to a smaller size in front to guide the gas piston into the gas block opening. I salvaged and used the rear ring of the AMD65 gas block to add the locking surfaces of the stock system to my custom gas tube.
I hope tightening up the gas piston/gas block clearances will bump up the recoil impulse without affecting reliability.
I'm begining to think that there are way too many varriables to really acurately calculate what will and won't work. Unfortunately, with all the data, pretty much nothing is remaining constant (barrel length, port distance, port diameter, spring length, carrier weight, ammuniton).
I think that If I start with a gas port which is really small and work my way up, I will probably be ok. I need the smallest possible port that will cycle the action reliably. The closer that port is to the breech, the more sensitve the whole system will be to changes.
I did mean to say "the rear of the gas tube" and not " the rear of the gas block". Its the thingy with the tab at the bottom and angled surface at the top so that the whatchamacallit can lock down the gas tube. There.. clear as mud.
I am working through a rough list of detail work to get mine cycling.
1. I worked out any resistance felt when sliding the bolt carrier and bolt back and forth.
2. Reworked the hammer face to allow the AK74 carrier to pass over the AMD hammer. (It was catching on the bottom front of the carrier after the bolt had passed over the hammer on its way forward.) The same mods gave the bolt slightly more leverage when recocking the hammer.
3. Made new lighter hammer springs. I also made a new spring to reset the trigger since the lighter hammer springs have the effect of lightening the force on the back of the trigger.
4. Shortened the recoil spring by 2". This compensated for the extra force the recoil spring was exerting in the 1" shorter bolt carrier tube.
5. Opened up the gas port in stages from 0.125" to 0.156" to 0.172".
6. Now I am tightening the piston to gas block clearance.
The last 3 steps were all accompanied by test firing.
I am getting close and it is getting exciting. I'll start my own thread when I get to the point of pictures.
Last edited by SW44; 12-11-2008 at 02:24 PM. Reason: english correction
not yelling just have bad eye strain at the end of the day. cant see the letters very well and I might be a little drunk as well.hey bub why are you yelling?
As far as adding orings and extra tight piston to tubes or gas blocks I played with that a bunch. my 223 varmit build has a custom GB were the piston rides in it for around 2" in a pretty snug bore. I was unable to get under a .075 port to cycle. I was attemting to run as small of a port as possable in hopes of accuracy gains. I felt I gained very little, possably was able to run the port .005 smaller than normal. I started at .040 and workd up in .005 incriments after I got the action to partally move at around .055" port. this is with a very smooth action polished and reprofiled hammer ect. I would be very carefull with very tight tubes ect. if the action were to jam or a case were to stick you are putting around 45,000 psi on the gas block, piston ect. some thing might give. I doubt a O ring would hold up to the heat and grime and pressure very long. I considerd cast iron piston rings but decided if a
.002 piston to bore fit didnt really help then nothing will.
heres a pic of the gas block I did. the tube is a short AMD 65 but the piston is full length rommy reaching past the tube into the gas block. this allowed for a snug fit in the block its self aprox .002 . I have had wolf ammo stick when the chamber gets really dirty. and when that happens I can tell you that the block sees some pretty high pressure. I get gas leaks between the barrel and block. not sure what the actuall pressure in the block was but the pressuer in the barrel is at over 50,000 PSI a sealed piston and up jamed up action must see a least close to that momentarily if nothing moves.
yea thats some rust after 1000 rounds on a damp day. it was initally cold blued but it didnt hold up. all better now.
I weighed a full length AK74 carrier with piston and bolt, and got about 1lb. Not a very exact scale, but for arguments sake lets say I am way off an it weighs 1/2 lb. 7,000 grains to a LB, so 3,500 grains for the carrier assembly as a very conservative measure. the pressure upon reaching the 6" mark on the barrel is propelling an 85 gr bullet to 1400 fps (also conservative measurement). (1400 fps x 12" = 16800" per second. 16800 / 11 inches of remaining barrel = 1/1518th of a second to leave the barrel.) the carrier is (3,500/85= 41.1) times heavier than the bullet. that same pressure would move the carrier 41 times slower than the bullet is moving. The carier travels backwards about 1/4" before begining to unlock the bolt. So the bullet will travel the remaining 11" in 1/1518th of a second. The carrier will travel (1400fps /41= 31.1) at 31.14 FPS, or (31x12"=372" per second) and so will cover 1/4" in (372" per second / 4 = 93) or 1/93 of a second. The fractions are really confusing, so let me convert them.
1/93= 0.01075 seconds for the carrier to unlock (1/4" travel)
1/1510= 0.00066 seconds for the bullet to leave the muzzle (11" travel)
or in laymans terms, the bullet will be long freaking gone by the time the bolt unlocks. Unless my math is complete bunk, it would take a much much lighter carrier or a much slower bullet to get a case head rupture because of early extraction. I used conservative measurements and didn't even factor in acceleration of the carrier, the hammer (which will be resting against the bolt) or hammer spring, or the recoil spring. It seems like IF my math is correct, that I am well within the safe zone.
say what you mean & mean what you say