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Ok the mission brief is a 2 day game in some open woodland. The field FPS limit is 350 with .20s for absolutely everything
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. Now I don't own a gun that is this low in FPS, even here in England where 328 +/- 10% is absolute max for an AEG. All my rifles are between 400 and 500FPS semi auto and bolty. So I had to buy :'( or make
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one 
.
I had this floating round the garage and only ever used it twice because it kept locking up on semi only
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Original spec Dboys SPR Mk12 Mod0 (400fps) Locked to semi only so as it was usable in this country
I wanted this rifle to be reasonably quiet when fired as I am expecting to be alot closer to the enemy than I'm used to and also my experience of sites that limit their FPS to 328/350 have a large amount of players that wont accept a hit unless it happens more than once or they see where it came from, pretty bloody useless if your a sniper using a bolt action, so a quick follow on shot had to be there as well .
To achieve this I striped and cleaned the gearbox, Incidental I changed the gearbox shell for a TM one I had floating about as the Dboys 1 had cracked badly at the front.
I then made an Air braked piston head
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The last pic shows version 1 but was found to be to short version 2 is now about an 1" long and covered in heat shrink tube to better fill the nozzle tube and be more effective. The foam on the back of the cylinder head is not really necessary but dampens the noise even further and also helps with the Angle Of Engagement on the piston (AOE)
The AOE was adjusted by adding spacers/washers between the piston and piston head so as the primary teeth of the sector gear and the piston are maximized at first contact, this normally means the removal of the second and about half the third teeth though so that you don't get pre-engagement.
Heres a pic to better describe whats going on
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Bare in mind that in the photo the piston is sitting hard against the cylinder head and the sector gear is in the actual position it will be in on first engagement, the tooth to tooth surface area has been maximized thus reducing the chance of a stripped piston to virtually nil
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Note also the sector gear has 2 teeth removed to shorten the overall stroke thus reducing windup time and therefore noise of the motor, there is a down side to this in that it deduces the FPS of the weapon due to not giving the piston its full stroke.
Next was to eliminate spring rattle. This was done on the spring so that just inside the end of the piston the spring is retained with no space to allow it to rattle when the compression is released and on the spring guide for the same reason. All I did was wrap Insulation tape round the spring so it is an interference fit in the piston and smear it in grease and the same on the spring guide.
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After shimming and greasing everything I reassembled the gearbox and then set about covering all the holes in it that weren't used with self adhesive foil tape. This helps to eliminate sound transference and whistling of fast moving air and reduce the 'echo box' effect by containing the sound to one area
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With the gearbox done it was time to do the barrel and hop. For this part of the build I wanted to experiment with some stuff and try out a couple of theories I had heard about :-/. I have read that all you need for good accuracy is a good stable hop and a stable straight and highly polished barrel, not necessarily a tight bore either For this project I used a 509mm 6.08 brass barrel out of a TM M16. and I cleaned it and then polished the life out of it Heres a pic of the products I used, some guys in the US use real steel cleaning and polishing compounds, but here in the UK we are gun free so make do and mend is the way forward
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Next the hop unit. An all metal TM unit was used in conjunction with a standard TM hop rubber, if this turns out to be poop I will try a Guarder Black. The biggest difference here is in the nub. Technically it hasn't got one. I have been thinking about getting one of these H nub doohickeys having read up on the whys and wherefores fore and against them, and the theory is to limit the amount of pressurized contact on the BB to 2 main points and to stabilize the flight path. Now I was thinking 'what if' and came up with this
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Poor photos I know but you may just get the idea It basically halves the H into more of an I. Care needs to be taken when sticking the piece of plastic to the arm, I used a section of sprue from a model kit, to make sure it is absolutely perfectly centre. The theory, if the only pressurized contact with the BB is directly on the top in the middle the is only one way it can go and it will be the same everytime, all the time, where as with the H nub there is two points, twice as likely for it to put uneven amounts of spin, remote I know, but still possible I then shimmed either side of the arm in the hop unit to prevent it leaning from side to side when in use then fitted the barrel with the usual Teflon tape, sheep shit and good will
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The gun was then re-assembled and the flash hider changed to 1 off my mates M4 so I could fit a QD suppressor to the end. As luck would have it we are not restricted here and are able to have functioning suppressors so its full of foam and baffles to deaden muzzle pop even further
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A Crane Stock was also fitted to eliminate the boxy noise that a solid stock gives when the weapon is fired, I know I could of stuffed the space round the battery full of foam but I also use this weapon with a different top reciever as my FIBUA/FISH weapon and it just looks better that way
The scope was chosen because its a) Mil dot and not 30/30 like my spare scope b) small, and c) its x2 to x6 with a good Field of view, so I thought ideal if the enemy are going to be all up close and personal
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The finished article minus gun rag, just need to touch up the paint on the scope and barrel now :
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Initial test indicate that the motor whine is the loudest noise, but I have yet to fiddle with the motor height so that can be reduced a little. Gearbox noise and muzzle pop are near inaudible under the motor whine but is still there as a dull thump now rather than a loud crack. The other major advantage to the air brake mod is that it will virtually eliminate the problem of the gearbox shell ever cracking at the front again because of the cushioning effect it gives
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The power of it is 340 average over a 10 shot string using .20s.
I'm quite happy over all with this project and think I have achieved what I set out to do, ie. make a quiet weapon. The true test will come when I get to skirmish it and test to see if the hop mod actually works or is just destined for the scrap heap....
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VA
.I had this floating round the garage and only ever used it twice because it kept locking up on semi only
Original spec Dboys SPR Mk12 Mod0 (400fps) Locked to semi only so as it was usable in this country
I wanted this rifle to be reasonably quiet when fired as I am expecting to be alot closer to the enemy than I'm used to and also my experience of sites that limit their FPS to 328/350 have a large amount of players that wont accept a hit unless it happens more than once or they see where it came from, pretty bloody useless if your a sniper using a bolt action
, so a quick follow on shot had to be there as well .To achieve this I striped and cleaned the gearbox, Incidental I changed the gearbox shell for a TM one I had floating about as the Dboys 1 had cracked badly at the front.
I then made an Air braked piston head
The last pic shows version 1 but was found to be to short version 2 is now about an 1" long and covered in heat shrink tube to better fill the nozzle tube and be more effective. The foam on the back of the cylinder head is not really necessary but dampens the noise even further and also helps with the Angle Of Engagement on the piston (AOE)
The AOE was adjusted by adding spacers/washers between the piston and piston head so as the primary teeth of the sector gear and the piston are maximized at first contact, this normally means the removal of the second and about half the third teeth though so that you don't get pre-engagement.
Heres a pic to better describe whats going on
Bare in mind that in the photo the piston is sitting hard against the cylinder head and the sector gear is in the actual position it will be in on first engagement, the tooth to tooth surface area has been maximized thus reducing the chance of a stripped piston to virtually nil
Note also the sector gear has 2 teeth removed to shorten the overall stroke thus reducing windup time and therefore noise of the motor, there is a down side to this in that it deduces the FPS of the weapon due to not giving the piston its full stroke.
Next was to eliminate spring rattle. This was done on the spring so that just inside the end of the piston the spring is retained with no space to allow it to rattle when the compression is released and on the spring guide for the same reason. All I did was wrap Insulation tape round the spring so it is an interference fit in the piston and smear it in grease and the same on the spring guide.
After shimming and greasing everything I reassembled the gearbox and then set about covering all the holes in it that weren't used with self adhesive foil tape. This helps to eliminate sound transference and whistling of fast moving air and reduce the 'echo box' effect by containing the sound to one area
With the gearbox done it was time to do the barrel and hop. For this part of the build I wanted to experiment with some stuff and try out a couple of theories I had heard about :-/. I have read that all you need for good accuracy is a good stable hop and a stable straight and highly polished barrel, not necessarily a tight bore either
For this project I used a 509mm 6.08 brass barrel out of a TM M16. and I cleaned it and then polished the life out of it Heres a pic of the products I used, some guys in the US use real steel cleaning and polishing compounds, but here in the UK we are gun free so make do and mend is the way forward 
Next the hop unit. An all metal TM unit was used in conjunction with a standard TM hop rubber, if this turns out to be poop I will try a Guarder Black. The biggest difference here is in the nub. Technically it hasn't got one. I have been thinking about getting one of these H nub doohickeys having read up on the whys and wherefores fore and against them, and the theory is to limit the amount of pressurized contact on the BB to 2 main points and to stabilize the flight path. Now I was thinking 'what if' and came up with this
Poor photos I know but you may just get the idea
It basically halves the H into more of an I. Care needs to be taken when sticking the piece of plastic to the arm, I used a section of sprue from a model kit, to make sure it is absolutely perfectly centre. The theory, if the only pressurized contact with the BB is directly on the top in the middle the is only one way it can go and it will be the same everytime, all the time, where as with the H nub there is two points, twice as likely for it to put uneven amounts of spin, remote I know, but still possible I then shimmed either side of the arm in the hop unit to prevent it leaning from side to side when in use then fitted the barrel with the usual Teflon tape, sheep shit and good will
The gun was then re-assembled and the flash hider changed to 1 off my mates M4 so I could fit a QD suppressor to the end. As luck would have it we are not restricted here and are able to have functioning suppressors so its full of foam and baffles to deaden muzzle pop even further
A Crane Stock was also fitted to eliminate the boxy noise that a solid stock gives when the weapon is fired, I know I could of stuffed the space round the battery full of foam but I also use this weapon with a different top reciever as my FIBUA/FISH weapon and it just looks better that way
The scope was chosen because its a) Mil dot and not 30/30 like my spare scope b) small, and c) its x2 to x6 with a good Field of view, so I thought ideal if the enemy are going to be all up close and personal
The finished article minus gun rag, just need to touch up the paint on the scope and barrel now :
Initial test indicate that the motor whine is the loudest noise, but I have yet to fiddle with the motor height so that can be reduced a little. Gearbox noise and muzzle pop are near inaudible under the motor whine but is still there as a dull thump now rather than a loud crack. The other major advantage to the air brake mod is that it will virtually eliminate the problem of the gearbox shell ever cracking at the front again because of the cushioning effect it gives
I'm quite happy over all with this project and think I have achieved what I set out to do, ie. make a quiet weapon. The true test will come when I get to skirmish it and test to see if the hop mod actually works or is just destined for the scrap heap....
VA
