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Crosman Phantom nitro piston conversion 
PostPosted: Sun Apr 03, 2011 10:04 pm Reply with quote
SMP
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I decided to convert my phantom .22 to a nitro piston to see just how much different it would be on a platform that I'm very familiar with. Why? Because I'm an engineer and we are genetically programed to do things just to see what will happen. Laughing
The gas ram was a full power crosman model ordered from Eric at scopesandammo.com
Supplied with the ram is a piston that has an extra thick base with a countersink drilled into it that mates with the end of the ram. I deburred the piston slot and cross honed the inside to a 800x finish and finished the outside in a similar manner to hold the lubricant.

There are two ways that the conversion can be done
1)Cut the spring guide off the existing end plug and use that. This is definitely the simpler method but I would recommend finding a nylon bushing 1/8 or 1/4 inch thick that is close to the diameter of the tube and drilling it to fit the ram. This would get secured to the end of the ram using one of the new nylon compatible super glues. This would keep the ram positioned in the center of the tube.
2)Use the end plug that is supplied with the gas ram.
The installed position works out the same with either method but the supplied end plug is drilled to support the gas ram in a socket. I chose to use the supplied end plug rather than cutting of the spring guide of the original. However using it requires a few modifications as follows.

The plastic dress cap needs to have an 1/8 trimmed off of it to bring the pin holes into alignment.



The threaded hole in the end plug for the trigger group bolt is 1/16 rearward from the original which requires the trigger group bolt hole in the tube to be opened up rearward to allow the bolt to be threaded in



This shifts the forward stock screws out of alignment by a 1/16 as well. Which is easily dealt with by a file or a small dremel rasp.

The amount of preload on the ram is very small, only about 3/16. This makes it much easier to slip the trigger group into it's slot before the compressor is tightened. Even though the preload is very small a spring compressor is an absolute must. I tried to compress the ram with all my 200lbs and I couldn't move it at all.



The rest of the assembly is as per normal

Subjective result
    Initially the cocking force was high but after 5 shots it settled down to just a bit harder than the full power spring
    Overall the shot cycle is greatly improved. The lock time is much faster than the spring version.
    The rearward recoil is heavy. Heavy enough to jar your cheek and shoulder
    The forward recoil is almost unnoticeable
    The gyrations from spring harmonics are completely gone
    The rifle is much less hold sensitive. I experimented with good hold technique and a tight grip and it made very little difference.
    Overall the accuracy is much improved. As seen in the 15m rested targets below.






Objective result
Here are the chrony results from 5 shot strings

RWS Hobby 11.9gr Avg V 755.5 fps

CPHP 14.3gr Avg V 720.5 fps

JSB Exact Jumbo 15.9gr Avg V 658.0

JSB Exact Jumbo Heavy 18.9gr Avg V 642.8

So consistent with the information from Eric, that the B18/19 only has enough swept volume to propel a 14.3gr to 720 fps, it's right on the nose.

Where I am seeing a difference is in the heavier ammo. I would have to look for my notes for exact numbers but I recall with the spring there was more spread between the 15.9gr and the 18.9gr JSB's.
As well the Velocity of the 18.9 is 40 fps faster than it was with the spring.
And the deviation of the 18.9 is 8fps or 1.2%

Overall I'm extremely happy with the mod.


Last edited by SMP on Mon Apr 04, 2011 8:11 am; edited 1 time in total
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PostPosted: Sun Apr 03, 2011 11:06 pm Reply with quote
AirGunEric
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Aha!

Finally, a conversion with chrony numbers- good info!

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PostPosted: Mon Apr 04, 2011 6:48 am Reply with quote
SMP
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Well, funny thing happened on Saturday.
My wife and I went out for an order of poutine and I came home with a chrony...I have no idea how the h$!! it happened. Wink
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PostPosted: Mon Apr 04, 2011 1:05 pm Reply with quote
AirGunEric
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SMP wrote:
Well, funny thing happened on Saturday.
My wife and I went out for an order of poutine and I came home with a chrony...I have no idea how the h$!! it happened. Wink


As long as your "significant other" isn't making an issue out of it- you're probably doing fine!

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PostPosted: Tue Apr 05, 2011 12:23 pm Reply with quote
SMP
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Haha

Nope she's fine with it. As soon as she found out that her quest was faster than the phantom she deemed it a good tool.

And we are not going to explain why it is faster are we Laughing
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PostPosted: Mon Apr 18, 2011 3:18 pm Reply with quote
domer_pyle
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Hell, why not?
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PostPosted: Thu Aug 18, 2011 10:34 am Reply with quote
Plasticman
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I did this conversion and now have a problem with the pump linkage bending from the extra force need to cock it.
Does anyone have any ideas on how to strengthen these parts? I was thinking of trying to weld on some sort of brace on them but don't know if there is enough room to do it.
Any ideas would be apreciated.
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PostPosted: Thu Aug 18, 2011 10:46 am Reply with quote
SMP
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Darn.

I didnt have this problem. After 4 or five cycles the cocking force settled down nicely. Did the bend happen during the first few cocking cycles?

There is all kinds of room in the phantom stock for a much beefier linkage. I would be inclined to remake it entirely before attempting to brace it. But silver soldering or brazing a small strip along the arm should work.

Post a pic if you could

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PostPosted: Thu Aug 18, 2011 11:59 am Reply with quote
Plasticman
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Darn thanks for the reply. I had over 200 shots when I had the problem.
After taking the gun apart this morning I found the arm bent where the roller is mounted to it. I think I can weld a brace on the side that won't interfear with the stock. The arm is pretty weak in that area because of the hole in it to mount the roller.
Time to phone my welding Buddy
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PostPosted: Thu Aug 18, 2011 12:18 pm Reply with quote
AirGunEric
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Funny- I came across something a few weeks ago (don't remember where, unfortunately) where someone had a bent cocking arm- on an NP rifle itself- not a conversion, but one straight from the factory.

Any way to post a picture of the bent arm?

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PostPosted: Thu Aug 18, 2011 12:29 pm Reply with quote
Plasticman
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Eric the bend is so slight I don't think it would show in a picture. It must bend more when I am cocking it for it to move out of the track.
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PostPosted: Thu Aug 18, 2011 3:14 pm Reply with quote
AirGunEric
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So it actually pulls itself out of the 'track' in the piston?

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PostPosted: Thu Aug 18, 2011 8:16 pm Reply with quote
Plasticman
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Eric there is a small roller on the side of the arm that runs along a track cast into the stock. It slips off there and jams against the stock.
I don't know how to post pictures on here so I will try to email you some later tonight
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PostPosted: Fri Aug 19, 2011 4:42 pm Reply with quote
Plasticman
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My idea to beef up the arm by welding a piece on the side won't work because it interfears with another part that slides along it. Now I have to come up with an other idea. I did take a few pictures and will try to post them.
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PostPosted: Fri Aug 19, 2011 4:46 pm Reply with quote
SMP
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depending on your location it may be just as easy and cheap to order a new cocking arm

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Crosman Phantom nitro piston conversion 
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