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Recon Straw Breech Mod

Custom Recon breech mod

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#1 2iko

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Posted 03 June 2009 - 09:08 PM

Alright, so here's my first writeup for you folks!

I recently got a Recon because I couldn't stop thinking about the clip potential, especially with a modded clip for more ammo. So I got to thinking of ways to make the breech better on this gun since the plunger assembly is really just crap with all that dead space.

So the idea came to me when I looked at the angel breach and realized what happens without a dart tooth, the dart pushes forward into the barrel. Sadly the Recon lacks this barrel making any breech mod without it mostly useless because of all the dead space in the bolt barrel. I solved this to a degree and added a barrel to the front end for the dart to slide into as you'll see.

Materials:
Recon
1' of 1/2" barrel materials. (I used 1/2" copper and PETG)
Pipe cutters (depending on barrel materials)
Plumber's goop
1 long-ass straw (or more if you can manage it, I found these pretty easily)
electrical tape
Exacto knife (box cutter, anything similar)
scissors

Sorry about the quality on some of this as it was a cellphone cam ;)

So here's the first pic:
Posted Image
Missing the barrel in this. Took out the dart tooth and other unnecessary pieces. As you can see I did some standard Recon mods to start, added a wire to the outside of the plunger mechanism to shorten the spring's length inside the gun of course, trigger pull mod and removed the ARs. (as you can obviously tell since I chopped the barrel off there)

Closeups!
Posted Image
Here's the straw mod, standard mods, again. (also replaced o-ring and used Silicone lube)

Posted Image
And here's where it gets interesting:
So I glued a piece of 1/2" copper to where the breech used to be. I measured the old one and cut a piece of copper to the length it was and also measured where the plastic top piece was located so I could place it properly on the copper (essential or it won't cock/load properly!) The main reasons you'd want to use new material here would be to make sure you've got a good seal between the straw hole end and the barrel you're going to need. The problem with the original barrel is you'd need to cut it down anyway and it's got an upwards curve on the bottom-front part which really would not help the seal with the barrel piece.

This is the point where I actually did the straw mod so it would go through from one end to the other.

I was planning on having another larger pipe to fit over the copper but it doesn't seem to work that well with the plastic sticking out top-front of it like that. (I'm planning on changing it to pump action so I will change the old cocking mechanism)

Posted Image
And this is the new PETG barrel I measured out to specifications based on measurements of the breech/plunger system:
Plunger tube:
Length:		 Inner Diameter:
9cm			  2cm

Straw:
L:				 ID:
17.75cm		6mm

PETG:
ID:
1.5cm

Equations:
Pi * (ID/2)^2 * Length = Volume

(Vpt-Vs)/PI * (ID/2)^2 = Length of barrel

Plunger tube:
Vpt = Pi * 1^2 * 9 = 28cm^3

Straw:
Vs = Pi * 0.3^2 * 17.75 = 5.0cm^3

Barrel:
L = 23/Pi * 0.75^2 = 13cm
So using these numbers I realized the barrel could be as long as 13cm and it would use all the air from the plunger! So I cut it a little short, wrapped it with electrical tape to fit it snug, gooped it up and shoved it in there tight.
Posted Image
And here's the back of the barrel. The tape on the other end is so it slides into the front attachment and aims properly, I'll show my front barrel and stock mods later as they are old news but I like them anyways. I measured the distance into the gun the barrel had to be by putting the new breech into the gun and pushing as far forward as it's going to go and then I pushed the barrel into the front until it's against the breech front and marked off where it's sticking out the front piece of the gun.

Sorry no ranges yet, still waiting for the barrel to dry completely. So far it seems to work fine otherwise and the only problem I ran into was the darts would catch on the PETG barrel but I can probably fix it up by making a bit of a ramp into the barrel leaving space so the copper bolt doesn't stop short of the PETG.

Also planning on doing other standard mods like the "extra" spring in the stock.

EDIT:
Ranges are stock so far but I am planning to add a stock spring and I should see considerable improvement with extra force on the plunger with this mod!

Questions? Comments? Suggestions? Flames?
Try not to be too harsh please, I can learn!

Edited by 2iko, 06 June 2009 - 10:00 AM.

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#2 Shwanky

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Posted 03 June 2009 - 09:14 PM

Does it feel like it shoots any farther? (You said you had no ranges I was hoping you had seen some gain or something)

Very nice. I'm guessing it does have better ranges since you said you were about all the air from the plunger.

You worked it down to the equations. Good job!

I might do this.
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I'm new to nerf but feel like I'm good at modding and at playing.

Currently modding every gun I can get my hands on.

Working on my own mod. I can't wait to finish and get the write-up...well written up. =)

#3 2iko

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Posted 03 June 2009 - 09:16 PM

The trick is to make a seal between the barrel and the breech end.

And I don't have ranges because as I said the glue on the barrel is still drying.
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#4 Shwanky

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Posted 03 June 2009 - 09:40 PM

The trick is to make a seal between the barrel and the breech end.

And I don't have ranges because as I said the glue on the barrel is still drying.


Oh, thank you.

Oh, alright.

It looks very nice and I can't wait to see what kind of ranges you get! (assuming you'll post them.)
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I'm new to nerf but feel like I'm good at modding and at playing.

Currently modding every gun I can get my hands on.

Working on my own mod. I can't wait to finish and get the write-up...well written up. =)

#5 Darth Freyr

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Posted 03 June 2009 - 11:02 PM

A 5" barrel may use all the air, but that air will not have enough force to continue to push the dart at the end. Try redoing the equations so the barrel ends when the force behind the dart is equal to the force of friction on the dart. The pressure behind the dart, in atmospheres, would be (plunger_volume + straw_volume + dead_space) / (straw_volume + dead_space + barrel_volume_behind_dart). The force would be pressure, in your preferred unit, times the cross-sectional area of your barrel. Friction can be found by placing the dart in your barrel, placing a short spacer in front, and pushing the assembly into an accurate scale. Use the highest reading as your friction. This approach, modified to fit your blaster, will work for optimum barrel length in any blaster.
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#6 2iko

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Posted 03 June 2009 - 11:26 PM

(plunger_volume + straw_volume + dead_space) / (straw_volume + dead_space + barrel_volume_behind_dart)


Okay, now this equation doesn't make sense to me. I thought the dead space was the space inside the straw because it never gets filled with anything other than air? Also with this rig, the dart is being pushed into a barrel by the end of the straw barrel so I don't see how this equation works.
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#7 Darth Freyr

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Posted 04 June 2009 - 08:44 AM

I stated the dead space and straw as two things because I thought it would be simpler that way, and I thought you may have some dead space in the barrel. Plunger_volume is the used volume of the outside part. Straw_volume is obvious. Dead_space is unused space behind the dart. Barrel_volume_behind_dart is the volume of the barrel the dart has already passed through, the length of which you are looking for. Use this big equation to find the pressure in the blaster, multiply by the area behind the dart, and you have the force behind the dart. When this force equals your friction, that is your barrel length. This approach uses only static pressure, not dynamic pressure, so you may find that the prediction is slightly lower.
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#8 2iko

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Posted 04 June 2009 - 12:09 PM

Barrel_volume_behind_dart is the volume of the barrel the dart has already passed through, the length of which you are looking for.


Does this mean it's the space behind the dart when it's pushed into the barrel from closing the breech? If so there's none. Or does it mean the length of the barrel (which you seem to mention)?

Assuming the first is true it's:
(28 + 5) / 5
= 33 / 5 = 6.6
So the friction needs to be less than 6.6? 6.6 what?

Edited by 2iko, 04 June 2009 - 12:10 PM.

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#9 Darth Freyr

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Posted 04 June 2009 - 12:31 PM

It is the volume of the entire barrel. You're looking for the length of the barrel. The main section gives you the pressure in atmospheres. Convert this to a force / area pressure scale, e.g. pounds / sq. inch. Multiply this by the cross-sectional area of your barrel, probably around 0.2 sq. inches. This resulting number is the force acting on the dart at the end of your barrel. You can find the friction by putting the dart in the barrel, placing a short spacer in front, and pushing the barrel into an accurate scale.

Friction = ((1.7 + 0.3 + 0) / (0.3 + 0 + 0.2 * barrel_length)) * 14.6 * 0.2

Find your friction, plug it in, and solve some equations (or just let me do it).
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#10 2iko

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Posted 05 June 2009 - 07:51 PM

Alright, I don't have any scales to measure the friction but I'm sure someone knows what the friction would be for a streamline going through a 1/2" PETG barrel from OMC.
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#11 Vinnie D

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Posted 06 June 2009 - 01:44 AM

I've tried straw modding a recon myself, and found that although you reduce dead space, you need more force to push the air through the now constricted area. The result is with only the recon's standard spring, roughly identical ranges. With your barrel addition it might improve though. You do start to see performance increase if you can improve the spring's power to force the air through. You start seeing some nice ranges when you use a powerstock, thus giving you enough force to push air through the tighter space and have enough power left over to push the dart.

Basically a straw mod increases air volume delivered to the dart, but decreases the air velocity, so you need more spring power to make up for it. The upside I've found, and you may notice it yourself, is that this increases resistance to the plunger when it hits the end of the tube thus giving you a slight air cushion reducing wear and tear on your plunger and silencing the recon a bit. I'd still advise adding a felt pad or a bit of foam to the back of the plunger though.
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#12 2iko

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Posted 06 June 2009 - 09:58 AM

Basically a straw mod increases air volume delivered to the dart, but decreases the air velocity, so you need more spring power to make up for it. The upside I've found, and you may notice it yourself, is that this increases resistance to the plunger when it hits the end of the tube thus giving you a slight air cushion reducing wear and tear on your plunger and silencing the recon a bit. I'd still advise adding a felt pad or a bit of foam to the back of the plunger though.


Very good point! And yes I have noticed that the gun is much quieter. Also if you noticed I did make the spring more compressed while in the gun, and I am planning on adding a stock spring soon when I can find piping to fit around the plunger end when it's out and hold the spring in there.

So basically this mod allows for other mods to be more useful, interesting concept.

Good news as well, I've tested ranges out and it's about stock (when it works, I'm still working out kinks like getting the darts into the barrel correctly.)

I can't wait to see ranges with proper barrel length and the stock spring mod... :3
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