Disco .177, .22, .25, caliber swaps. These parts are needed.

[lloyd-ss]

A lot of questions about what is needed to convert a disco to various calibers with good answers sprinkled about.  I have tried to compile it all and add a little more info.

To do a basic caliber switch between any of the 3 calibers you will need 3 parts for each caliber:
1) barrel
2) bolt (or probe, or bolt probe, whichever you prefer to call it)
3) o-ring for the bolt (this o-ring seals the breech end of the barrel for firing.

Mustang Mike pointed out that you can find the parts for the .177 and .22 on the Crosman site at:
http://www.crosman.com/pdf/manuals/BP1K77xx%20&%20BP9M22xx%20EVP.pdf
BP1K77xx Unique Parts List, those 3 items are what you need to convert a 22 to 177
BP9M22xx Unique Parts list, those 3 items for converting a 177 to a 22
(Look on the crosman site under customer service, manuals, and then look in the Benjamin-Sheridan section)

The similar 3 parts would be needed for converting to .25 cal, but Crosman does not offer those parts so you have to find them aftermarket.

So how come you don't have to change the breech block??  The stock Disco breech block (in both .177 and .22) has a hole for the barrel that is about .438 diameter, So any barrel of that O.D. will fit.  The bolt end of the block has a hole for the bolt of about .250 diameter, so the hole is already big enough for a .25 cal pellet.  The important part is how the probe end of the bolt, and its o-ring, mates up with the breech end of the barrel.  A .177 cal bolt won't seal the end of a .25 cal barrel. 

One thing you must remember about this type of conversion is that as you go up in caliber, the FPE will only go up a very tiny bit, and the velocity will drop significantly.  That is becasue even though you have a bigger hole in the barrel, the stock valve is still flowing the same, small amount of air.  The stock disco was made to get lots of shots, not to be a power-house.  To increase the FPE as you increase the caliber, internal mods to the gun must be done.  The first, and easiest to do is a heavier (stronger) hammer spring like what rsterne makes, and that will add enough FPE to make most people happy.  What the heavier hammer spring does is make the hammer hit the valve stem a little harder to open the valve faster, farther, and for a little longer dwell. In other words, it puts more air behind the pellet.

After that, there is a list of at least 10 additional mods you can do to boost the power.  As rsterne has shown many times, an RVA (spring adjuster) works very well to tune the preload on the heavier spring.  But most of these additional mods have to do with streamlining the airflow and reducing choke points for the airflow inside the valve and transfer port, and where the air is approaching the valve.  So this group of mods keeps the pressure up as the air runs thru the valve, and doesn't allow the air pressure to trail off real quickly. 

By going all-out with the power mods I was able to peak at 80 fpe with a .25 cal shooting heavy Eun Jins. 

The real point is that this is a very versatile platform and you can easily convert from .177 to .22, to .25 and even to .308 if you want, plus gets a lot more power if that is what you want.
Lloyd

[grumpy]

Excellent post Lloyd. Thanks for posting it.

Dave

[airpuffhunter]

great info
thank you

[william71743]

Lloyd you and Bob are always a power house of great information ! thanks for all that you do.......

[Lambchops]

Thanks Lloyd, just what I was looking for.

[rsterne]

Great post, Lloyd.... I would add that unless you are a glutton for punishment (hand waving madly in the air here) I wouldn't go bigger than .25 cal on a Disco (and you can't without a breech change).... It isn't too difficult to make the Disco valve flow enough air to get about 45 FPE in .25 cal (without hammer mods, just a spring).... which puts it in MRod territory, although the shot count will be around 10 shots.... Stick one of Lloyd's Disco Double kits on it, and you've got a great, hard hitting, light, hunting rifle.... The .25 Auto barrel liners from TJ's are 7/16" OD and slide right into a Crosman breech.... The smallest LW barrel in .25 cal is 12mm OD and needs a short section turned down to 7/16" to fit, but the barrel still clears the main tube with a stock breech.... Both work well, and will require a chamber, leade, and barrel port machined into them.... I would suggest you simply set the gun up for a poly port (0.166" ID) as it will flow enough air and it's easy to do.... and you will need a new bolt with a bigger O-ring.... Last I looked, Mountain Air sold LW barrel/bolt kits for the Disco ready to install....

Bob

[lloyd-ss]

Good points, Bob, thanks for joining in.  I agree that even though the Disco, with a fair amount of work, has enough power for a  .308 barrel, going bigger than .25 cal is a much more challenging task. If you can stick with a stock breech, you can really keep the cost down.
By poly port, you are referring to a short length of thick wall .166 ID polyethylene tubing, correct?

Lloyd

[Lambchops]

Why is Lloyd not a moderator! He's so helpful

[lloyd-ss]

Why is Lloyd not a moderator! He's so helpful

Ha, Ha, thanks, but my boss says I've got to leave the airguns at home and give him an honest 9 hours a day. 
That's one of the many great things about the GTA, there are LOTS of helpful people on here, and the best moderators keeping this a fun and helpful place.
Lloyd

[rsterne]

Yes, a "Poly Port" is a short length of 1/4" OD x 0.166" ID polyethylene tubing.... IIRC, Mountain Air sends a short piece out with their .25 cal kit.... Cut square and to the correct length (critical) they work well and seal great.... If you don't get the ends square or if it's too short, they leak.... If it's too long, they kink from the compression, or you can't tighten the breech down properly.... About 0.005"-0.010" longer than the space between the top of the valve and the shoulder on the barrel port is best.... While they can be made to work sitting against the curved OD on a barrel (so I've been told) I always use a 1/4" mill to cut a shallow square shouldered counterbore in the barrel for the tubing to sit against.... You only need to go deep enough to have a complete circular recess....

Bob